Page 631 - Basic Electrical Engineering

Removing the extra variable resistance from the circuit when it is not
2
required is essential to avoid unnecessary wastage of energy as I R loss.

7.10 SPEED CONTROL OF DC MOTORS

The basic equations for a dc motor are

V − E = I R (7.7)
b
a
a
from (7.6) and (7.7),

From this expression for speed, we can say that the speed N of the motor can
be changed by any or a combination of the following three methods.

7.10.1 Voltage Control Method

By changing the supply voltage V, the speed can be changed. As supply
voltage can only be reduced, speed N can also be reduced from its rated value

by this method.

7.10.2 Field Control Method

By varying the flux ϕ, and hence this method is called the field control

method. By putting a variable resistance in the field circuit, field current can
be reduced, and hence flux produced can be reduced. When flux is reduced,

speed is increased. Thus, by the field control method the speed of the motor
can only be increased. See eq. (7.8).
By a combination of voltage control and field control methods, the speed

of the motor can be increased and also decreased above and below its normal
speed.