GATE EE 2014 Set 3 — Question 22
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Electrical Machines → Single-Phase Induction Motors → Starting Methods
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Question
A single phase induction motor is provided with capacitor and centrifugal switch in series with auxiliary winding. The switch is expected to operate at a speed of , but due to malfunctioning the switch fails to operate. The torque-speed characteristic of the motor is represented by
Correct answer
(C) [figure]
Solution
In a single-phase induction motor with a capacitor and centrifugal switch in series with the auxiliary winding, the auxiliary winding (with its capacitor) produces a phase-shifted current, so together with the main winding it creates a rotating field and a large starting torque. Normally the centrifugal switch opens at about and the motor then runs on the main winding alone.If the switch fails to operate, the auxiliary winding and capacitor remain connected at all speeds. The motor therefore follows the two-winding (capacitor) characteristic over the entire speed range: a single continuous curve that starts from a non-zero torque at zero speed, with no step at .Checking the options:
- (A) shows the normal behaviour: the two-winding curve up to and then a drop onto the main-winding-only curve when the switch opens. That requires the switch to work.
- (B) starts from zero torque at standstill and shows a step at ; neither fits a motor whose auxiliary winding stays in circuit.
- (D) is continuous but starts from zero torque at zero speed. That is the main-winding-only characteristic: a single winding gives a pulsating field and no starting torque.
- (C) is continuous, starts from a non-zero torque at zero speed and has no step at — the capacitor-motor characteristic throughout.
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