Electrical Control Circuit Schematic Diagram of Capacitor Start Motor

Figure 1: Typical capacitor start motor

Standard Terminal Markings of DC (Direct Current) Motors

Standard terminal markings, as developed by NEMA, are given for series, shunt and compound motors in figure 1, figure 2 and figure 3 respectively. These figures all show compensating-field as well as commutating-field windings since they are intended to cover both fractional horsepower and integral horsepower motors. Fractional horsepower motors do not generally use compensating windings and they may not use commutating-field (interpole) windings.

The markings are combinations of letters and arabic numerals, generally, these are as follows:

- Armature - A1, A2
- Control signal lead - C
- Series field - S1, S2
- Shunt field - F1, F2

Reversing DC Motors

Procedure for reversing direct current motors

Direct current motors are reversed by changing the direction of current flow through the armature or through the field. In series motors, the usual procedure is to reverse the current through the armature. Figure 1 shows this method.

All that is necessary is to interchange the leads on the brush holders. Figure 2 shows the series motor reversed by changing the current in the field circuits.

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.

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.