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, the auxiliary winding with a capacitor is used to provide starting torque. A centrifugal switch is typically used to disconnect this auxiliary winding once the motor reaches approximately to of its synchronous speed (). If the centrifugal switch fails to operate and remains closed, the auxiliary winding and capacitor stay in the circuit for all speeds. Consequently, the motor will follow the combined (starting) characteristic curve throughout its entire speed range, without the transition (jump) to the main-winding-only curve that normally occurs at . Looking at the options:
- Curves (A) and (B) show a transition at , which would only happen if the switch functioned correctly.
- Curves (C) and (D) show a single continuous curve.
- Curve (C) correctly represents the high starting torque characteristic of a capacitor-start motor.
- Curve (D) represents a characteristic with low starting torque, which is not typical for a motor with a starting capacitor.
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