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					               LOW COST SWITCHED RELUCTANCE DRIVES

                                         Dr. C. Pollock
                                  Department of Engineering,
                        University of Warwick, Coventry CV4 7AL


                                          ABSTRACT

Switched reluctance motors offer a simple and rugged motor solution for many variable speed drive
applications. The low manufacturing cost of the motor has not however, been matched by low cost
electronics to allow the widespread commercial exploitation of this exciting motor technology. This
paper will present the results of recent research and development work which has focused specifically
on reducing the cost of power electronic converters for switched reluctance drives without reducing
the performance of the motor. The paper will present low cost switched reluctance drives which
although using the minimum of electronics are still superior to existing technology. This has been
achieved through innovative design of the motor, its windings and the power electronic circuit
configuration. New solutions are proposed to the problems of starting and reversing low phase
number motors. The paper will also show how innovation in winding design has led to a completely
new class of switched reluctance drive employing the absolute minimum of power electronic
circuitry.




                                     I. INTRODUCTION

1.1     History of the Switched Reluctance Motor


The reluctance motor operates on the principle that a magnetically salient rotor is free to move to a
position of minimum reluctance to the flow of flux in a magnetic circuit. The phenomena has been
known ever since the first experiments on electromagnetism. In the first half of the 19th century,
scientists all over the world were experimenting with this effect to produce continuous electrical
motion. In 1838, W. H. Taylor obtained a patent for an “electromagnetic engine” in the United
States and subsequently on 2nd May 1840 he was granted a patent [1] in England for the same engine.
The engine was composed of a wooden wheel on the surface of which were mounted seven pieces of
soft iron equally spaced around the periphery. The wheel rotated freely in a framework in which four
electromagnets were mounted. The electromagnets were connected to a battery through a mechanical
switching arrangement on the shaft of the wheel such that excitation of an electromagnet would attract

                                                  1
the nearest piece of soft iron, turning the wheel and energising the next electromagnet in the sequence
to continue the motion. However this motor and other subsequent inventions all suffered from torque
pulsations and were soon superseded by the invention of the d.c. machine and the a.c. induction
machine.


Over 140 years after these early experiments, the advent of suitable power electronic switches has
meant that the mechanical commutator of the early reluctance motors can be replaced by an
electronic one. Improved magnetic materials and advances in machine design have brought the
switched reluctance motor into the variable speed drive market [2,3].             The simple brushless
construction of the motor makes it cheap to build and very reliable in operation. The unipolar current
requirements of the phase windings results in a simple and very reliable power converter circuit.


In this paper Dr. Pollock will explain why research is now focusing on switched reluctance motors and
drives with only one or two phase windings so that applications for the technology are being created in
low cost, high volume markets such as domestic appliances, heating ventilation and air conditioning
and automotive auxiliaries. The paper will go on to show that through innovative design, new classes
of electronically controlled motors have been invented which have an exciting future in the cost
competitive world of high volume manufacturing.


Whilst the initial thrust of most of the current research is into these high volume applications the next
ten years will see a dramatic increase in the number of switched reluctance drives and they will also
start to be employed in more routine applications including the marine industry. With a current trend
in the variable speed drive market to package motors with electronics there is a tremendous
opportunity for switched reluctance technology to be used in place of other electronically controlled
motors since it will almost always be able to offer superior performance at a lower price.


1.2 Principle of Operation


A schematic representation of the lamination pattern of single phase, two phase and three phase
switched reluctance motors is shown in Figure 1. In each of the motors shown in Figure 1 a coil is
wound around each stator pole and is connected, usually in series with the coil on the diametrically
opposite stator pole to form a phase winding. The reluctance of the flux path between the two
diametrically opposite stator poles varies as a pair of rotor poles rotates into and out of alignment.
Since inductance is inversely proportional to reluctance, the inductance of a phase winding is a
maximum when the rotor is in the aligned position, and a minimum when the rotor is in the non-
aligned position. A pulse of positive torque is produced if current flows in a phase winding as the
inductance of that phase winding is increasing. A negative torque contribution is avoided if the current
is reduced to zero before the inductance starts to decrease again. The rotor speed can be varied by


                                                   2
changing the frequency of the phase current pulses while retaining synchronism with the rotor
position.




