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DC motor drives ..................................................................................................................... 1
Fields of Application .............................................................................................................. 1
DC motor types ...................................................................................................................... 1
Permanent magnet DC motor ............................................................................................. 1
DC motor with separately excited field winding ............................................................... 1
Power Supplies of the DC motor Drives ................................................................................ 1
Diode Rectifiers.................................................................................................................. 2
Controlled rectifiers ............................................................................................................ 2
DC/DC converters .............................................................................................................. 3
Characteristics of the DC motors ........................................................................................... 3
Permanent-magnet DC motors ........................................................................................... 3
DC motors with a separately excited field winding ........................................................... 4
Transfer functions .................................................................................................................. 6
DC motor drives
Fields of Application
Traditionally, dc motor drives have been used for sped and position control applications. In
the past few years, the use of ac motor servo drives in these applications is increasing. In spite
of that, in applications where an extremely low maintenance is not required, dc drives
continue to be used because of their low initial cost and excellent drive performance.
DC motor types
Permanent magnet DC motor
Often in small dc motors, permanent magnets on the stator produce a constant field flux. The
torque-speed characteristics of this type can be shifted along the speed axis by controlling the
applied terminal voltage Vt. Therefore the speed of a load with an arbitrary torque-speed
characteristics can be controlled by controlling Vt in a permanent-magnet dc motor with a
constant field flux.
They have limited ratings of a few horsepower and also have a maximum speed limitation.
DC motor with separately excited field winding
The limitations of the previous type can be overcome if the field flux is produced by means of
a field winding on the stator, which is supplied by a dc current If.
At this type of dc motor both the terminal voltage Vt and the field flux f can be controlled to
yield the desired torque and speed.
Power Supplies of the DC motor Drives
There are some devices, which can be used as DC motor power supplies.
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Diode Rectifiers
The trend is to use the inexpensive rectifiers with diodes to convert the input AC into DC in
an uncontrolled manner.
Fig. 5.1 Block diagram of a rectifier.
In such diode rectifiers, the power flow can only be from the utility ac side to the dc side.
They are used in switching dc power supplies, ac motor drives, dc servo drives and so on.
These rectifiers draw highly distorted current from the utility. Now and in the future,
harmonic standards and guidelines will limit the amount of current distortion allowed into the
utility, and the simple diode rectifiers may not be allowed.
Controlled rectifiers
As the name of these converters implies, the line-frequency voltages are present on their ac
side. In these converters, the instant at which a thyristor begins or ceases to conduct depends
on the line-frequency ac voltage waveforms and the control inputs.
They are used in dc motor and ac motor drives, where it is necessary or desirable to be able to
control the power flow in both directions between the ac and dc sides. (regenerative
capabilities)
A fully controlled converter is shown in Fig. 6-1a in a block diagram form.
Fig. 6.1a Line-frequency controlled converter.
For given ac line voltages, the average dc-side voltage can be controlled from a positive
maximum to a negative minimum value in a continuous manner. The converter dc current Id
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(or id on an instantaneous basis) cannot change direction. Thus, a converter of this type can
only operate in two quadrants (of the Vd and Id plane).
In some applications, such as in reversible-speed dc motor drives with regenerative braking,
the converter must be capable of operating in all four quadrants. This is accomplished by
connecting two two-quadrant converters in antiparallel or back to back.
DC/DC converters
The dc-dc converters are widely used in regulated switch-mode dc power supplies and in dc
motor drive applications. As shown in Fig. 7.1, often the input to these converters is an
unregulated dc voltage, which is obtained by rectifying the line voltage, and therefore it will
fluctuate due to changes in the line-voltage magnitude. Dc-to-dc converters are used to
convert the unregulated dc input into a controlled dc output at a desired voltage level.
Fig. 7.1 A dc-dc converter system.
These converters are very often used with an electrical isolation transformer in the switch-
mode dc power supplies and almost always without an isolation transformer in case of dc
motor drives.
Characteristics of the DC motors
Permanent-magnet DC motors
Often in small dc motors, permanent magnets on the stator as shown in Fig. 13-1a produce a
constant field flux f. In steady state, assuming a constant field flux f:
Tem = kTIa (13-10)
Ea = kEm (13-11)
Vt = Ea+RaIa (13-12)
where kT = ktf and KE = Kef. Equations 13-10 through 13-12 correspond to the equivalent
circuit of Fig. 13-4a. From the above equations, it is possible to obtain the steady-state speed
m as a function of Tem for a given Vt:
1 R
m Vt a Tem (13-13)
kE
kT
The plot of this equation in Fig. 13-4b shows that as the torque is increased, the torque-speed
characteristic at a given Vt is essentially vertical, except for the droop due to the voltage drop
IaRa across the armature-winding resistance. This droop in speed is quite small in integral
horsepower dc motors but may be substantial in small servo motors. More importantly,
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however, the torque-speed characteristics can be shifted horizontally in Fig. 13-4b by
controlling the applied terminal voltage Vt.
