6

Speed Control of D.C. Motors

VINTRODUCTION
Athough a far greater percentage of electric motors in service are a.c. motors, the d.c.
motor is of
considerable industrial importance. The
Changed over a
principal advantage of a d.c. motor is that its
be speed can
wide range
by variety of simple methods. Such a iine speea
a
possible with a.c. motors. In fact, fine Control generally not
is

control is one of the reasons for the strong

speed
position of d.c. motors in the moden
industrial applications. In this chapter, we shall competitive
discuss the
various methods of
speed control of d.c. motors.
6.1 SPEED CONTROL
OF D.C. MOTORS
The speed of a d.c. motor is
given by:
Na

O N -K r.p.m. .(i)
where R = Ra
.for shunt motor
From exp.
R,+ Rge for series motor
(i), it is clear that there are three main methods of
namely controlling the speed of a d.c. motor,
) By varying the flux per pole (o). This is known as
flux control method.
(i) Byvarying the resistance un the armature circuit. This is
known as armature control method.
(ii) By varying the applied voltage V. This is known as
voltage control method.
6.2 SPEED CONTROL OF D.C. SHUNT MOTORS
The speed of a shunt motor can be
changed by () flux control
(ii) voltage control method. The first method (i.e. flux control method (i) armature control method
is simple and inexpensive. method) is frequently used because it
/1. Flux control method. It is based the fact that
on
(N 1/0) can be changed and hence the name flux control by varying the flux o, the motor speed
method. In this method, a variable
tance (known as shunt field rheostat) is
placed in series with shunt field resis
sh winding as shown in Fig. 6.1.

a Speed with
field rheostat
Rah Ra V
in field circuit

Speed without
field rheostat
Ln in field circuit
Field
rheostat
Fig. 6.1
Fig. 6.2
190
 Speod Control of D.C. Motors 191
The shunt tield rhevstat reduees the shunt field current and hence the (lux p. Therefore, we
ean only 'raIse the speed of the nmotor above the noral speed (See Fig, 6.2). Generally, this method
pernuts to nerease the speed in the ratio 3: 1. Wiier speed ranges tend to produce instability and
poor commutatioDn.

Advanlages
() This is an easyy and convenient mcthod.
(i) lt is an inexpensive method since very little power is wasted in the shunt ficld rhcostate
to relatively small value of /st
(i) The specd control exercised by this method is independent of load on the machine.
Disadvantages
() Only speeds higher than the normal speed can be obtained since the total field circuit
resistance cannot be reduced below R,h the shunt ficld winding resistance.
(i) There is a limit to the maximum specd obtainable by this method. It is because ifthe flux is too
much weakened, commutation becomes poorer.
Note. 7hefield ofa shunt motor in operation should never be opened because its speed will increase to an
extremely high valhue.
Armature control method. This method is based on the fact that by varying the voltage
available across the armature, the back e.m.f. and hence the speed of the motor can be changed. This
is done by inserting a variable resistance Re (knownm as controller resistance) in series with the
armaturc as shown in Fig. 6.3.
sh (WITHOUT
CONTROLLER
a RESISTANCE

fWITH
CONTROLLER
Ash RESISTANCE
Ra

L
Fig. 6.3 Fig. 6.4
N o V-I, (R, +R)
where R= controller resistance
Due to voltage drop in the controller resistance, the back e.m.f. (E,) is tdecreased. Since N« E
the specd of the motor is reduced. The highest specd obtainable is that corresponding to R=0 i.e.,
normal speed. Hence, this method can only provide speeds belowthe normal speed (See Fig. 6.4).
Disadvantages
(i) A large amount of power is wasted in the controller resistance since it carries full armature
current
When resistance in the ficld circuit is increased, the current (,h) in the field circuit decreases. This causes
reduction in the field flux. Thus fewer magnetic lines are cut by the armature and, therefore, less back
em.f. (E) is generated. As a result, morecurrent( ) flows through the amature. The increase in armature
current is greaterthan the decrease is fhux. Therefore,thetorque (7 o d/,) is greater and the amature speed
increases. In doing so, the back c.m.f. increases and reduces the amature current. The speed increases
until the developed torque becomes equal to load torque and then the motor again runs at a constant speed.
Therefore, pcculiar as it may seem, the speed of a shunt motor is increased by increasing the resistance of
the field circuit.
EV-1 (R, + R). Since V is constant, E, will decrease.
192 Principles of Electrical Machines

(i) The speed varies widely with load since the speed depends upon the voltage drop in the
Controller resistance and hence on the armature current demanded by the load.

(ir) The output and efficiency of the motor are reduced.

(iv) This method results in poor speed regulation.
Due to above disadvantages, this method is seldom used to control the speed of shunt motors

Note. The amature control method is a very common method for the speced control of d.c. series moto
The disadvantage of poor speed regulation is not important in a series motor which is used only where varying

speed service is required.

3. Voltage control method. In this method, the voltage source supplying the field current is
different from that which supplies the armature. This method avoids the disadvantages of poor speed
regulation and low efficiency as in armature control method. However, it is quite expensive. Therefore,
this method of speed control is employed for large size motors where efficiency is of great importance.
() Multiple voltage control. In this method, the shunt field of the motor is connected perma-
nently across a fixed voltage source. The armature can be connected across several different
voltages through a suitable switchgear. In this way, voltage applied across the armature can
be changed. The speed will be approximately proportional to the voltage applied across the
amature. Intermediate speeds can be obtained by means of a shunt field regulator.
(i) Ward-Leonard system. In this method, the adjustable voltage for the amature is obtained
from an adjustable-voltage generator while the field circuit is supplied from a separate source.
This is illustrated in Fig. 6.5. The armature of the shunt motor M (whose speed is to be
controlled) is connected directly to a d.c. generator G driven by a constant-speed a.c. motor
A. The field of the shunt motor is supplied from a constant-voltage exciter E. The field of
the generator G is also supplied from the exciter E. The voltage of the generator G can be
varied
bymeans of its field regulator. By reversing the field current of generator G by
controller FC, the voltage applied to the motor may be reversed. Sometimes, a field regu-
lator is included in the field circuit of shunt motor M for additional speed adjustment. With
this method, the motor may be operated at **any speed upto its maximum
speed.
Motor-generator Set

.- - M) Shunt
Motor
A.C. motor

Fe
Ward-Leonard speed-control system
Fig. 6.5
Advantages
(a) The speed of the motor can be adjusted through a wide range without resistance losses
which results in high efficiency.

A more stable operation can be obtained by plac1ng a diverter across the armature in addition to the
control
ler resistance. A change în armature current (due to change in load torque) will not now be so effective in
changing the voltage drop across the armature (See Example 6.28).
** Since we can vary the voltage ofthe generator G applied to motor M. Note that field circuit of the motor is
supplied form a separate source. Therefore, the field circuit current is not affected by the variation of
generator voltage.