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Control & Protection of Motor (no 165)

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Control & Protection of Motor (no 165) Powered By Docstoc
					                                        n° 165
                                        monitoring and
                                        protection of
                                        HV motors
J.Y. Blanc

He graduated as an engineer from
SUPELEC in 1979. He joined Merlin
Gerin in 1981 as Medium Voltage
dielectric engineer. Still in the
MV Division, he was then in turn as
project manager for the development
- Fluair range primary substations
and equipment,
- SF set built-in protection circuit-
He is currently in charge of
development of MV primary
distribution and switchgear.

E/CT 165 first issed, May 1995
Cahier Technique Merlin Gerin n° 165 / p.2
control/monitoring and protection of                                                Unit power of rotating machines
                                                                                    frequently exceeds 100 kW in industry
HV motors                                                                           and large tertiary. In such cases and/or
                                                                                    if the length of the supply line is
                                                                                    particularly long (voltage drops,
                                                                                    losses…), use of high voltage motors is
                                                                                    The purpose of this Cahier Technique
                                                                                    is to analyse and compare these
                                                                                    motors, their starting systems and the
                                                                                    various protection devices which may
                                                                                    be used, in order to simplify technical
contents                                                                            choices.

1. Reminder of the various types of      Asynchronous cage motors           p. 4
AC motors                                Asynchronous slipring              p. 5
                                         rotor motors
                                         Synchronous motors                 p. 6
                                         Sizing tolerances                  p. 6
                                         Dielectric withstand and tests     p. 7
2. Classical HV starting processes       Direct stator starting on          p. 8
                                         full voltage
                                         Stator starting on reduced         p. 8
                                         Startor starting with capacitors   p. 9
                                         Rotor starting                     p. 10
                                         Choosing the starting mode         p. 11
3. Control and monitoring equipment      Electromechanical solutions        p. 13
                                         Electronic solutions               p. 15
4. Protection of HV motors               Main fault types                   p. 18
                                         Protection principles              p. 18
                                         Technological evolution            p. 22
Appendix 1: determining motor            Calculation hypotheses             p. 23
starting mode                            Overall approach                   p. 23
Appendix 2: coordination of protection devices                              p. 25
Appendix 3: bibliography                                                    p. 26

                                                                                    Cahier Technique Merlin Gerin n° 165 / p.3
1. reminder of the various types of AC motors

Both high and low voltage AC motors,
offer a large variety of electrical,
dynamic and technological                         C     I
characteristics. However, except for a            Cn   In
small number of motors used for highly
specific applications, they can be
divided into three families, namely:                                        current
c asynchronous cage motors;
c asynchronous slipring rotor motors;
c synchronous motors.                                  4
They differ from each other in:
c starting current and torque values;
c speed variation in normal operation;
c power factor and efficiency values as
a function of load.
HV motors are supplied with a voltage                  2
rarely exceeding 7.2 kV, their power               1
ranges from 100 kW to over 10 MW,
with an average of 800 kW.

asynchronous cage motors
These HV motors fall into two main
categories according to their rotor                0                 25 %              50 %             75 %        100 %    (N/Ns)
composition which can be single or
double cage.                                  fig. 1: curves C (N) and I(N) of an asynchronous single cage motor.
This enables choice of starting current
and torque characteristics:
c single cage rotors have:                       C       I
v a relatively low starting torque (0.6          Cn     In
to 1 Cn),
v a maximum torque of around 2
to 2.2 Cn,
v a starting current ranging from 4.5                  6
                                                              Id            current
to 5.5 In,
(Cn: rated torque, In: rated current);
c double cage or deep slot rotors have:                5
v a slightly higher starting torque (0.8
to 1.2 Cn),
v a maximum torque of around 2
to 2.2 Cn (slightly higher for deep slots),
v a starting current ranging from 5                    3                    torque
to 6.5 In.
Figures 1 and 2 show the form of these             1                                                                    Cn
curves as a function of speed (N/Ns).                  2

Note that:
c single cage motors have a minimum                    1                                                                In
torque (0.5 to 0.6 Cn), whereas the
torque curve, varying according to the
speed of the double cage or deep slot
motors, continues to increase up to                0                 25 %              50 %             75 %        100 %    (N/Ns)
maximum torque.
c these motors are ideal for intensive use    fig. 2: curves C(N) and I(N) of an asynchronous double cage motor.
and dangerous environments, due to:

Cahier Technique Merlin Gerin n° 165 / p.4
v the simplicity of rotor design in
short-circuit providing them with an
excellent mechanical and electrical                 η      cos ϕ
robustness,                                                                                       η
v absence of brushes.                             0.9
These two features allow maintenance
to be reduced to a minimum.                       0.8
The torque characteristics of                                                                     cos ϕ
asynchronous cage motors are                      0.7
especially suitable for machines such
as centrifugal pumps, compressors,                0.6
converter sets, machine-tools and fans.
However, all these motors have the                0.5
drawback of a relatively low power
factor, around 0.8 to 0.9 on full load,
and even less when they are running
on low load (see fig. 3).
If asynchronous motor installed power
is high, reactive power compensation is
required. This may either be global, for
each set of motors or for each motor
(large units).

asynchronous slipring                                0                                    2/4                 3/4                      4/4   (P/Pn)

rotor motors                                 fig. 3: efficiency curves η (P) and power factor curves cos ϕ (P) of an asynchronous double
The rotor winding of these motors            cage motor.
connected to sliprings means the
resistance of this circuit can be
modified by introducing external                                                 C
In the motor stability zone,                                                                Rr          R'r > Rr          R''r > R'r
corresponding to the positive slope of
curve C = f (g) (see fig. 4), the slippage                                                                         C''d
"g" is proportional to the rotor                                                 CM
g =     Rr C                                                                     Cn
      A                                                  rotor stability zone                                      Cd
                Ns − N                                   in short-circuit
where g % =             100
                  Ns                                            1          0.5
where:                                                                                0           0.5               1                         2       g
Ns : synchronous speed,
N : operating speed.
             p M
A = 3V 2             = constant
             ω L1
V: phase to neutral supply voltage,
p: number of pole pairs,
ω: pulsation of supply currents,
M: reciprocal stator-rotor inductance,                      operation                           operation                 operation in
L1: total stator choke,                                     on generator                        on motor                  regenerative
(L1 = M + Ls)                                                                                                             braking
Rr: rotor resistance = rotor inherent
resistance + external resistances,
                                             fig. 4: curves C (g) of an asynchronous slipring rotor motor.
C: motor torque.