                 A                                 A



            R                                               R’
                                        B                        B’
                      R’                       R


                 A’                                A’




(a) 2/2 Single Phase Switched               (b) 4/2 Two Phase            (c) 6/4 Three Phase
      Reluctance Motor               Switched Reluctance Motor        Switched Reluctance Motor


        Figure 1 Schematic of Switched Reluctance Laminations all with two poles per phase


The absence of permanent magnets or coils on the rotor means that torque is produced purely by the
saliency of the rotor laminations. The torque produced is in the same sense irrespective of the
direction of the flux through the rotor, and hence the direction of current flow in the stator phase
windings is not important. The need for unipolar phase current in the reluctance motor results in
simpler and more reliable power converter circuits.


1.3 Power Electronic Control of the Switched Reluctance Drive


Unlike induction motors or d.c. motors the reluctance motor cannot run directly from an a.c. or d.c.
supply. A certain amount of control and power electronics must be present. The power converter, is
the electronic commutator, controlling the phase currents to produce continuous motion. The control
circuit monitors the current and position feedback to produce the correct switching signals for the
power converter to match the demands placed on the drive by the user. The purpose of the power
converter circuit is to provide some means of increasing and decreasing the supply of current to the
phase winding.        Many different power converter circuits have been proposed for the switched
reluctance motor [5-12].


By far the most common power converter for the switched reluctance drive is the asymmetric half-
bridge, shown in Figure 2 for a single phase and a two phase motor. Each asymmetric half-bridge has
three main modes of operation. The first, a positive voltage loop, occurs when both switching devices
associated with a phase winding are turned on. The supply voltage is connected across the phase


                                                        3
winding and the current in the phase winding increases rapidly, supplying energy to the motor. The
second mode of operation is a zero voltage loop. This occurs if either of the two switching devices are
turned off while current is flowing in a phase winding. In this case the current continues to flow
through one switching device and one diode. Energy is neither taken from or returned to the d.c.
supply. The voltage across the phase winding during this time is equal to the sum of the on-state
voltages of the two semiconductor devices. This voltage is very small compared to the supply voltage
and so the current in the phase winding decays very slowly.



  +V s                                      +V s



  0V                                        0V

  Figure 2. Power converter for single phase and two phase motors with the asymmetric half-bridge


The final mode of operation is a negative voltage loop. Both the switching devices are turned off.
The current is forced to flow through both the freewheel diodes. The current in the phase winding
decreases rapidly as energy is returned from the motor to the supply. The asymmetric half-bridge thus
offers three very flexible modes for current control. The zero voltage loop is very important in
minimising the current ripple at any given switching frequency. The zero voltage loops also tend to
reduce the power flow to and from the motor during chopping by providing a path for motor current
to flow without either taking energy from or returning it to the supply capacitors.


The major advantage with this circuit is that all the available supply voltage can be used to control the
current in the phase windings. As each phase winding is connected to its own asymmetric half-bridge
there is no restriction on the number of phase windings. However, as there are two switches per phase
winding it is best suited to motors with few phase windings.




                                                    4
1.4     Purpose of this paper


Reluctance motors can operate with any number of phase windings, however, for conventional
machines there are some guidelines governing the choice of stator and rotor pole numbers [2, 4]. This
paper describes the design and construction of switched reluctance motors and drives of one and two
phase windings as these are particularly suited to applications requiring a variable speed drive with a
competitive manufacturing cost. Novel topologies of both motor and power electronic controller are
described which result in very low cost variable speed drive systems.


Single phase and two phase switched reluctance motors benefit most from the machine’s advantages of
constructional simplicity, robustness and low cost. Despite the discontinuous nature of its torque, the
single phase switched reluctance motor may compete well with the ac. commutator motor in high
speed applications in which starting requirements are relatively easy.


