0195136047.pdf

13.4 DIRECT-CURRENT MACHINES 605

overall efficiency. The speed of the machine is governed by the value of the voltage drop in the

series resistor and is therefore a function of the load on the machine. The application of this method

of control is thus limited. Because of its low initial cost, however, the series-resistance method,

or a variation of it, is often attractive economically for short-time or intermittent slowdowns.

Unlike shunt-field control, armature-resistance control offers aconstant-torque drivebecause

both flux and, to a first approximation, allowable armature current remain constant as speed varies.

Theshunted-armature methodis a variation of this control scheme. This is illustrated in Figure

13.4.11(a) as applied to a series motor, and in Figure 13.4.11(b) as applied to a shunt motor.

ResistorsR 1 andR 2 act as voltage dividers applying a reduced voltage to the armature. They offer

greater flexibility in their adjustments to provide the desired performance.

The overall output limitations are as shown in Figure 13.4.12. With base speed defined as the

full-field speed of the motor at the normal armature voltage, speeds above base speed are obtained

by motor-field control at approximately constant horsepower, and speeds below base speed are

obtained by armature-voltage control at approximately constant torque. The development ofsolid-

state controlled rectifierscapable of handling many kilowatts has opened up a whole new field of

solid-state dc motor driveswith precise control of motor speed. The control resistors (in which

energy is wasted) are eliminated through the development of power semiconductor devices and

R 1

+

−

+

−

R 2

Vt

Series field

(a)

Armature

R 1

R 2

Vt

Shunt field

(b)

Armature

Figure 13.4.11Shunted-armature method of speed control.(a)As applied to a series motor.(b)As applied

to a shunt motor.

Approximate

allowable

horsepower

Constant horsepower

Constant

torque

Base

speed

Maximum

speed

Armature voltage

control

Motor field

control

Approximate
allowable
torque

Constant horsepower

Constant torque

Base

speed

Maximum

speed

Armature voltage

control

Motor field

control

Figure 13.4.12Output limitations combining the armature voltage and field rheostat methods of speed control.