Page 569 - The Mechatronics Handbook

0066_Frame_C20 Page 39 Wednesday, January 9, 2002 5:49 PM

Speed Control of dc Motors
Equation (20.24) shows that the speed of a dc motor is influenced both by the applied voltage and the
magnetic flux. A change in either one of these parameters will therefore effect a change in the motor
speed.
Field Regulator
For shunt-wound and compound-wound motors a variable resistor, called a “field regulator,” can be
incorporated in series with the field winding to reduce the flux. For the series-wound motor the variable
resistor is connected in parallel with the field winding and is called a “diverter.” Figures 20.64–20.66 show
the various methods of weakening the field flux for shunt-, compound-, and series-wound motors.
In all of the above methods, the flux can only be reduced and from Eq. (20.24) this implies that the
speed can only be increased above the rated speed. The speed may in fact be increased to about three or
four times the rated speed. The increased speed, however, is at the expense of reduced torque since the
torque is directly proportional to the flux which is being reduced.
Variable Armature Voltage
Alternatively, the speed can be increased from standstill to rated speed by varying the armature voltage
from zero to rated value. Figure 20.67 illustrates one method of achieving this.

I a I f

R a

Supply
V
E M

FIGURE 20.64 Speed control by flux reduction: shunt-wound motor.

I a

R a
I f
Supply
V
E M

FIGURE 20.65 Speed control by flux reduction: compound-wound motor.

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