                                                                                                      Cahier Technique Merlin Gerin n° 165 / p.5
By decreasing external resistance on          c the power factor which may be set by       These motors can supply reactive
starting, the characteristic C (g) is         the exciting current.                        energy by increasing the exciting
translated and the starting torque            Technologically, they are identical to       current. This characteristic, which
adapted to the torque of the machine          AC generators.                               enables compensation of network
being driven. Note that maximum               In order to obtain asynchronous torque       reactive loads, is one of the main
torque value does not depend on rotor         and avoid oscillations, the rotors are       reasons these motors are chosen.
resistance.                                   equipped with a damping cage. This           The curves in figure 5 show stator
Moreover, for small slippages, rotor          cage means synchronous motors can            current variation as a function of
current is inversely proportional to rotor    be started with a low load torque in         exciting current for a given constant
resistance. Its amplitude is given by:        similar fashion to asynchronous single       load (Mordey curves). Use of this type
          g                                   cage motors (they have practically the       of motor for small powers is fairly rare.
I2 = B                                        same characteristic torque and current       On the other hand, it is frequently used
                                              curves). To avoid surge voltages in the      above 2,000 kW for its excellent
where                                         exciting circuit, this circuit is shunted    efficiency and control of its power
          M                                   during starting and on tripping by a         factor. In the case of highly regular
B = V        .
          L1                                  resistance with a value chosen               movements, synchronous motors are a
                                              between 5 and 10 times the resistance        necessity. However, the machines
Stator current follows the same law,          of the exciting circuit.                     being driven must have a relatively low
except for the winding ratio and the
                                              In view of the fact that the                 load torque during starting, and sizing
magnetising current.
                                              asynchronous torque tends to zero on         of the dampening cage limits starting
Consequently, the choice of initial rotor     approaching synchronous speed,               rate.
resistance solves virtually all the           coupling to the network when motor
problems concerning high starting             starting is complete cannot take place
torque and inrush current on the              at synchronous speed as is the case for      sizing tolerances
network and ensures these two                                                              The electromechanical characteristics
                                              AC generators. The result is invariably
requirements are satisfied. The various                                                    of motors are defined by standard
                                              a transient state varying according to
possibilities for using asynchronous                                                       IEC 34-1. In the case of certain rated
                                              the speed acquired at the end of
slipring rotor motors make them                                                            characteristic values, the standard
                                              starting and motor power. To limit this
suitable for driving machines with high                                                    defines the tolerances to be complied
                                              transient state:
starting torque such as crushers,
                                              c either use a relay to monitor slippage     with by the manufacturer. It is useful to
mixers, conveyors, etc.
                                              by measuring frequency of the rotor          be familiar with these tolerances since,
Moreover, machines requiring high             current passing through the starting         for certain characteristics, they directly
regenerative braking also use this type       resistance. This relay controls exciting     affect the choice of motor and
of motor.                                     circuit supply when slippage is at its       equipment power and the setting of the
Just as for asynchronous cage motors,         lowest.                                      protection devices.
the power factor in normal operation is       This device is practically indispensable     The table in figure 6 gives the
relatively low: this characteristic and the   when the synchronous motor accounts          tolerances of the main characteristic
presence of sliprings and rotor               for a large part of total installed power.   values.
resistances mean these motors are             c or apply the exciting current in two
increasingly replaced by double cage or       stages, automatically or manually.
deep slot motors.                             The exciting sources can be either
Figure 4 represents the characteristic        separate:                                       (I) stator
C (g) curves, according to rotor              c motor set, exciter;
resistance value and the stability            c thyristor rectifier;                                                    full load

                                                                                                                           1/2 load

zones. These curves show the                  or placed at the end of the motor shaft:                                         1/4 load

advantage of introducing a high               c reversed generator;

resistance in the rotor circuit as a

                                              c rotating "diode rectifier and armature"
means of obtaining efficient                  reversed AC generator.
regenerative braking.
                                              The techniques most frequently used
                                              are the thyristor rectifier and the
synchronous motors                            "rotating diodes".                              cos ϕ AR cos ϕ AV
The main differences with                     The latter does away with brushes,                   cos ϕ = 1
asynchronous motors are:                      removes the exciting cubicle and, in            0                         (i) excitation
c their constant speed (synchronous           addition, is often fitted with a
speed);                                       synchronisation and recoupling
                                                                                           fig. 5 synchronous motor: Mordey curves.
c the rotor circuit supplied with DC;         mechanism should synchronism fail.

Cahier Technique Merlin Gerin n° 165 / p.6
dielectric withstand and                      Two types of tests are stipulated in the            test voltage applied to the rotor will be
                                              standards: standard frequency tests                 4 U + 1,000 V.
tests                                         and impulse withstand tests.                        Impulse withstand test
Motors, just like all electrical network
components, are subjected to a variety        Standard frequency test                             This test consists in applying an
of surge voltages. They are particularly      Withstand to switching surge voltages               impulse voltage representative of
sensitive to steep front surge voltages       is checked in compliance with standard              lightning:
or high frequency since they are              IEC 71, by the standard frequency                   c buildup time: 1.2 µs;
"jammed" by the first turns of the stator     withstand test. Testing commences                   c dropdown time at Upeak/2: 50 µs;
windings.                                     with a voltage of less than U/2 which is            c test voltage:
Switching surge voltages                      gradually increased up to 2 U + 1,000V,             Upeak = 4 U + 5,000 V.
These are the result of transient             at which level it is applied for one                The windings are subjected to a
phenomena occurring during changes            minute.                                             number of positive and negative waves.
in status in the supply network.              For the stator, U is the specified supply           Impulse withstand tests are not
The following phenomena, specific to          voltage. For the rotor, U is the voltage            currently mandatory in standards, as
inductive circuits and thus to motors,        which appears, with the rotor circuit               they can lead to early ageing of
must be taken into consideration:             open, when the specified stator                     armature and winding insulation.
c current pinch-off on current breaking;      supply voltage is applied with the                  More generally, the dielectric tests
c multiple re-ignitions on breaking and
                                              rotor locked in rotation. If the motor              must not be repeated; if a second test
prearcing on current making if the
                                              is reversible (change of rotation                   is performed, it will be carried out at
breaking device is capable of breaking
the high frequency currents                   direction of motor already started), the            80 % of the voltages indicated above.
corresponding to these phenomena.
Steep front surge voltages
These are the result of direct or indirect    value                                   tolerance
lightning strokes. They spread onto the
network, creating a dielectric stress         asynchronous motors
which, even when limited by the use of        current with locked rotor               + 20 % of current
surge arresters, can be considerable.         and short-circuited                     (no lower limit)
Surge voltage is studied in detail in         torque with locked rotor                - 15 % to + 25 % of torque guaranteed
"Cahier Technique" n° 151
                                              minimum torque during starting          - 15 % of torque guaranteed for cage motors Cd u at
"Overvoltages and insulation                                                          a third of rated torque and i at half of torque with
coordination in MV and HV" and                                                        locked rotor, at full voltage
special motor sensitivity in
                                              maximum torque                          - 10 % of torque guaranteed provided that once this
"Cahier Technique" n° 143 "Behaviour
                                                                                      tolerance is applied, the torque remains u 1.6 times
of the SF6 MV circuit-breakers Fluarc                                                 rated torque
for switching motor starting currents".
To check the motor’s capacity to              synchronous motors
withstand to these various surge              current with locked rotor               + 20 % of value guaranteed
voltages, the motors undergo                  torque with locked rotor                - 15 % to + 25 % of value guaranteed
standardized tests performed as
                                              pull-out torque                         - 10 % of value guaranteed provided that once this
defined by IEC 34-1 standard.                                                         tolerance is applied, the torque remains u 1.35 times
The test voltage is applied between the                                               rated torque (1.5 for synchronous motors with salient
winding being tested and the body of                                                  poles)
the machine to which the magnetic
circuits and all the other stator and rotor   fig. 6: tolerances on the main characteristic values as in standard IEC 34-1.
windings are connected.

