Types of DC Motors

There are three basic types of DC motors such as series motors, shunt motors and compound motors. They differ largely in the method in which their field and armature coils are connected.

Series DC Motor

In the series motor, the field windings consisting of a relatively few turns of heavy wire are connected in series with the armature winding. Both diagrammatic and schematic illustration of a series motor is shown in the following figure. The same current flowing through the field winding also flows through the armature winding. Therefore, any increase in currents strengthens the magnetism of both the field and the armature.

FIGURE

Because of the low resistance in the field winding, the series motor is able to draw a large current in starting. This starting current produces a high starting torque, which is the series motor's principal advantage.

The speed of a series motor is dependent upon the load. Any change in load is accompanied by a substantial change in speed. A series motor will run at high speed when it has a light load and at low speed with a heavy load. If the load is removed entirely, the motor may operate at such a high speed that the armature will fly apart. If high starting torque is needed under heavy load conditions, series motors have many applications. Series motors are often used in aircraft as engine starters and for raising and lowering landing gear, cowl flaps, and wing flaps.

Shunt DC Motor

In the shunt motor, the field winding is connected in parallel or in shunt with the armature winding. The resistance in the field winding is high. Since the field winding is connected directly across the power supply, the current through the field is constant. The field current does not vary with motor speed as in the series motor and, therefore, the torque of the shunt motor will vary only with the current through the armature. The torque developed at starting is less than that developed by a series motor of equal size.

FIGURE

The speed of the shunt motor varies very little with changes in load. When all load is removed, it assumes a speed slightly higher than the loaded speed. This motor is particularly suitable for use when constant speed is desired and when high starting torque is not needed.

Compound DC Motor

A compound motor has two field windings, as shown in the following figure. One is a shunt field connected in parallel with the armature, the other is a series field that is connected in series with the armature.

The shunt field gives this type of motor the constant speed advantage of a regular shunt motor. The series field gives it the advantage of being able to develop a large torque when the motor is started under a heavy load. It should not be a surprise that the compound motor has both shunt-motor and series-motor characteristics.

FIGURE

When the shunt field is connected in parallel with the series field and armature, it is called a long shunt as shown in view A of the figure. Otherwise, it is called a short shunt, as shown in view B.

The shunt winding is composed of many turns of fine wire and is connected in parallel with the armature winding. The series winding consists of a few turns of large wire and is connected in series with the armature winding. The starting torque is higher in the shunt motor but lower in the series motor. Variation of speed with load is less than in a series-wound rid motor hut greater than in a shunt motor. The compound motor is used whenever the combined characteristics of the series and shunt motors are desired.

Like the compound generator, the compound motor has both series and shunt field windings. The series winding may either aid the shunt wind (cumulative compound) or oppose the shunt winding (differential compound).

The starting and load characteristics of the cumulative-compound motor are somewhere between those of the series and those of the shunt motor.

Because of the series field, the cumulative-compound motor has a higher starting torque than a shunt motor. Cumulative-compound motors are used in driving machines which are subject to sudden changes in load. They are also used where a high starting torque is desired, but a series motor cannot be used easily.

In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor. These two tend to offset each other and the result is a practically constant speed. However, since an increase in load tends to decrease the field strength, the speed characteristic becomes unstable. This type of motor is rarely used in aircraft systems. A graph of the variation in speed with changes of load of the various types of DC motors is shown below.

FIGURE