Commutating Field (Interpole) of DC Motors

Nearly all shunt and compound motors of 1/2 horsepower or more have commutating fields or interpoles located between the main poles. These interpoles have one winding of heavy wire and are connected in series with the armature as shown in figure 1. The purpose of the interpole is to prevent sparking.

There are usually as many interpoles as main poles, although half as many may be used without causing inefficient operation. Although the interpoles are connected for alternate polarity, just as the main poles, they also have a definite polarity with respect to the main poles.

The polarity of an interpole in a motor is the same as the main pole behind it. This means that if a motor is viewed from the commutator and is rotating clockwise, the polarity of the interpole must be the same as that of the main pole which precedes it in the direction opposite to rotation. Figure 1 shows a schematic diagram of a compound-interpole direct current motor.

Figure 1: Schematic diagram of a compound-interpole direct current motor

A two-pole, compound-interpole motor connected for counter-clockwise rotation is shown in figure 2.

Procedure for connecting compound-interpole motor:

Connect the shunt-field coils in series for proper polarity and bring the two lead wires out of the DC motor.

Note  the polarity of one pole. Perform the same operation for the series field coils, and bring out two wires. Connect the interpoles in series for alternate polarity, then connect them in series with the armature, bringing out one interpole lead and one armature lead. Six leads have been brought out of the motor, two shunt-field leads, two series-field leads, and two armature-interpole leads. Connecting the six leads as shown in figure 2 results to a compound motor in counter-clockwise rotation.

Figure 2: A two-pole compound interpole DC motor for counter-clockwise rotation