While the use of a polyphase as opposed to a single phase switched reluctance drive is generally
advantageous in terms of reduced peak current per switching device, non-discontinuous torque,
improved iron utilisation, reduced rotor D2 L per mean torque, and easier heat dissipation (smaller
coils), these advantages become less pronounced when low power (less than 700 W say) high speed
(>8000 rev/m) applications are involved.


The relatively low power means that peak currents in the case of single phase switched reluctance
drives are low enough to be handled by relatively small devices. The motor’s small size means that the
penalties on motor size and cost for single phase operation are small in absolute terms, while coil
surface to volume ratios are relatively high, enabling the coils’ heat dissipating properties to be
acceptably good. There remains the question of the single phase motor’s discontinuous torque. This is
undoubtedly unacceptable for critical applications such as capstan drives and speed servos, and is likely
to give insuperable problems when starting requirements are severe.        However many applications
remain for which discontinuous torque poses no difficulties, a typical example being sub-kW blower
drives where the starting load is purely inertial and the speed ripple under normal running conditions is
of zero consequence.


The principal advantages of the single phase drive for low power applications lie, of course, in the cost
savings associated with the small component count in the power electronic controller. However, where
starting is more arduous, there is little alternative but to increase the complexity of the drive to say a
two phase motor. This does not necessarily result in a large increase in cost. The matching choice of
motor phase and pole numbers to the application is crucial for the optimum system design. This will
be explained in the next Section. Section III and IV give some examples of single phase and two phase
switched reluctance drives constructed by the author. In Sections V, VI and VII some completely new
topologies of switched reluctance drive are described before Section VIII draws some conclusions.

                                                    5
          II. IDEAL NUMBERS OF PHASE WINDINGS, STATOR AND
                 ROTOR POLES FOR LOW COST APPLICATIONS

The main factors concerned with the choice of phase number and stator/rotor pole combination are
the required starting performance the rated output power; the maximum speed and the requirement
for reversing.


2.1     Required starting performance and rated output power


These two factors influence the choice of phase number rather than the number of stator and rotor
poles. Single phase motors are less attractive when the required rated power is significantly greater
than 1 kW because of the impact of single phase operation on device peak current and the size of
the d.c. link capacitor. Single phase motors must also be ruled out when difficult loads are concerned,
for instance, loads which combine direct-drive and high friction. Two phase motors can be started
under difficult load conditions and they will generally be cheaper than three phase equivalents due to
the lower component count in the power electronic drive. Since the two phase switched reluctance
drive has two quadrature phases the current drawn from the power supply can have a relatively low
ripple and so there is no upper restriction on the power level of a two phase drive.         Where a
specification combines a high speed requirement (for which single phase is advantageous due to its
low pulsing frequency) and difficult starting requirements, a hybrid solution may be appropriate in
which the motor operates as a polyphase machine at starting and low speeds and as a single phase at
medium and high speeds.


2.2     Maximum speed


This factor influences the choice of number of stator and rotor poles. In order to avoid excessive
core losses it is desirable to choose a stator/rotor pole combination to limit the maximum phase
switching frequency (and hence flux frequency) to around 600 Hz. Core losses can be reduced by
using a higher grade steel or by reducing the magnetic loading in the motor, the latter resulting in a
larger motor for a given torque.


The switching frequency is given by:
                                             f = m ⋅ Nr ⋅ n
where m is the number of phases, Nr is the number of rotor poles and n is the speed in rev/sec. Each
phase sets a frequency of Nr.n.


                                                  6
2.3     Requirement for reversing


The majority of starting techniques for single and two phase switched reluctance motors are
inherently unidirectional but there are some exceptions such as the use of hybrid single / three phase
and offset pole arrangements (see later). The requirement for reversibility is therefore a crucial
factor in determining the optimum pole geometry.



