Question

(a) Which of the following three motors has the poorest speed regulation:

(i) Shunt motor

(ii) Series motor

(iii) Cumulative compound motor

Explain (6)

(b) A 440V shunt motor takes a armature current of 30A at 700 rev/min. The armature resistance is 0.7 ohm. If the flux is suddenly reduced by 20 percent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transition from initial to final steady-state conditons (10)

Answer 1

Part (a)

Series motor has the poorest speed regulation among the three motors.

Speed regulation refers to the change in motor speed from no load to full load, expressed as a percentage of the full-load speed:

$$\%\:Speed\:regulation=\frac{No\:load\:speed\:-\:Full\:load\:speed}{Full\:load\:speed}\:\times100$$

Shunt Motor: The field windings are connected in parallel (shunt) with the armature. Shunt motors have a nearly constant speed regardless of load changes, offering excellent speed regulation. This is because the field current remains relatively stable, keeping the magnetic flux constant.

Series Motor: The field windings are connected in series with the armature. Series motors exhibit significant speed variation with load changes. At no load, they can reach dangerously high speeds, while under heavy load, the speed drops considerably. This results in poor speed regulation, making them unsuitable for applications requiring constant speed.

Cumulative Compound Motor: Combines both series and shunt field windings, with the series field aiding the shunt field. Cumulative compound motors offer a compromise between shunt and series motors. They provide better speed regulation than series motors but are not as precise as shunt motors. The combination of windings helps moderate speed variations with load changes.

✓ Verified Model Answer (Text Solution)

Series motor has the poorest speed regulation among the three motors.