                                                                                                  Cahier Technique Merlin Gerin n° 165 / p.7
2. classical HV starting processes

The main HV motor starting processes         Voltage reduced by reactance               Starting voltage is determined by the
are as follows:                              This starting mode (refer to power         maximum current inrush l'd authorised
c direct stator starting on full voltage;    diagram, figure n° 8) is the one which     on the network:
c stator starting on reduced voltage by      reduces current inrush on the network                         I'd
star-delta connection, by reactance or       in the simplest manner. Since motor        Ud = Un                .
                                             starting torque is low, the machines                          Id
by autotransformer;
c stator starting by capacitors;             being driven must have a relatively low    The phase-to-phase voltage drop in the
c rotor starting.                            load torque during start up:               reactance has the following value:
                                             compressors, centrifugal pumps,            →        →                           →
                                             converter sets, etc.                       Un − Ud = j                3 L ω I'd .
direct stator starting on full               In point of fact, asynchronous motor
                                             torque varies according to square of the   The diagram in figure 9 shows that this
voltage                                                                                 relationship can be expressed in an
                                             supply voltage , whereas the absorbed
This starting mode is used for                                                          arithmetic form for asynchronous
                                             current remains proportional to this
asynchronous motors with cage rotor                                                     motors, since the power factor at the
and for synchronous motors.                  voltage.
                                                                                        initial moment start up moment is
Current peak on starting is around 4                   U                              initiated is virtually the same as the
to 7 In, according to motor                  C' d = Cd  d                             power factor of the starting inductance.
                                                        Un 
characteristics, and can last for roughly                                               Therefore:
1 to 10 seconds depending on the             where :
                                                                                                     Un − Ud
moment of total inertia (motor +             C'd: starting torque with reduced          L ω =                .
machine), motor torque and load              voltage,                                                  3 I'd
torque.                                      Cd: starting torque with full voltage,     Knowledge of start up time and
If this starting mode is used, the           Ud: starting voltage,                      operation rate is required to determine
network must be able to withstand the        Un: rated operating voltage.               reactive power.
above current overload without                            Ud
disturbing the other loads, and the          I'd = I d                                  Voltage reduced by autotransformer
machine being driven must be able to                      Un                            This starting mode sometimes makes it
withstand the mechanical impact due to       where:                                     possible to reconcile reduction of
the motor torque. The simplicity of both     I'd: starting current with reduced         current inrush on the network and
equipment and motor and the resulting        voltage,                                   motor torque value. In point of fact, it
savings mean that this mode is very          Id: starting current with full voltage.    has the advantage of reducing current
popular and even recommended                 These relationships can also be            inrush according to the square of the
provided that voltage drop on the            expressed using the rated                  winding ratio:
network on start up is acceptable. The       characteristics as follows:
decisive factor lies in the motor power/
                                             I'd   I Ud
short-circuit power ratio.                       = d     .
                                             In    In Un                                        Ud
                                             The curves in figure 7 give the ratio              Un
stator starting on reduced                   variations as a function of
voltage                                                  Ud                             0.8
                                             the ratio      .                                                  Cd
Star-delta starting                                      Un
This starting mode is used to reduce:                                                   0.6
                                             Voltage at the motor terminals
c current in a ratio of e;                   increases gradually during starting. The
c starting torque by a third.                                                           0.4                            I'd
                                             resulting start up is smooth.
It is used in LV and for low powers, but                                                                               Id
                                             c operation and schematic diagram:
rarely in HV due to the high current         v first stage:                             0.2
peaks when moving to delta. In this          operation on reduced voltage by                                                      I'd     C'd
case it is replaced by reactance             closing the line contactor CL,                                                       Id      Cd
starting.                                    v second stage:                                0        0.2       0.4     0.6       0.8   1
Voltage reduced by resistance                normal operation by closing the short-
Commonly used in LV, it is rarely used       circuit contactor CC;                      fig. 7: graphs showing start-up on reduced
in HV due to the Joules to be dissipated     c determining a starting reactance         voltage (by reactance or star-delta).
and resistance insulation problems.          (see fig. 9)

Cahier Technique Merlin Gerin n° 165 / p.8
                     2                       rating of the neutral point contactor to         stator starting with
I'd   I U 
    = d  d                                 be reduced;
In    In  Un                               c there is an alternative version of this        capacitors
                                             configuration in which the neutral point         This process enables motor full voltage
C' d   Cd  Ud                              contactor is removed. This version is            starting characteristics to be
     =                                     not recommended since the move from              maintained. It is mainly used for
Cn     Cn  Un                              reduced to full voltage necessites the           keeping the starting torque of
where:                                       motor-network link be cut. In view of the        synchronous motors constant during
                                             relatively short switching time, and the         start up, for example in cement works
I'd: starting current on network side with
                                             residual voltage at the motor terminals,
reduced voltage.                                                                              and crushing plants.
                                             a current inrush exceeding starting
These relationships are used to              current can ocur on transfer to normal           Capacitors, as well as motor, supply
determine the value of reduced voltage       operation. This is unacceptable for both         part of the reactive energy during the
authorised on the network, as a function     the network and the motor and can lead           starting phase: the motor power factor
                                             to the tripping of the protection devices.       is small at this stage. Power inrush on
               Ι'd                 C' d
of the ratio       or of the ratio
               Ιn                  Cn
authorised by the machine being driven.
The curve in figure 10 gives the                                                              1
                                                       3L                                             Un
variation of      as a function
               Un                                                                             0.8

     Ι'd    C' d
of       or      .                                                                            0.6
     Ιn     Cn
c operation and schematic diagram                                              CL
(see fig. 11)
CL: line contactor,
CC: short-circuit contactor,                                                                  0.2
                                                                                                                                     I'd     C'd
CPN: HV neutral point formation                                                                                                      Id      Cd
contactor,                                                                     CC       L
AT: autotransformer                                                                               0        0.2    0.4        0.6    0.8      1
v first stage
                                                                                              fig. 10: graphs showing start up by
operation on reduced voltage by                                                               autotransformer on reduced voltage.
closing CPN which causes CL to close,
v second stage                                                            M
operation in inductance by opening CPN,
v third stage
operation on full voltage by closing CC.     fig. 8: power diagram: starting by reactance.             3L
c in theory the second stage is short
(around one second) since in most
cases it is a slowing-down time;
Use of an autotransformer with air gaps                                                                                            neutral point
considerably reduces this fault, but                              Jj L ω I'd        3
requires knowledge of the value of the
current absorbed by the motor at the
end of the first stage;
c the move to full voltage invariably                                                                 CL         CC                        CPN
results in a transient state whose
duration varies according to the speed                                                                                  AT         3L
acquired at the end of the first stage
and the value of the current absorbed;
c the current flowing through the                           ϕd                                                        M
neutral point on start up is equal to the
difference between the motor current                                                    I'd
and the line current, excluding the                                                           fig. 11: power diagram: starting by
magnetising current of the                   fig. 9: vectorial diagram to determine L.        autotransformer.
autotransformer. This enables the

                                                                                              Cahier Technique Merlin Gerin n° 165 / p.9
the network is reduced accordingly                    Moreover, control equipment must be                        c third stage
(see fig. 12).                                        chosen especially for capacitor                            short-circuiting of the second section of
This technique is tricky to implement                 switching.                                                 the rotor resistance by closing C2;
and calls for a study of the motor and                                                                           c Nth stage
capacitors to avoid resonance and                                                                                short-circuiting of the n-1 section of the
surge voltage (due to motor self-                     rotor starting                                             rotor resistance by closing Cn-1.
excitation), as well as mechanical                    This starting mode solves virtually all
                                                                                                                 The number of stages, or notches, n is
oscillations on the transmission                      the problems which may occur on
                                                                                                                 always greater by 1 than the number of
system.                                               starting, namely:
                                                                                                                 sections or contactors.
                                                      c reduction of current inrush on the
                                                      network with increase in motor torque;                     This number n is determined
                                                      c adaptation of motor torque to load                       approximately by the formula:
 inductive                                            torque;                                                            log gn
 reactive                                             c long, progressive starting (e.g. for                     n =
 power          apparent                              loads with high inertia).                                         log n
                motor power                                                                                                  Cp
                on starting                           This mode can only be used for
                                                      asynchronous slipring rotor motors and                     or by n = n gn
                                                      for synchronous induction motors (its                             Cp
                                                      use is increasingly rare in industry).                     where:
                              power supplied          It is particularly used for load starts.                   Cp= peak torque,
                              by capacitors                                                                      gn = rated slippage.
                                                      Example of a rotor start in n time
                                                                                                                 According to each case, n can be
                                                      This start up is illustrated in figures 13                 deduced from Cp, and vice-versa.
                                                      and 14. Motor torque varies between
                                                      two values at each notch. The lower                        Complete determination of rotor starting
                                                      value is taken as being equal to the                       equipment calls for knowledge of
                          power supplied              rated torque. At each notch, rotor                         operation (hourly rate and starting
                          by the network              resistance changes value and the                           time). Lack of standards for HV motors
                                                      torque-speed characteristic evolves. In                    means this equipment is determined in
                                                                                                                 each individual case by specialists.
                                 active power         the last stage, rotor resistance is simply
                                                      equal to the rotor internal resistance.                    Remark
fig. 12: vectorial diagram for starting by            c first stage                                              A linear start is sometimes required.
capacitor.                                            stator supply and starting on full rotor                   This calls for power electronics
                                                      resistance by closing CL;                                  enabling monitoring of rotor energy: for
                                                      c second stage                                             example using a Graëtz bridge and a
                                                      short-circuiting of the first section of the               chopper to make a continually variable
                                                      rotor resistance by closing C1;                            resistance (see fig. 15).