            III. SINGLE PHASE SWITCHED RELUCTANCE DRIVES

The single phase switched reluctance drive is the simplest brushless electric motor in existence today.
The stator and rotor have typically the same number of stator and rotor poles with the most common
designs having two (figure 1(a)) or four poles. Single phase motors with higher number of poles are
possible and an eight pole rotor and stator is shown in Figure 3. This design was constructed for a low
speed domestic fan and was designed to compete with an induction motor with an external rotor. The
rotor of this particular design is therefore outside the stator and the stator windings are wound around
the central core.
                               Rotor Outside diameter provides mounting point for fan




                                                                        Rotor


                                                                        Stator




         Figure 3 An eight pole single phase switched reluctance motor with an external rotor.




                                                   7
Single phase motors are most ideally suited to applications such as fans and some pumps. Providing the
starting torque is predominantly inertial with minimal friction it is possible to use a small magnet to
park the rotor in a position where a pulse of stator current will produce torque in the required direction.
The torque produced by this magnet can be small such that it does not impinge on the running torque
of the motor. It is common for most low speed fans and pumps to be driven by induction motors
which are usually single or split phase motors. A switched reluctance replacement for such a motor can
offer significantly higher efficiency, variable speed control over a very wide speed range and much
greater control over the mechanical layout.        However, small fans and pumps are often used in
applications where acoustic noise levels must be minimised. In such cases the design of the switched
reluctance motor and its mechanical support structure is vital if the acoustic noise levels achievable
with an induction motor are to be matched.


Another very attractive application area for single phase switched reluctance motors is in high speed
motors for blowers. Such units typically use series universal motors e.g. vacuum cleaners and hand
dryers. Such motors can run at high speeds to develop the higher pressure or higher volume flow
required of such applications. In these applications acoustic noise is less important since the blower
itself is not quiet and the series universal motor has significant brush noise. The single phase switched
reluctance motor can outperform a series universal motor in terms of acoustic noise, efficiency and
hence power to weight and being brushless has no reliability problems.          The cost of the power
electronics for a single phase switched reluctance drive is minimised as only one or two power switches
are required for the complete drive.



               IV. TWO PHASE SWITCHED RELUCTANCE DRIVES

In an application where the load has some inherent torque at starting such as a traction drive the
parking magnet is no longer sufficient to guarantee that the rotor is in a correct position for starting.
The design must be such that excitation of the stator windings alone can produce the required torque.
This means that the motor needs to be designed to include a second phase winding arranged
approximately in quadrature to the first phase winding.


The basic form of such a machine was shown in Figure 1(b) with four poles on the stator and two poles
on the rotor. However, it will be observed that with the rotor exactly aligned with respect to the stator
poles of one phase, the quadrature phase can produce no torque. A perfectly symmetric two phase
motor is therefore not inherently self starting from all positions. This problem can be avoided by
adding some asymmetry to either the stator or the rotor.




                                                    8
4.1     Two Phase Motors with Rotor Asymmetry
Figure 4 shows three different two phase switched reluctance motors in which the rotor has been
modified to create asymmetry.




                  (a)                                  (b)                                      (c)


                                 Figure 4 Self-starting two phase rotors
                          (a) Snail-Cam (b) Stepped Rotor (c) Saturating Pole




                                               Torque vs Position (0=unaligned, 109 or 180=aligned)
                                            150


                                                                                                  Standard
                                            100
                                                                                                  Self-starting

                                             50


                                              0
                                                   0    20   40     60      80      100   120    140      160     180

                                             -50
                                        Torque
                                        (Nm/m)
                                           -100


                                           -150
                                                                  Position (mechanical
                                                                  degrees)

      (a) Graded Air Gap Rotor                         (b) Variation of Torque with Position


                Figure 5 Graded Air-Gap Two phase Motor and its Torque Output




                                                        9
In recent research at University of Warwick an improved version of the “snail-cam” rotor has been
designed and constructed and referred to as a “graded” rotor design [13], shown in Figure 5(a). The
effect of this is to make the torque production of the rotor asymmetric; the slowly increasing air-gap
producing a positive for more that half the rotor pole pitch. A comparison of the torque produced by
this “graded” rotor and a symmetrical rotor is shown in Figure 5(b).


