Types of Direct Current (DC) Compound Motors

Figure 1: Two-pole compound DC motor

The motor shown in figure 1 above is one of the four different types of compound wound DC (direct current) motors. Although this type is known as the most common form of connection, it is the one used most often and the one which should be used unless otherwise specified. It is essential also to discuss the other types of compound wound DC motors.


Four types of compound motors:

1. long-shunt cumulative
2. long-shunt differential
3. short-shunt cumulative
4. short-shunt differential

In a long-shunt cumulative motor, the current flows through the series-field and shunt-field coils of a pole in the same direction as indicated in figure 1.

If the shunt-field connection of a compound motor is reversed with respect to the series-field, the current will flow through it in the opposite direction as shown in figure 2. This produces bucking fields and the motor is known as a differentially connected motor. Motors of this type are rarely used and the only chance for it to be used is in special applications.

Figure 2: A long-shunt, differentially connected compound motor with the current flow in opposite directions in the fields.

When the shunt field of a compound motor is connected to the armature terminals instead of across the line, the motor is known as a short-shunt motor. This motor can also be either cumulative or differential depending upon the direction of current through the shunt and series fields as described. See figure 3 and figure 4.

Figure 3: A short-shunt cumulatively compound DC motor.

Figure 4: A two-pole, short-shunt differentially compounded DC motor