                         neutral point





                                             Cn - 1
              M                                                                4th        3rd        2nd stage             1st stage
                                                                     0   gn g4       g3         g2                 g1                      1     g

fig. 13: power diagram: rotor starting.               fig. 14: symmetrical rotor starting diagram.

Cahier Technique Merlin Gerin n° 165 / p.10
choosing the starting mode                            Starting conditions                                c motor torque is always greater than
                                                      In view of the required starting                   load torque (see fig. 17);
Choice of starting mode is conditioned                                                                   c current inrush on the network and the
                                                      characteristics, it is necessary to check
by ensuring that the motor torque and                                                                    corresponding voltage drop are
                                                      that starting can take place in correctly
the load torque of the load are properly              as far as motor torque, current inrush             acceptable to the network;
matched.                                              and starting time are concerned, for the           c starting time is compatible with the
Knowledge of the load torque is                       starting techniques under                          equipment used.
required (see fig. 16).                               consideration:                                     Approximative calculation of starting
                                                                                                         Operation of the motor-driven machine
                          tachometer dynamo                                                              assembly is governed by the following
                                                                                                         mechanical equation:
                               N                    I0 +                                                 Cm − Cr = J
                                            +                                                                                 dt
                                        -       N                      I
                                                           -                                             where:
                                                                                                         Cm: motor torque with Un voltage,
                                                                                                         Cr: load torque,
                                                                                                         J: inertia of rotating frames (motor and
                                                         I                                               machine driven),
          3L                                                                                                  : angular acceleration.
                                                                                                         Angular velocity varies from 0 to n
                                                                                                         throughout starting time ∆t. Moreover, a
                                                                                                         mean accelerating torque Ca, equal to
                                                                                                         the mean difference between Cm and
                                                               R1          R2                            Cr can be defined:
               M                                                                                         This results in Ca = (Cm - Cr) mean
                                                                                                              (ωn − 0)
                                                                                                         = J
                                                                                                                J ωn
                                                                                                         ∆t =        .
                                                                                                         Bearing in mind that real motor torque
fig. 15: setting speed by rotor chopper.                                                                 varies according to the square of its
                                                                                                         supply voltage:
                                                    characteristics             examples                 C'm   U 
                                                                                                             =  real  with real voltage.
    C                                                                                                    Cm     Un 
                                                                                                         C’m = motor torque with real tension.
                                                    constant torque             piston compressor        Reducing this voltage will thus reduce
                                                                                                         Ca, hence increasing starting time.



                                                                                                          C     torque
                                                    parabolic torque            pump:                     Cn
                                                                                          valve opened   1.8                          Cm
                                                                                ----      valve closed


    C                                                                                                                                            C'r
                                                                                                         0.9                          C'm

    Cn                                              high breakaway              crusher



                                    n                                                                       0
fig. 16: rreminder of load torque curves of machines to be driven (loads).                               fig. 17: case of non-starting (C'm < Cr).

                                                                                                         Cahier Technique Merlin Gerin n° 165 / p.11
Starting mode selection table                       c starting by stator impedance;                   c motor torque and current at full
The table in figure 18 summarises the               c direct starting.                                voltage as a function of rotation speed;
advantages and disadvantages of the                                                                   c load torque of the machine driven
                                                    Choice of starting mode calls for good            (see fig. 16);
main starting modes for the various                 communication between the electrical              c moment of inertia of the rotating
applications.                                       energy supplier and the manufacturer              frames.
For a given torque, the current                     of the motor and machine being driven.
                                                                                                      If the supply network power/motor power
absorbed on the network is established              The vital characteristics for making this         ratio is less than 5, particular care must
in the following increasing order:                  choice are:                                       be paid to choice of starting mode and
c rotor starting;                                   c supply network power and maximum                choice of coordination of the protection
c starting by autotransformer;                      authorised current inrush;                        devices (see appendices 1 and 2).

application                 application                  starting mode            controlled by                   advantages
needs                       characteristics                                                                       disadvantages

permanent or                machines requiring           direct                   1 or            1               simplicity,
quasi-permanent             high start torque                                                                     less investment.
process i 1/jour                                                                                                  on starting:
frequent starts > 1/day     motors with low current      direct                                                   c high torque;
                            inrush or low power                                                   1               c high current inrush,;
                                                                                                                  c high mechanical stresses.
pumps, fans,                machines starting            stator                                   2               reduction of current inrush
compressors,                with low torque              by reactance                                             on starting
frequent starts                                                                                                   (possible adjustment).
optimisation of             when starting current        stator by                                3               optimisation of torque
starting characteristics    must be reduced, but         autotransformer                                          (reduced) and of current inrush
                            the necessary starting                                                                on starting
                            torque maintained                                                                     (possible adjustment).
optimisation of             the most difficult starts    rotor                                    generally       small current inrush and
high torque                                                                                       3               high starting torque.
starting characteristics

fig. 18: starting mode selection table for the most common applications.

Cahier Technique Merlin Gerin n° 165 / p.12
3. control and monitoring equipment

The function of this equipment is             Circuit-breakers                                Fuses
threefold:                                    Circuit-breakers are generally used for         Fuse rating is determined according to:
c energising and stopping (control);          high motor powers of more than 300 A,           c rated current In;
c disconnecting the motor should a            with a small operation rate, and for            c the Id/In ratio (Id = starting current);
fault occur (protection);                     operating voltages of more than 6.6 kV.         c starting time determined using the
c monitoring the motor (monitoring).          Their use can naturally be extended to          chart in figure 20 page 14.
                                              lower powers, operable by switch or             Finally, it should be pointed out that
When we talk about monitoring, this           contactor.
implies the equipment is capable (or                                                          fuses protect the motor against
                                              Fuse-contactors                                 overcurrents roughly five times greater
not) of:
                                              c operation rate                                than motor rated current and that they
c initializing the start (starting sequence
                                              Their simple control mechanism                  must be combined with additional
automation);                                  combined with the robustness and                protection devices (thermal relays, etc.
c acting on motor speed;                      simplicity of their contacts mean               see "protection device" chapter).
c supplying information on motor              contactors have a high operation rate.          Current transformers
electrical status and contributing to         This rate cannot be withstood by circuit-
                                                                                              The increasing use of digital protection
protection.                                   breakers, even special ones, and even
                                                                                              devices means unconventional current
The monitoring function chiefly relies on     less so by switches.
                                                                                              sensors can be used (e.g. Rogowski
power electronics and low currents            Some installations use contactors with          toroids). These sensors have the
(digital technology); it is currently being   mechanical latching to do away with             advantage of being linear and thus
fully developed. Protection of                permanent consumption of the closing            delivering an accurate signal
HV motors will be dealt with in the next      electromagnet. This may reduce                  throughout the useful current range.
                                              endurance as a result of the greater
chapter.                                                                                      They present no saturation or thermal
                                              complexity of the kinematic chain.
                                                                                              problems, as is sometimes the case
                                              c network short-circuit power                   with classical Current Transformers
electromechanical                             This factor does not really affect              (see "Cahier Technique" n° 112 "The
                                              contactors thanks to the presence of            breaking process with a Fluarc SF6
solutions                                     fuses placed immediately after the
                                                                                              puffer-type circuit-breaker").
The choice between the various                isolating switch or next to the
devices (switch, circuit-breaker or           disconnecting contacts on the busbar            Special features due to use of fuses
contactor) depends on:                        side. These fuses with their high               or circuit-breakers
c operation rate;                             breaking capacity, limit the short-circuit      c operation in single-phase due to fuse
                                              current.                                        melt, with the striker not working.
c electrical endurance;
c motor power.                                This special feature means that,                Today striker reliability is such that the
                                              if network power is increased, the              risk is slight. Their dependability may
The main breaking device
                                              motor feeder cubicles can be                    be increased still further by use of an
characteristics are summarised in the         maintained. The busbar supports are             additional protection device
table in figure 19.                           strengthened if required.                       (undervoltage or unbalance relay).
By their very design, the breaking
capacity, mechanical and electrical           device                   mean rate            endurance              acceptable motor
endurance of switches is low. This                                                          nb operations          power
limits their use to small powers              fuse-switch              low: 2-3/day         2,000                  low i 50 A
(In = approx. 50 A - 5,500 V) and             circuit-breaker          low: 10/day          10,000                 high
to rates of two to three operations a                                                                              u 7.2 kVA
day.                                                                                                               > 300 A
                                              fuse-contactor           high > 10/h          > 100,000              average i 300 A
Moreover, the low breaking capacity of
these devices makes choice of                 fig. 19: breaking device application field.
protection devices tricky.

