While DC motors are applied mostly in special applications, knowing how to properly connect them is important to their required performance.

You're about to connect a newly repaired or purchased DC motor on your industrial vertical boring machine or planer. What should you know about the motor's leads? What do the lead designations mean? How do you determine the correct direction of rotation?

Let's walk through some basics and discuss the answers to these questions.

DC motor connections

There are many ways in which a DC compound motor can be connected. One such connection is called a short shunt connection. As shown in Fig. 1A, the shunt field spans only the armature. With the motor short shunt connected, the shunt field is added to the series field current, resulting in the motor having stronger torque characteristics.

Another connection is when the shunt field spans both the armature and the series field, as shown in Fig. 1B. This is called a long shunt connection and permits the motor to have better speed regulation.

Suppose we make the motor lead connections, as shown in Fig. 2A, so that the series field magnetism strengthens the magnetism produced by the shunt field. The motor then becomes what is known as a cumulative compound motor.

If we reverse the above connection so that the magnetism of the series field opposes or weakens the magnetism of the shunt field, as shown in Fig. 2B, the motor then becomes what is known as a differential compound motor.

While a differential compound motor provides a more constant speed at all loads, it's somewhat unstable. Therefore, most applications use cumulative compound motors.

DC motor terminal identification

DC motor terminal leads are labeled for easy identification in the motor's terminal box. For example, leads labeled "A1" and "A2" are connected to the armature through the motor's brushes; leads labeled "S1" and "S2" are the ends of the series field; and leads labeled "F1" and "F2" are the ends of the shunt field.

When interpoles are present, these windings will have their leads brought out to the terminal box and labeled "C1" and "C2," or "S3" and "S4." In fact, most machines designed to be used as motors or generators will have these leads readily accessible.

Determining direction of rotation

You can find out the direction of rotation of a DC motor by facing its commutator end, which usually is the back or rear of the motor, and noting the lead designation. Let's suppose you have a series motor, as shown in Fig. 3 (on page 36). If its windings are labeled in a standard manner, you can reverse the direction by simply reversing the armature leads. The same can be done with a shunt motor, as shown in Fig. 4 (on page 36), and a compound motor, as shown in Fig 5 (on page 36).

You also can reverse direction by reversing field leads; however, you run the risk of changing a cumulative compound motor, which is inherently stable, into a differential compound motor, which is somewhat unstable.

RELATED ARTICLE: SOME DC MOTOR BASICS

A rotating armature and stationary field are the major components of a DC compound motor. The armature has a winding that connects to a rotating commutator. Riding on this commutator are carbon brushes. The stationary field serves as the housing or frame.

A compound wound motor has two separate field windings: A shunt winding, which is wound with smaller wire and has thousands of turns, and a series winding, which is wound with large wire and only has a few turns. These field windings, or coils, are placed on pole pieces that are attached to the motor's frame.

Parts of the series and shunt field windings are wound on each pole piece, with each winding made in opposite clockwise and counterclockwise directions. (All DC compound motors have an even number of poles; smaller motor sizes usually have two or four poles while larger sizes have a larger number of poles.) Thus, each pole piece is alternately magnetized north and south. The two ends each of the shunt and series windings are brought out to the motor terminal box.

Commutating poles, or interpoles, are small pole pieces placed midway between the main pole pieces. They are permanently connected in series with the armature brushes and considered part of the armature circuit. Interpoles also are wound clockwise/counterclockwise alternately and counteract field magnetism distortion caused by the rotation of the heavily magnetized armature in the field flux. Thus, brush sparking or arcing is eliminated.