Two phase, self-starting, switched reluctance drives have been successfully constructed for a number of
applications and some of these have employed a novel power converter circuit known as the shared
switch asymmetric half bridge [14]. This circuit, shown in Figure 6, requires only three power switches
to control a two phase motor rather than four for the conventional asymmetric half bridge shown in
Figure 2. The switch S 2 is used to control the current in both phases but since the two phase currents
occur at different times this does not cause any significant problem to the operation of the drive.



                 +Vs
                                                ph1                S2
                              Cs                                        ph2

                                           S1                                       S3
                 0V

                       Figure 6. Two phase shared switch asymmetric half bridge


4.2     Two Phase Motors with Stator Asymmetry


As an alternative to creating an asymmetric rotor the stator can have some asymmetry introduced.
An example of one method which has been developed recently at University of Warwick is shown in
Figure 7. The stator poles of one phase are not positioned mid way between the stator poles of the
other phase. As a result the entire torque production cycle of one phase is shifted, thus ensuring that
there is torque available from either phase at all rotor angles.


In this offset stator pole design there is no asymmetry in the rotor and as a result the motor will rotate
with equal performance in either direction. At most rotor angles there is a choice of either positive or
negative torque. However, at some angles both phases produce torque of the same polarity. At these
positions it may be necessary to start the rotation in the wrong direction to move the rotor into a
position where torque of the required polarity is available. This design of stator therefore allows two
phase switched reluctance motors to start and run with equal performance in either direction, a feature
not possible with previous designs. This technology is very suitable for traction drives as employed in


                                                     10
electrically powered vehicles and small trucks where a low cost alternative to the d.c. series motor is
sought. This topology has also been successfully employed for a drive for a fan in which reversibility
was required.


                                         100



                                          50


                                                                                                         Phase A
                                           0                                                             Phase B
                                                0        10     20         30         40       50   60

                                          -50
                                      Torque (mNm/mm)


                                         -100                 Angle (Degrees : 0° = Aligned)

       (a) Lamination
                                                    (b) Torque variation with angle for each phase


                 Figure 7 Two phase switched reluctance drive with offset stator poles
                                       used to improve starting



                           V. HYBRID SINGLE/THREE PHASE
                            SWITCHED RELUCTANCE DRIVE

In an application such as a power tool there is a requirement to be able to start in either direction from
any rotor position against high torques. Also many power tools run up to very high speeds to achieve
the power required for machining and drilling. The high speed (possibly 30,000 r/min) requires a low
number of phases and poles to minimise flux frequencies in the iron whereas the starting performance
demands a higher number of phases. For such an application a new design of hybrid single/three phase
switched reluctance drive is proposed. The motor is fundamentally single phase with two main stator
poles and two main rotor poles. Additionally there are two further auxiliary phases which are designed
to run for short periods at starting and at low rotational speeds. A diagram of the laminations of such a
machine is shown in Figure 8. In the position shown in Figure 8(a) the rotor is in the zero torque
position with respect to the horizontally orientated main stator poles. The main stator phase would
not produce any torque in this position. but excitation of either of the auxiliary phases will produce
either clockwise or anti-clockwise torque as required. The main phase can then be excited to continue
the motion. In order to avoid the cost of a three phase power converter this new type of motor is
controlled by an innovative power electronic circuit. Since the auxiliary phases are only used at start-

                                                        11
up and at low speed the can be controlled by two line-commutated thyristors as shown in Figure 8(b).
The chosen thyristor conducts during a positive half cycle of the mains supply to provide a 10 ms
pulse of current in the appropriate auxiliary phase. Due to their short operating cycle, the windings on
the auxiliary phases can be made up of a large number of turns of thin wire, thus creating a large torque
from a relatively small current.