                                                                                              Cahier Technique Merlin Gerin n° 165 / p.13
                      t seconds

                                                                        rating of 7.2 kV fuses
                                                                        100A 125A 160A 200A 250A

                10                            motor In = 100 A
                                              Id = 6 . In
                                              td = 5 s
                                              fuse rating = 200 A



                                                      Amp In


               0.1                              90
              0.05                              50






                                                       d =

                                                 10                                                I
fig. 20: fuse determination chart.

Cahier Technique Merlin Gerin n° 165 / p.14
c discrimination with upstream
equipment.                                                      main
This can be hard to achieve when both:                          circuit breaker
v the fuses used have a high rating
(200 A or 250 A),
v and the feeder protected by these
fuses accounts for a large fraction of
the power supplied by the main circuit-
breaker (see fig. 21 and 22).
However, the high breaking capacity of
the Rollarc contactors combined with                      overcurrent
these fuses allows the use of slightly                    relay
time delayed overcurrent relays, thus
ensuring discrimination. Discrimination

is easier to achieve if the motor feeder
is protected by circuit-breaker but                                                                                    overcurrent
since, for high short-circuit currents the                                                                             relay
current is not limited, there is an
increase in thermal stresses.
c surge voltages
Some types of device, in particular                                                                    M
vacuum breaking devices, generate
surge voltages on motor energising and
stopping (due to their capacity to break     fig. 21: protection diagram showing a high current motor feeder.

high frequency currents, resulting for
example from the current pinch-off
phenomenon - see "Cahier Technique"                  t
n° 143).                                                                          circuit-breaking
In order to prevent these surge                                                   melting curve
voltages progressively damaging motor
insulation, manufacturers place ZnO                                                       remain circuit-breaker
type surge voltage limiters in the                                                        overcurrent relay
equipment if required.
It can be concluded that today's                          contactor                                                   contactor
electromagnetic solutions are reliable,                   breaking                                                    breaking
robust, economic and entirely suitable                    time                                                        capacity
for most applications.                                                                               discrimination
electronic solutions
These provide users with additional                      contactor
possibilities and advantages such as:                    overcurrent
c variable speed;                                        relay
c possibility of speed regulation;
c high operation rate;
c energy savings.
                                             fig. 22: discrimination diagram for a high current motor feeder.
The electronic solution is rarely used
just for starting.
Before dealing with standard cases of        c safety margin in temperature rise due             c reinforced insulation between turns
electronic devices, it should be pointed     to harmonics: a 15 % margin on current              due to the high voltage gradients
out that their use calls for a certain       is generally sufficient;                            generated by thyristor switching (which
number of constructive precautions at        c forced ventilation is recommended                 may reach the magnitude of impulse
motor level:                                 (motors can run at low speed);                      withstand test ones).

                                                                                                 Cahier Technique Merlin Gerin n° 165 / p.15
motor               loads                        speed variations            power                   overall                 speed controller
                                                                                                     efficiency              type
asynchronous        pumps, fans,                 0 % to over 100 %           a few 10 kW             0.85 to 0.90            autonomous
or synchronous      compressors, extruders                                   to a few 100 kW                                 rectifier/inverter
asynchronous        same                         60 % to 100 %*              a few 100 kW            0.90 to 0.95            subsynchronous
slipring                                                                     to a few MW                                     cascade
synchronous        same                         0%                           100 kW to a             0.90 to 0.95            selfcontrolled
                   centrifugal machines         to several times 100 %       few 10 MW                                       rectifier/inverter
                   TGV bogies
                   (high speed)
asynchronous       crushers, rolling            0 % to ± 33 %                100 kW                  0.85 to 0.90            cyclo converter
or synchronous     mills, cement kilns                                       to a few 10 MW
                   (low speed)
*: 100 % corresponds to relative speed at 50 Hz

fig. 23: application areas of electronic speed controllers for AC motors.

The devices described below are the                   U
ones most frequently used in medium
voltage. Only the general principles are
The table in figure 23 gives an idea of                                                                                                     t
the adequations between the type of
speed controller, the type of motor and
the type of load driven.
Autonomous rectifiers/inverters
With their capacity to deliver variable               U
voltage and frequency, they guarantee
complete control of motor speed and
A reminder for asynchronous motors:                   0
c torque is proportional to U2 if f and N
are constant;
c torque is inversely proportional to f for
a given voltage and speed.                          fig. 24: generation of variable voltages and frequency with PWM speed controllers.
There are three types of autonomous
                                                    with variable period and amplitude (see           v enables regenerative braking if the
c voltage rectifier/inverter
                                                    fig. 24).                                         rectifier is reversible.
v the thyristor rectifier sets voltage,             c current rectifier/inverter (switch)             c the PWM inverter
v the thyristor inverter supplies                   v the thyristor rectifier combined with a         v allows a wide range of speeds,
AC current at variable frequency.                   smoothing choke acts like a                       v maximum speed limited by the
This configuration is also used by                  DC generator,                                     maximum switching frequency
Uninterruptible Power Supplies (UPS)                v the inverter switches current in turn in        authorised by the inverter thyristors.
used in LV to supply computers. The                 the motor windings using capacitors.
                                                                                                      Use of power transistors (IGBT) makes
only difference is that frequency and               Motor frequency and thus speed
                                                                                                      it possible to work at far higher
voltage are fixed.                                  depend on switching speed.
                                                    A few elements of comparison                      frequencies but at lower powers,
c rectifier/inverter with Pulse Width               between these three types of                      v reversible operation possible (two
Modulation (PWM)                                    rectifiers/inverters:                             rotation directions).
v the diode rectifier supplies the                  c voltage inverter                                c the current switch
inverter,                                           v suitable for high reactance motors,             v suitable for low reactance motors,
v the inverter generates voltage pulses             v often requires a filter between inverter        v enables operation in the four
enabling reproduction of a sine wave                and motor,                                        quadrants.