Fig 13-4 Permanent-magnet dc motor: (a) equivalent circuit; (b) torque-speed characteristics:
Vt5>Vt4>Vt2>Vt1, where Vt4 is the rated voltage; (c) continuous torque-speed capability.
Therefore, the speed of a load with an arbitrary torque-speed characteristic can be controlled
by controlling Vt in a permanent-magnet dc motor with a constant f.
In a continuous steady state, the armature current Ia should not exceed its rated value, and
therefore, the torque should not exceed the rated torque. Therefore, the characteristics beyond
the rated torque are shown as dashed in Fig. 13-4b. Similarly, the characteristic beyond the
rated speed is shown as dashed, because increasing the speed beyond the rated speed would
require the terminal voltage Vt to exceed its rated value, which is not desirable. This is a
limitation of permanent-magnet dc motors, where the maximum speed is limited to the rated
speed of the motor. The torque capability as a function of speed is plotted in Fig. 13-4c. It
shows the steady-state operating limits of the torque and current; it is possible to significantly
exceed current and torque limits on a short-term basis. Figure 13-4c also shows the terminal
voltage required as a function of speed and the corresponding Ea.
DC motors with a separately excited field winding
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Permanent-magnet dc motors are limited to ratings of a few horsepower and also have a
maximum speed limitation. These limitations can be overcome if f is produced by means of a
field winding on the stator, which is supplied by a dc current If. To offer the most flexibility in
controlling the dc motor, the field winding is excited by a separately controlled dc source vf,
as shown in Fig. 13-5a. The steady-state value of f is controlled by If (= Vf/Rf), where Rf is
the resistance of the field winding.
Fig 13-5 Separately excited dc motor: (a) equivalent circuit; (b) continuous torque-speed
capability.
Since f is controllable, Eq. 13-13 can be written as follows:
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Vt Ra Tem (13-14)
m
k e f
k t f
recognizing that kE = kef and kT = ktf. Equation 13-14 shows that in a dc motor with a
separately excited field winding, both Vt and f can be controlled to yield the desired torque
and speed. As a general practice, to maximize the motor torque capability, f (hence If) is kept
at its rated value for speeds less than the rated speed. With f at its rated value, the
relationships are the same as given by Eqs. 13-10 through 13-13 of a permanent-magnet dc
motor. Therefore, the torque-speed characteristics are also the same as those for a permanent-
magnet dc motor that were shown in Fig. 13-4b. With f constant and equal to its rated value,
the motor torque-speed capability is as shown in Fig. 13-5b. where this region of constant f is
often called the constant-torque region. The required terminal voltage Vt in this region
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increases linearly from approximately zero to its rated value as the speed increases from zero
to its rated value. The voltage Vt and the corresponding Ea are shown in Fig. 13-5b.
To obtain speeds beyond its rated value, Vt is kept constant at its rated value and f is
decreased by decreasing If. Since Ia is not allowed to exceed its rated value on a continuous
basis, the torque capability declines, since f is reduced. In this so-called field-weakening
region, the maximum power EaIa (equal to mTem) into the motor is not allowed to exceed its
rated value on a continuous basis. This region, also called the constant-power region, is shown
in Fig. 13-5b, where Tem declines with m and Vt, Ea, and Ia stay constant at their rated values.
It should be emphasized that Fig. 13-5b is the plot of the maximum continuous capability of
the motor in steady state. Any
operating point within the regions shown is, of course, permissible. In the field-weakening
region, the speed may be exceeded by 50- 100% of its rated value, depending on the motor
specifications.
Transfer functions
Figure 13-6 shows a dc motor operating in a closed loop to deliver controlled speed or
controlled position.
Fig 13-6 Closed-loop position/speed dc servo drive
To design the proper controller that will result in high performance, it is important to know
the transfer function of the motor. It is then combined with the transfer function of the rest of
the system in order to determine the dynamic response of the drive for changes in the desired
speed and position or for a change in load.
The equations for the motor-load combination can be represented by transfer function blocks,
as shown in Figure 13-7.
Fig 13.7 Block diagram representation of the motor and load (without any feedback).
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The inputs to this system are the terminal voltage Vt(s) and the load torque TWL(s). If we
consider just the motor without the load, then the transfer function will be:
(s) 1
G1 ( s ) m
Vt ( s ) k E ( s m 1)( s e 1)
where
R J
m a m = mechanical time constant, and
kT k E
La
e = electrical time constant
Ra
The electrical time constant determines how quickly the armature current builds up, in
response to a step change vt in the terminal voltage, where the rotor speed is assumed to be
constant.
The mechanical time constant determines how quickly the speed builds up in response to a
step change vt in the terminal voltage, provided that the electrical time constant is assumed
to be negligible and, hence, the armature current can change instantenously.