    (a) Lamination                                 (b) New Low Cost Power Converter


           Figure 8 New Low Cost Hybrid Single/Three Phase Switched Reluctance Drive



            VI. DUAL VOLTAGE SWITCHED RELUCTANCE DRIVES

A new inexpensive class of variable speed drive has been developed with dual voltage capability [15].
The concept of this new drive allows it to operate at rated power from substantially different voltage
supplies without modification. For example, the drive could be supplied with either a mains a.c. supply
or a low voltage battery supply and function as a battery charger from the mains supply without the
need for additional components. Rechargeable power tools, electric stair lifts, wheelchairs and other
small electric vehicles are thought to be ideal applications to benefit from this operational duality.


With conventional technology this dual voltage operation can only be achieved at high cost. The
options are a dual wound motor which makes inefficient use of materials or a single voltage motor
and a full power rated power supply to match the voltage differential. The new class of variable
speed drive can be designed to have inherent dual voltage operation with optimal use of materials
and electronic components, minimising cost. This will be achieved through the design of a switched
reluctance drive in which the power electronic circuit topology is designed in unison with a new
motor configuration. The result will be drive which retains the high efficiency capability of the
switched reluctance drive, essential for battery powered applications, while also offering dual voltage
and integral battery charging functions at no extra cost.

                                                    12
The use of switched reluctance motor windings within a battery charging circuit is not new [16].
However, previous systems have not used suitable electronics to drive the motor at full rated power
from either supply source. They have employed mechanical switching to reconfigure the motor
winding arrangement for motoring or battery charging. This will not be necessary in the proposed
system.


Figure 9 shows one of the proposed new class of switched reluctance motors. A conventional 2 pole
switched reluctance stator is configured so that the motor phase winding has both a high voltage and
a low voltage coil, magnetically coupled on the same motor poles. With an appropriate turns ratio
this dual voltage winding will allow the energisation of the respective poles from either a high
voltage supply (derived from the a.c. supply) or a low voltage source (a battery). A proposed dual
voltage power converter is shown in Figure 10. It comprises a bridge rectifier and smoothing capacitor;
two switches S1 and S2 , connected respectively to the high voltage and low voltage coils. An LC filter
smoothes the current into and out of the battery.



                                Low voltage coil
                                High voltage coil
                                                           AC                              battery




                                                                           S1    S2



  Figure 9 Single phase switched reluctance                     Figure. 10 Single phase
stator configured for dual voltage operation.               dual voltage power converter



The switched reluctance motor is unique in that the direction of torque is not dependent on the
direction of current flow in the windings but only on the timing of current pulses with respect to
rotor position. It is this feature which is exploited in the new dual voltage switched reluctance drive.
In Figure 10 energisation of the dual voltage phase winding from either supply source will pull a rotor
into the aligned position with the stator poles. The switches S1 and S2 control the power flow to and
from the motor. The two switches S1 and S2 are never conducting simultaneously. The circuit can
operate in one of three main modes.
(i)       Motor rotating with energy supplied by the AC supply - with S2 open and S1 controlled by
          rotor position sensor(s), the motor will run from the a.c. supply, with the freewheel energy
          being used to trickle charge the battery.


                                                      13
(ii)    Motor rotating with energy supplied by the battery - with S 1 open and S2 controlled by
        rotor position sensor(s), the motor will run from the battery. Freewheel energy is stored in the
        high voltage capacitor and periodically this energy must be used to drive the motor so that the
        capacitor does not become overcharged.
(iii)   Motor stationary; charging the battery from the AC supply - with S2 open and S1
        chopping the circuit operates as a flyback converter, charging the battery through the output
        filter. The rotor will sit in the aligned position but will not turn.


Regenerative operation of the drive is possible, either continuously or to effect a rapid deceleration.
The energy is transferred to the battery. The circuit shown in Figure 10 is a single phase drive. Other
phases could be added and controlled in an identical manner. Alternatively, the additional phases could
use conventional power converter topologies, placed on either the low voltage or high voltage side.