Cahier Technique Merlin Gerin n° 165 / p.16
Subsynchronous cascade                     Cyclo converter                                  The second bridge is used for reverse
Asynchronous slipring rotor motors are     Each motor phase is "supplied" by a              current during negative alternation to
normally supplied by the network.          three-phase double bridge.                       one of the phases.
Speed is adjusted using the rotor          The first bridge is used to draw off             The cyclo converter produces a three-
current by means of a rectifier/inverter   current throughout the positive cycle on         phase pseudo mains requiring filtering
set. The rectifier draws off energy from   each of the network phases according             and with a frequency varying between
the rotor circuit, thus increasing         to the required frequency.                       0 % and a third of network frequency.
slippage. The energy drawn off
depends on the conduction setting of
the thyristors of the inverter which
reinjects energy into the network (see
fig. 25). The subsynchronous cascade                               3L    3L
enables continuous speed variation
with a maximum slippage of
around 40 %.
Converter set power is low compared
                                                                                      Pr        3L
with motor power, and the energy
regenerated results in outstanding
overall efficiency.
Note that the converter is put into
operation after starting by rotor
This assembly can operate at speeds
higher than synchronous speed                                  P
(supersynchronous) in the case of
driving loads.
Selfcontrolled rectifier/inverter
The motor stator phases (in this case
synchronous) are supplied in turn just
as for the current switching
autonomous rectifier/inverter.
Switching from one stator phase to the
next is selfcontrolled by motor speed by
means of a "notched disk" sensor.
There is thus a correspondence
between the excitation flux and the
armature flux, as for DC machines, and
the pull-out risk is zero.
Since switching presents problems both
on starting and at low speed, the con-
verter control system must be modified.
This solution is ideal for synchronous     fig. 25: subsynchronous cascade power diagram.

                                                                                            Cahier Technique Merlin Gerin n° 165 / p.17
4. protection of HV motors

"Motor protection" groups all the             The detection/protection processes for      Short-circuits
protection devices used to prevent            the main fault types are studied in the     Short-circuits on circuit-breaker
serious damage due to abnormal                paragraph below.                            equipment are detected by
operating conditions at supply, motor or                                                  instantaneous operation overcurrent
process level.                                                                            relays, set above the starting current.
The protection devices to be installed
                                              protection principles                       On fuse-contactors, short-circuits are
are chosen according to the following         Overloads                                   cleared by the fuses.
criteria:                                     Overloads can be detected by reverse        However, an useful solution is to add
c operating conditions;                       time overcurrent relays, thermal image      slightly time delayed overcurrent relays
c importance of the operation                 relays or heat sensors.                     to the fuses. This means that the
performed by the motor;                                                                   contactor can be used right up to its
                                              The relays process the information
c the degree of dependability required;                                                   breaking capacity.
                                              "current absorbed by the motor" which
c the relative cost of the protection
                                              is generally detected by current            Phase breaking, reversal and
device with respect to the motor;
                                              transformers.                               unbalance
c the likelihood of the faults considered
                                              The heat sensors are inserted in the        These faults are detected by a filter
                                              live parts of the motor.                    which highlights the negative phase
As well as:                                                                               sequence components.
c the type of load driven;                    c reverse time overcurrent relays
                                                                                          It is vital to monitor phase breaking and
c disturbances which could occur on           Their use requires:
                                                                                          unbalance since these faults give rise
the network;                                  v either an operating curve I(t) allowing
c the type of motor protected.                starting, or a device blocking the relay    c increased current, in the stator,;
The faults listed below may therefore         during starting,                            c additional temperature rise by Joule
require use of a protection device.           v an operating threshold I0 close to the    effect, due to the fact that all out-of-
                                              rated current In of the motor               balance states result in the appearance
main fault types                              I0 ≈ 1.10 In .                              of reverse currents flowing through the
                                              These relays do not memorise the            rotor at twice supply frequency in the
Asynchronous motors                                                                       rotor.
c overloads;                                                                              Phase reversal is detected either by
c short-circuits;                             c thermal image relays
                                                                                          currents or voltages:
c phase breaking, reversal and                These relays are certainly the most
                                                                                          c by currents: this reversal is detected
unbalance;                                    suitable since they ensure the greatest     after contactor closing: the driven
c insulation fault between turns;             possible advantage is derived from          machine receives the fault;
c stator frame;                               motor overload possibilities without        c by voltages: this means contactor
c under and overvoltage;                      damage.                                     closing can be prohibited, if necessary,
c incomplete starting.                        The operating curve I(t) of the relay       if phase order is not the normal network
Synchronous motors                            must enable the starting current to flow    one.
All the above faults, plus:                   without tripping and be approved by the     Insulation fault in winding
c loss of synchronism;                        motor manufacturer.                         Stator windings may have faults
c loss of excitation;                         c heat sensors                              between turns on the same phase or
c rotor frame;                                These are resistors, the ohmic value of     between windings of different phases.
c prolonged operation in asynchronous         which varies with temperature.              As a result of its electrical position, the
mode on starting;                                                                         fault may not be detected quickly
                                              In theory these devices are not used by
c overloads and short-circuits in the                                                     enough by the overload protection
exciting winding;                             themselves, but rather back up the
                                              relays using the current absorbed as a      device, thus causing serious damage.
c reverse power (operation on                                                             These faults are normally detected by
AC generator).                                measuring means.
                                                                                          current comparison.
Other faults linked to the process or         Overloads due to temperature rise of a
                                                                                          c longitudinal earth leakage protection
load                                          bearing are, in theory, insufficient for
                                                                                          Provides protection against faults
c over frequent starts;                       detection by the overload relays.           between windings of different phases.
c locked rotor;                               Bearings must therefore be protected        For this, the ends of the motor windings
c underpower or undercurrent.                 by thermostats or heat sensors.             must be accessible on the neutral side.

Cahier Technique Merlin Gerin n° 165 / p.18
Faults are shown up by comparing the
input and output currents of the same
phase (see fig. 26).
If there are no faults, these currents are
identical and the protection relay is not                                                           M                                  N
tripped. It trips when the difference
between these currents reaches a
value set by the relay setting.
c transverse earth leakage protection
Provides protection against faults
between turns of the same phase.
It applies to machines with divided
phases, i.e. with two windings per                                                               protection
phase.                                                                                           relay
The operating principle is the same as
above, that is the currents of each
winding are compared (see fig. 27).          fig. 26: diagram showing a longitudinal earth leakage protection.
Stator frame
This protection device is vital to comply
with the statement of the 14.11.1988 on
workers' protection. It is chosen
according to the earthing system of the
                                                                                                                        M                N
network supplying the motor.
c motor protection supplied by network
with earthed or impedance-earthed
The fault is detected by measuring the
zero sequence current formed between
the faulty phase and the network frame.
Low threshold overcurrent relays are
used for measuring.
The zero sequence current is delivered
by three parallel-connected current
transformers or, better still, by a toroid
(see fig. 28).
                                             fig. 27: diagram showing a transverse earth leakage protection.
Use of a toroid prevents the
appearance of a false zero sequence
component due to unequal saturation of
current transformers on motor starting,                                                                                           CT
and allows a relatively low operating
These relays must operate for a fault
current value such that frame potential
compared with earth is never raised to                                    toroid
more than 24 V in conductive
environments, with frames
interconnected, or 50 V in other
installation cases.
Knowledge of earth connection value
and of the frame interconnection                 zero                                                   zero
configuration is therefore necessary to          sequence                                               sequence
determine this setting point.                    current relay                                          current relay
If the frames are not interconnected,
the value of the operating threshold is                                                                                     eventually toroid
given by:
       24 or 50 V                            fig. 28: diagram showing a stator frame zero sequence protection with toroid sensor or CT +
IF i              .                          earthed or impedance-earthed neutral.

