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Centrifugal switch disconnect starting winding at
20 to 30 % speed
30 to 40 % speed
50 to 70 % speed
100% speed
50 to 70 % speed
In a single-phase split-phase or capacitor-start induction motor, a centrifugal switch is connected in series with the starting (auxiliary) winding ┬╖ The switch automatically opens and disconnects the starting winding when the rotor reaches approximately 70% to 80% (typically specified as 50% to 70% or 75% in textbook standards) of its synchronous speed ┬╖ This prevents the auxiliary winding, which is designed only for short-time duty, from overheating and burning out.
In a single-phase split-phase or capacitor-start induction motor, a centrifugal switch is connected in series with the starting (auxiliary) winding ┬╖ The switch automatically opens and disconnects the starting winding when the rotor reaches approximately 70% to 80% (typically specified as 50% to 70% or 75% in textbook standards) of its synchronous speed ┬╖ This prevents the auxiliary winding, which is designed only for short-time duty, from overheating and burning out.
NsтАЛ=P120fтАЛ тАФ Synchronous speed formula
% NsтАЛ = \left(NsтАЛNтАЛ \right) ├Ч100тАФPercentage speed at which switch opens
Single-phase induction motors are not self-starting because a single-phase alternating current creates a pulsating magnetic field rather than a rotating one ┬╖ An auxiliary winding (often with a capacitor) is used to create a phase shift, producing a revolving magnetic field to initiate rotation ┬╖ As the motor accelerates, centrifugal forces act on spring-loaded weights attached to the rotor shaft; once the speed reaches 50тАУ70% of synchronous speed, the outward force overcomes the spring tension and opens the electrical contact, removing the auxiliary winding from the circuit.
The centrifugal switch is connected in series with the starting (auxiliary) winding.
If the centrifugal switch fails to open, the auxiliary winding will remain energized, causing overheating and potential winding burnout.
If the centrifugal switch fails to close, the motor will not be able to develop starting torque and will hum without rotating.
It operates based on centrifugal force developed on weights attached to the rotor shaft.
Automatically isolates short-duty rated auxiliary winding without external controls.
Simple mechanical construction and high reliability during normal operation.
Moving mechanical parts are subject to wear, tear, and sparking at contacts.
Cannot be used in explosive or hazardous environments due to potential arcing.
Split-phase induction motors used in small tools and fans.
Capacitor-start induction-run motors used in compressors, pumps, and conveyors.
Option A (20 to 30 % speed) is too low because the motor has not developed sufficient main-winding torque yet.
Option B (30 to 40 % speed) is insufficient to ensure smooth transition to running torque.
Option D (100% speed) is incorrect because an induction motor never runs at 100% synchronous speed (NsтАЛ) under load due to slip.
C is correct тАФ The centrifugal switch disconnects the auxiliary winding when the motor reaches 50 to 70 % of synchronous speed.
In competitive exams like GATE/SSCJE, remember that split-phase and capacitor-start motors use a centrifugal switch, whereas capacitor-start capacitor-run motors use it to disconnect only the starting capacitor, keeping the running capacitor permanently connected.