             VII. TWO PHASE SWITCHED RELUCTANCE MOTORS
                            WITH FULLY PITCHED WINDINGS

A conventional switched reluctance motor has the phase windings wound in a short pitched manner
around each stator pole. The concept of fully pitched windings, where the coil of each phase spans a
number of pole pitches equal to the number of phase windings was first proposed for three phase
switched reluctance motors by B.C. Mecrow [17]. The idea of fully pitched phase windings in a two
phase reluctance actuator has been known for much longer [18] as the Law’s relay. When this coil
arrangement is combined with a completely new topology of power electronic controller a new class of
electric motor results which has the potential to be one of the simplest and most versatile brushless
electric motors of the future [19].


Figure 11 shows the lamination of a 4 pole stator and a 2 pole rotor. The windings of phase A and
phase B span across the motor occupying diagonally opposite slots. In Figure 11(a) both windings are
excited with current, the polarity of which has the combined effect of creating a horizontally
orientated field, holding the rotor in the horizontal position. In Figure 11(b) the current direction in
phase winding B has been reversed but the current direction in phase winding A is unchanged. The flux
axis is now vertical holding the rotor in a new position.




                                                      14
                A                       B                      A                     B




                B                       A                      B                     A




            (a) Stator excitation horizontal :              (b) Stator excitation vertical :
                 A positive, B positive                            A positive, B negative


                    Figure 11 : Winding Arrangement in 4-2 Fully Pitched Motor


This winding arrangement brings about three particular features of the two phase fully pitched motor
which make it substantially different to conventional two phase switched reluctance motors.
(i)     Excitation current is supplied to both windings for either rotor alignment position. Since
        each winding occupies half the total stator slot area, the fully pitched two phase motor can
        achieve 100% utilisation of its slot area. This is in contrast to the 50% achieved by a
        conventionally wound two phase motor, where only one phase winding can be energised at a
        time.
(ii)    It can be seen from Figure 13 that the direction of current in winding A is the same for both
        alignment positions. Winding A can therefore be driven by a dc current without any
        switching, which is of great significance for the design of power converters for this motor.
        The phase winding which is supplied with direct. current will subsequently be referred to as the
        ‘field’ winding of the motor.
(iii)   The direction of current in winding B in Figure 13 determines the orientation of the stator
        flux and hence selects whether the rotor is attracted to the horizontal or vertical positions.
        In normal operation the winding (B) must be supplied with an alternating current which is in
        synchronism with the rotor position. This winding will therefore be referred to as the
        'armature' winding


Since the excitation mode of this motor is substantially different to conventional motors, several
novel power converters have been developed. Any form of power converter to be used with this
motor must provide dc current to the field winding and bi-directional mmf. in the armature winding,
synchronised to the rotor motion. It is also important that the magnitudes of the magnetomotive
forces associated with each of the windings are kept as similar as possible to avoid inadvertent
negative torque generation.
                                                  15
The motor is particularly suited to connection to a full-bridge or half-bridge current source converter
employing thyristors. The field winding acts as the dc current source and the armature winding forms
the load of the inverter. A full bridge current source inverter would use four switching devices to
reverse the current in a single armature winding. Although thyristors offer high current switching
and overload capabilities at lower cost than other currently available power semiconductors, they
suffer from a need to be force commutated when used in dc circuits.


The additional cost of the commutation electronics is reduced if a half bridge current source
converter is employed as shown in Figure 12. The armature winding is made up of two coils,
arranged such that when current flows in one coil a positive mmf. is produced, while current flowing
in the second armature coil produces a negative mmf. Commutation in synchronism with the rotor
position is achieved by firing the non-conducting thyristor.        The voltage across the capacitor
reverses, and the current switches over to the reverse polarity. Very high rates of change of current
are possible with this circuit with modest input voltages, since the voltage available at commutation
is controlled by the capacitor. No dc link capacitance or inductance is required with this combination
of motor and power converter offering substantial cost savings.



