How do compound, series and shunt motors differ?

Question

On the basis of

a. starting torque

b. starting current

c. efficiency

d. speed regulation

Answer 1

Speed regulation is generally defined as the percentage speed change resulting from a specified percentage load change. If you look at the torque vs speed characteristic curves for the three motor types, you will see that the shunt motor has a relatively flat curve and thus provides the best speed regulation. The series motor has a very high speed when lightly loaded with the speed dropping rapidly as load is applied. That illustrates a very poor speed regulation. The slope of the curve for a compound motor is between the other two.

The current that can be tolerated by the power source and the comutator will limit the available starting torque. Since the series motor has the highest torque for a given starting current, it has the highest starting torque. The shunt motor has the lowest starting torque and the compound motor is intermediate.

The starting current is normally limited by inserting a series resistor or electronically reducing the voltage during starting. However, since the series motor offers the lowest starting current at a given torque, it can be said to have the lowest starting current. The shunt motor has the highest starting current and the compound motor is intermediate.

The armature and rotational losses for the three types of motors would be similar. The series field losses are higher than the losses for the shunt field. Since compound motors have both a shunt field and a series filed, their losses are the highest. That makes the shunt motor the most efficient, the compound motor the least efficient and the series motor intermediate.