                                                                                                  Cahier Technique Merlin Gerin n° 165 / p.19
RTM is the value of the earth resistance
of the frame in question.                       diagram 1
Note that the lower the threshold, the
earlier the detection, and the smaller
the risk of damage to the magnetic
c unearthed neutral
The fault is detected by continuous
measurement of global network
insulation compared to earth by means
of DC injection devices such as
continuous insulation monitors (see               DC injection
diagram 1 in figure 29) or by zero                insulation                                                                           surge
sequence overvoltage relays where this            monitor                                                                              voltage
overvoltage is delivered by three                                                                                                      limiter
                                                                                 R            R             R
voltage transformers with the
secondary winding in open delta (see
diagram 2 in figure 29).                                                    R = load resistance
Under and overvoltage                           diagram 2
(see fig. 30)
c undervoltage
This relatively common protection
prevents the motor from working on
overload and waiting for tripping by the
overload protection device. Moreover, if
the contactor coil is supplied by an
auxiliary LV source not from the
network, the latch undervoltage
protection device is vital to prevent
untimely startup on restoration of
power.                                             zero sequence
The "voltage" information is provided by           voltage relay
a voltage transformer and processed by
a time-delayable threshold device.             fig. 29: insulation fault monitoring diagram with continuous insulation monitor or zero sequence
c overvoltage                                  voltage relay - unearthed neutral
This protection is necessary if there is a
risk of strong fluctuations occurring on       Prolonged use of the starting system,
the supply network. It means it is no          calculated to run for a set time, is thus
longer necessary to wait for the               avoided.
overload relays to trip, since
                                               Loss of synchronism
overvoltage results in motor overcurrent
                                               This protection is vital for synchronous
and increased motor torque which
could be detrimental for the machine           In point of fact, the dampening cage
being driven.                                  of a synchronous motor is relatively
The fault is detected by time delayed          fragile. If the motor jams, induced                                             voltage
overvoltage measuring relays.                  currents will flow through this cage and                                        relay
                                               could destroy it if the motor is not
Incomplete or overlong starting
This protection is justified for starting in   Jamming may occur following a                               M
several stages.                                mechanical overload, undervoltage, a
It is provided by a time delay relay put       loss of or drop in excitation.
                                                                                                  fig. 30: diagram showing under and
into operation at the beginning of             This fault is detected by under                    overvoltage protection. In most cases, two
starting and removed at the end. The           impedance or power factor relays                   VTs supply the relay phase-to-phase
value monitored may be speed or                supplied by voltage transformers                   voltages.
current.                                       and current transformers (see fig. 31).

Cahier Technique Merlin Gerin n° 165 / p.20
Loss of excitation                            Either the "incomplete starting"                 On starting, it is used as an exciting
This loss due, for example, to a break in     protection described above is used or a          voltage presence relay and authorises
rotor winding, causes the motor to jam.       thermal device adapted to the rotor              closing of the exciting contactor at
It can be detected by:                        thermal time constant, placed in series          the end of starting in the
c the "loss of synchronism" protection        with the inductance during starting (see         asynchronous mode (see fig. 32
described above;                              fig. 32 element b).                              element g).
c or an exciting undercurrent or              Overload and short-circuit in the                Reverse power
undervoltage relay.                           exciting winding                                 This protection particularly applies to
Rotor frame of a synchronous motor            These protection devices prevent                 synchronous motors.
This protection is determined according       damage due to temperature rise of the
                                                                                               When the supply circuit-breaker trips, it
to the supply configuration and the           exciting winding and its power supply.
                                              The fault is detected by an exciting             prevents energy from being sent back
DC production mode.                                                                            to the loads connected on the same
                                              overcurrent relay. Moreover an exciting
If the entire DC exciting circuit is          undervoltage relay is normally used,             busbar. It also prevents a fault on this
unearthed, an insulation fault will not       tripping on voltage drop, caused for             busbar from being supplied by the
affect motor operation.                       example by a short-circuit.                      motor.
However, should a second fault occur,                                                          The protection device has to detect a
it may give rise to an overload or short-                                                      reversal in current or power direction.
circuit with all its consequences. The                                                         Its function is therefore performed by a
relays used to detect this type of fault                                                       directional power relay (see fig. 33).
are generally devices injecting low                  a
frequency 10 Hz or 20 Hz AC current                                       b                    Frequent starts
(see fig. 32).                                                       I>                        Too many starts over a given period of
                                                                                               time may cause motor damage if the
The 50 Hz frequency is also used,                                                              motor is not designed for such an
which requires that no 50 Hz                                                                   operation.
components must be present in the
exciting circuit.                                                         c                    This protection is provided by a relay
                                                                                               performing metering and time delay
Prolonged operation in the                                                                     functions, which automatically limits:
asynchronous mode on starting                                                          d
                                                                                               c either the number of starts over a
Overlong starting on synchronous                                                               given period of time;
motors causes excessive temperature

                                                                                               c or the time interval between start ups
rise of the damping cage.                                                                      over a given period of time.



                                                                              U< g

                             or cos ϕ relay   a. exciting winding                                                         directional power
                                              b. thermal protection: prolonged operation in                               relay
                                              asynchronous mode
                                              c. starting resistance
                                              d. exciting contactor
                                              e. rotor frame protection
                                              f. overcurrent protection
         M                                    g. undervoltage protection
                                              h. to tripping of motor feeder contactor                  M
                                              s. continuous source
fig. 31: protection against loss of           fig. 32: rotor protection devices on startup
synchronism.                                  and in operation.                                fig. 33: protection against reverse power.

                                                                                               Cahier Technique Merlin Gerin n° 165 / p.21
Locked rotor                                  after a time delay initiated when the     protection devices. This is a the
Jamming of a motor for mechanical             motor is energised. This time delay is    traditionally used term (relay = a type of
reasons causes an overcurrent roughly         set at a value greater than or equal to   protection device) which goes back to
equivalent to starting current. The           normal starting time.                     the time when motor protection
resulting temperature rise is                 Undercurrent or underpower                required the use of separate "relays"
considerably greater since rotor losses       Pumps may be damaged when                 each with a single protection function.
are maintained at their highest value                                                   In the seventies, manufacturers brought
                                              unpriming occurs. Unpriming could also
throughout jamming and ventilation is                                                   out RACKS able to house a number of
no longer present if connected to rotor       result in a drop in the active power
                                              absorbed by the motor. Protection         different protection devices in order to
rotation. As a result, when there is a
                                              against this fault is provided by an      meet needs more closely.
risk of jamming, the "locked rotor"
protection is necessary since overload        undercurrent relay.                       In the eighties digital technology
relays sometimes have an excessively                                                    increased adaptation possibilities still
long response time.                                                                     further. Thus a single programmable
This fault is detected by an overcurrent
                                              technological evolution                   device performs the various protection
relay set at a value less than the            The term "relay" is often used in the     and control/monitoring functions
starting current. This value is validated     above description of the various          required in each specific case.

Cahier Technique Merlin Gerin n° 165 / p.22
appendix 1: determining motor starting mode

The purpose of the example below is                                                             →
                                                    overall approach                           OA = 1,200 kVA : transformer power
not to fully deal with a problem but
                                                    The approach chosen by the designer        used on other feeders.
rather to illustrate, through an example,
                                                    is to look for the best technico-           →
an approach wich helps to choose the
                                                    economic choice. For this, first try and    OB = St = 6,000 kVA : maximum
appropriate starting mode for a given
                                                    validate the easiest and most              apparent power authorised. The
                                                    economical solution. Then, if this does    apparent power available for starting
                                                    not work, follow the order shown in the    (motor + reactance) is graphically
calculation hypotheses                              table in figure 34.                        deduced.
Asynchronous motor with:                            Direct starting                             →
c rated power Pn= 1,500 kW;                         Motor apparent power at the beginning       AB = Sd = 5,300 kVA .
c rated voltage Un = 5,500 V;                       of starting:                               Power reduction to be caused by the
c efficiency x power factor:                                     Pn    Id                      reactance:
η x cos ϕ = 0.84;                                    Sm =
                                                               η cos ϕ In                      Sd
                                                                                                          ≈ 0.6 .
c starting torque/rated torque ratio on
full voltage:                                                                                  Sm   8,925
                                                     Sm =           5 = 8,925 kVA
 Cd                                                          0.84
       = 0.8                                        where, with power factor on starting:
                                                    cos ϕd = 0.15, i.e. : ϕd = 81°.
c starting current/rated current ratio on
full voltage:                                       This power is vectorially added to the
                                                    power delivered by the transformer on
   = 5;                                             the other feeders (see fig. 35).
In                                                  The total value of the apparent power
c breakaway torque of the driven                    required by the transformer is                  S = 9,580*
machine: 0.2 Cn;                                    graphically deduced from the following:
c main supply transformer power:                    S ≈ 9,580 kVA.
Pt = 3 MVA;
c maximum apparent power inrush                     The maximum inrush authorised is
authorised by the transformer network:              6,000 kVA: direct starting is thus not            B
St = 6 MVA.                                         possible.
Other data necessary for calculation:               Starting by reactance
                                                    Introduction of a reactance reduces the                       Sm = 8,925*
c torque-speed C (N) characteristic of
the motor;                                          apparent power absorbed by the motor.
c load torque-speed Cr (N);                         The available starting power is                               St = 6,000*
characteristic of the driven machine                graphically determined (see fig. 35).
c transformer delivery on feeders other             When the motor starts, the presence of                        Sd = 5,300*
than the motor one: 1,200 kVA under                 the reactance means that the power
cos ϕ' = 0.87.                                      factor approaches zero. Thus ϕd ≈ 90°.

