Question

Which of the following three motors has the poorest speed regulation: shunt motor, series motor, or cumulative compound motor? Explain.

Graph of speed regulation of shunt, compound, and series-connected DC motors

Speed regulation is defined as the change in speed when the load on the motor is reduced from rtated value to zero, expressed as % of the rated load speed.

Speed regulation = no load speed – full load speed / full load speed x 100%

Shunt motor has the poorest speed regulation.

N α V/ Φ

If Φ is constant, assuming constant applied voltage V, then N may be considered as constant over the load range.

Speed is unaffected by Ia and theoretical graph is N1

In practice, speed reduces slightly with load as shown by N

This is because back emf reduces slightly due to the armature voltage drop IaRa increases

Fig 1

Fig 2

Fig 3

Series motor. Load is same for field current as well as armature current. Since Φ increases with load and N α 1/ Φ [ as Eb α N Φ]

No load flux is small and speed can be excessive

Therefore the motor should never run light

By observing fig 2 between N and Ia, this machine is also called as variable speed machine.

Cumulative compound motor

In this motor there are 2 variations

Strong shunt – weak series,

Strong series – weak shunt.

In case of speed regulation, the characteristic of this motor is a combination of shunt and series motor.

Used where high starting torque is needed or pulsating loads.

[In SERIES MOTORS, the field windings are connected in series with the armature coil. The field strength varies with changes in armature current. When its speed is reduced by a load, the series motor develops greater torque. Its starting torque is greater than other types of dc motors. Its speed varies widely between full-load and no-load. Unloaded operation of large machines is dangerous.

In SHUNT MOTORS, the field windings are connected in parallel (shunt) across the armature coil. The field strength is independent of the armature current. Shunt-motor speed varies only slightly with changes in load, and the starting torque is less than that of other types of dc motors.

In COMPOUND MOTORS, one set of field windings is connected in series with the armature, and one set is connected in parallel. The speed and torque characteristics are a combination of the desirable characteristics of both series and shunt motors.