DC Compound Motors

The DC compound motor consists of both series and shunt field windings.

Long shunt compound motor

In long shunt dc compound motor, the shunt field winding is connected across both armature and series field winding.

Short shunt compound motor

In short shunt dc compound motor, the shunt field winding is across the armature and series field winding is connected in series with this combination.

Classification of Compound Motor

• Cumulative Compound Motor
• Differential Compound Motor

Cumulative Compound Motor

If series excitation helps the shunt excitation i.e) series flux is in the same direction, then the dc motor is said to be cummulatively compound. Cumulative compound motors are used where series characteristics are required and where, in addition, the load is likely to be removed totally such as in coal cutting machines etc. Due to shunt windings, speed will not become excessively high but due to series windings, it will be able to take or operate under heavy load.

Application of cumulative Compound Motor

• Cumulative compound motors are used wherever high starting torque or pulsating loads are required.
• Cumulative compound motors are used to drive electric shovels, metal-stamping machine, reciprocating pumps, hoist, compressors etc.

Differential Compound Motor

In differential compound motor, series field opposes the shunt field. As series field opposes the shunt field, the flux is decreased as load is applied to the motor. This results in the motor speed remaining almost constant or even increasing with increase in load. Due to this reason, there is a decrease in the rate at which the motor torque increases with load.

Application Differential Compound Motor

• Differential compound motors are used wherever the constant speed is required with irrespective of load.
• Differential compound motors are mostly used in Elevator, Escalator etc.