Electrical Power Systems Technology

Mechanical Systems 359

nent-magnet motor can be reversed by reversing the two power lines.

Series-wound DC Motors—The manner in which the armature and

field circuits of a DC motor are connected determines its basic character-

istics. Each type of DC motor is similar in construction to the type of DC

generator that corresponds to it. The only difference, in most cases, is that

the generator acts as a voltage source, while the motor functions as a me-

chanical power-conversion device.

The series-wound motor, shown in Figure 14-5, has the armature and

field circuits connected in a series arrangement. There is only one path for

current to flow from the DC voltage source. Therefore, the field is wound

of relatively few turns of large-diameter wire, giving the field a low resis-

tance. Changes in load applied to the motor shaft cause changes in the cur-

rent through the field. If the mechanical load increases, the current also in-

creases. The increased current creates a stronger magnetic field. The speed

of a series motor varies from very fast at no load, to very slow at heavy

loads. Since large currents may flow through the low resistance field, the

series motor produces a high torque output. Series motors are used when

heavy loads must be moved, and speed regulation is not important. A typ-

ical application is automobile starter motors.

Figure 14-5. Series-wound DC motor

Shunt-wound DC Motors—Shunt-wound DC motors are more com-

monly used than any other type of DC motor. As shown in Figure 14-6,

the shunt-wound DC motor has field coils connected in parallel with its

armature. This type of DC motor has field coils that are wound of many

turns of small-diameter wire and have a relatively high resistance. Since

the field is a high-resistant parallel path of the circuit of the shunt motor, a