                                         Field
                                        Windings



                                                                        Armature
                                                                        Windings

                     DC Supply




      Figure 12 A Low Cost Current source converter for a two phase switched reluctance motor
                                      with fully pitched windings




                                                  16
A prototype of this new motor concept has been built at the University of Warwick and was designed
to produce 400 W of output power at 4000 r/min. The windings were designed to run from a 24 V
supply. The major advantage of this circuit is that the thyristors can carry very large currents as the
motor accelerates resulting in an acceleration time of less than one second. The total cost of the
electronics is however minimal since no control is needed other than a position sensor and an inverter
for one of the thyristors. Figure 13 shows the current waveforms in the field winding and combined
positive and negative current in the armature when the motor was running at rated speed..




                      Field




                   Armature




                        Figure 13 Current waveforms in the new motor circuit


The performance and capability of this new motor can be further enhanced by replacing the thyristors
with controlled switching devices such as IGBTs as shown in Figure 14. The turn-on and turn-off
points of the switches can now be independently controlled to deliver any chosen operating point. An
RC snubber can be added to capture any energy which is not transferred from one winding to the other
at the point of commutation.


A further innovation which is also implemented in figure 14 is that the field winding has been
connected in shunt rather than in series, thus giving control over the operating characteristics of the
motor at the design stage. Active research work is being carried out at Warwick on this new class of
motor and the results are very promising indeed. Furthermore the mathematical analysis which has
been developed to date has shown that this motor can be analysed, modelled and designed using the
mathematics of the conventional d.c. series and shunt motors. This is therefore a d.c. machine in
which both the field and armature windings are located on the stator. This new motor is a true
brushless d.c. machine without any permanent magnets.



                                                  17
                                                            RC Snubber

                                                            R       C



                                                       Phase 1          Phase 2
              L


                                       Field             Cs
              N




     Figure 14 A second implementation of the new motor with IGBTs and a shunt field winding



                                    VIII. CONCLUSIONS

This paper has described the work of the author to develop switched reluctance drives for low cost
applications. Whilst the production of torque from the variable reluctance principle was first achieved
over 160 years ago, the switched reluctance drive is only now emerging as a serious alternative to
existing brushed and brushless motor technology. This paper has shown that the design of the motor
and the power electronics can be closely matched to the requirements of an application so that an
absolute minimum cost is achieved for the complete drive system.


In the pursuit of lower manufacturing cost Dr. Pollock has shown that innovative design can lead to
new solutions which have not been previously investigated. New methods of achieving starting and
reversing of low phase number switched reluctance motors have been described. The proposed designs
are all easy to implement with the minimum of cost.


Two completely new drive concepts have been described. A dual voltage drive system capable of
operating from an a.c. supply or a battery supply while also being able to charge the battery through
the motor itself will create some exciting new applications for switched reluctance technology. The
new class of brushless motor derived from a two phase switched reluctance motor with fully pitched
windings is very innovative and will undoubtedly have a role in many applications in the future. The
low cost of the power electronics combined with the effective use of the motor windings combine to
make a variable speed drive with enormous potential at all power levels.

                                                  18
                                  ACKNOWLEDGEMENTS

Dr. Pollock acknowledges the support of a consortium of 21 companies who participated in a project
on Low Cost Switched Reluctance Drives, together with the University of Warwick and the University
of Cardiff. The project was part of the LINK Programme on Power Electronics and Derived Systems
and the financial support of the Engineering and Physical Sciences Research Council is acknowledged.
Professor Hugh Bolton of the University of Cardiff played an integral part in the work and his efforts
together with the Research and Technical staff of the two Universities are acknowledged.



                                         REFERENCES

1. W.H.Taylor, "Obtaining Motive Power", Patent No.8255, England, 2nd May 1840.
2. P.J. Lawrenson, J.M. Stephenson, P.T. Blenkinson, J. Corda and N.N. Fulton, "Variable-speed
switched reluctance motors," IEE Proc., vol. 127, pt. B, no. 4, pp. 253-265, July 1980.
3. T. Miller, " Brushless reluctance-motor drives," Power Engineering Journal, pp. 325-331, November
1988.
4. R. Krishnan, R. Arumugam and J.F. Lindsay," Design procedure for switched reluctance motors,"
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                                                  19
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Italy, June 24-27 1996, pp. 1798 - 1803.




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