starting solutions                                  main acceptance criteria
direct                                              power inrush compatible with network                      ϕd = 81°
reactance                                           c starting torque greater than breakaway
                                                    torque                                       1,200*
                                                    c current peak (for full voltage                 ϕ' = 29.5°
                                                    resumption) acceptable by network
                                                                                                                                  active power
autotransformer                                     the same                                   *: in kVA
fig. 34: decisive criteria for choosing motor starting mode.                                   fig. 35: starting power diagram.

                                                                                               Cahier Technique Merlin Gerin n° 165 / p.23
I'd is the new starting current value.                    1,500                                                   2
                                              Smg = 0.4           = 720 kVA .                            U 
Sd = Un I' d      3                                        0.84                                   hence:  d             = 0.513
                                              The power reduction factor is then:                         Un 
Sm = Un I d       3                                                                               i.e. Ud = 0.718 Un
                                              Sd − Smg               5,300 − 720
thus: I'd = 0.6 Id.                                              =               = 0.513
                                                     Sm                 8,925                     New starting torque:
          I' d   Ud
Moroever:      =    = 0.6 .                   At constant voltage Un, the current                                     2
          Id     Un                           inrush on the network side is therefore:                   U 
                                                                                                  0.8 Cn  d              =    0.41 Cn .
The value of voltage at the motor             0.513 Id.                                                   Un 
terminals is Ud = 0.6 Un. Once the            Determining the reduced starting                    This value is sufficient for starting to
electrical problem has been solved, it        voltage Ud.
remains to be seen if this solution is                                                            take place.
                                              Equality of primary and secondary                   Remark 2
valid from the mechanical standpoint. In
the case of direct starting, the starting     autotransformer powers results in the
                                                                                                  For a breakaway torque greater than
torque equals : Cd = 8 Cn (see fig. 36).      following:
                                                                                                  0.41 Cn, stator starting with this motor
In the case of starting by reactance, the      0.513 I d Un = I "d Ud                             is no longer possible.
starting torque C'd equals:                   I"d is the reduced voltage starting                 Either a slipring rotor motor with rotor
                                              current on the motor side                           starting must be used, or a motor with a
              U                                                          Ud                     special cage possessing a high starting
C' d = 0.8 Cn  d          = 0.288 Cn        0.513 I d Un = I d              Ud
               Un                                                        Un                     torque.
This value is compatible with the
breakaway torque of the machine being
driven.                                        C       I
One last point remains to be checked:          Cn     In
if the point of mechanical balance                                                                      (a)
Cm = Cr is placed at an excessively low
speed, a current peak may occur on the
move to full voltage. If this peak is too
high for the network, the starting mode
will have to be reconsidered and
starting by autotransformer, for                      5                    (c)
example, chosen (see fig. 36).                  1
Remark 1
Let us assume that the value of the           0.8                                                                                        point of balance
breakaway torque of the driven                                                                                                           at reduced
machine is 0.35 Cn instead of 0.2 Cn.                                      (d)                                                           voltage
Starting by reactance is then                                                                                                            current peak
incompatible with the breakaway                                                                                                          on move
torque.                                                                                                                                  to full voltage
The solution of starting by                                                (b)
autotransformer must then be                                                                                                   In
envisaged.                                    0.2     1
The apparent power available remains                                       (e)
Sd = 5,300 kVA. From this value the
magnetising force of the auto-
                                                 0         0.1       0.2                0.5                   0.8                   1               (N/Ns)
transformer Smg must be deduced
which, at the initial starting moment, is
arithmetically added to motor apparent        a: curve C (N) on full voltage
power. Smg is around 0.2 to 0.4 times         b: curve C (N) on reduced voltage (0.6 Un)
motor apparent rated power.                   c: curve I (N) on full voltage
                                              d: curve I (N) on reduced voltage (0.6 Un)
i.e. with the factor 0.4:
                                              e: curve Cr (N)
             Pn    
Smg = 0.4                                   fig. 36: torque and current curves for starting by reactance.
           η cos ϕ 

Cahier Technique Merlin Gerin n° 165 / p.24
appendix 2: coordination of protection devices

Once the protection devices have been
chosen according to operating                                 60
requirements, they must be coordinated
to obtain the maximum advantage from
their possibilities. A balance must be                        30
sought between untimely tripping and                          20
delay on fault removal. The problem of
protection device coordination is solved
by studying the curves t (I) of the
                                                minutes       8
relays, of the circuit-breaker and of the                     6
breaking capacity of the contactor. The
characteristics of the motor shown in                         4
the figure are as follows:                                    3
                                                                                   thermal relay
c Pn = 550 kW;
                                                              2                    operating zone
c Un = 3150 V;
c In = 130 A;
                                                              1                      cold
c Id = 5 In .
The contactor is of the Rollarc fuse type.                    40
Type of protection devices
c thermal relay with indirect tripping,                       20
set at In = 130 A for overloads;
c positive phase sequence component
relay set at 6 In, time delayed at 0.05 s                     10
for balanced faults;
c negative phase sequence component                           5
relay set at 0.3 or 0.4 In, time delayed
at 0.6 s.
In the case of networks with quasi-
permanent unbalances, a two-threshold
relay is used:
c a lower time delayed threshold, set                         1
just above the permanently accepted
negative phase sequence component
c an upper instantaneous threshold for
                                                                       phase sequence
phase break protection.
Instantaneous tripping by the positive                                 negative                                           Rollarc contactor
phase sequence component relay                                         positive                                           breaking capacity
enables best use to be made of the                          0.1
contactor’s breaking capacity and
avoids fuse melting. Analysis of the                        0.05
curves in figure 37 shows that the             contactor
motor and network are protected                breaking     0.03
against:                                       time
c unbalances of approximately 0.3 In
to 10 In,
c balanced faults from 6 In to 28 In.                       0.01
The fuses only take action beyond 15 In                            1         2          5           10 20       50            100          k In
for unbalanced faults and 25 In for                                                                                                        In (A)
balanced faults.                                                  130        260       650     1,300 2,600     6,500
The maximum current the contactor
may have to break is 28 In = 3,640 A.                                                                   3,640 A
This value is considerably lower than its    fig. 37: minutes thermal relay operating zone.
breaking capacity of 10 kA.

                                                                                                     Cahier Technique Merlin Gerin n° 165 / p.25
appendix 3: bibliography

c IEC 34-1: Rotating electrical
c IEC 71: Insulation coordination.
Merlin Gerin's Cahier Technique
c The breaking process with a Fluarc
SF6 puffer-type circuit-breaker,
Cahier Technique n° 112 -
c Behaviour of the SF6 MV circuit-
breakers Fluarc for switching motor
starting currents,
Cahier Technique n° 143 -
c Overvoltages and insulation
coordination in MV and HV,
Cahier Technique n° 151 -

Cahier Technique Merlin Gerin n° 165 / p.26
Cahier Technique Merlin Gerin n° 165 / p.27
                                              Réal. : Illustration Technique - Lyon
Cahier Technique Merlin Gerin n° 165 / p.28   DTE 05/95 - 2500 - Printing : Clerc - 38600 Fontaine

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