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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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In single-phase induction motor. The starting winding is cut out of from the circuit when ______________________
when the motor speed reaches 25 % of the full load speed
when the motor speed reaches 50 % of the full load speed
when the motor speed reaches 75 % of the full load speed
when the motor speed reaches 70 to 80 % of the full load speed
when the motor speed reaches 70 to 80 % of the full load speed
Quick Summary: In a single-phase induction motor, the centrifugal switch is designed to disconnect the auxiliary (starting) winding once the motor gains sufficient momentum. This disconnection typically occurs when the motor reaches approximately 70% to 80% of its rated synchronous speed to prevent overheating of the starting winding.
In a single-phase induction motor, the centrifugal switch is designed to disconnect the auxiliary (starting) winding once the motor gains sufficient momentum. This disconnection typically occurs when the motor reaches approximately 70% to 80% of its rated synchronous speed to prevent overheating of the starting winding.
Ns=P120f — Synchronous speed of the rotating magnetic field
s=NsNs−Nr — Slip of the induction motor
Single-phase motors are not inherently self-starting; the auxiliary winding provides a phase shift to generate a rotating magnetic field. Once the rotor accelerates due to this field, the centrifugal switch physically opens the auxiliary circuit via a mechanical mechanism. If the switch remains closed, the high-resistance starting winding would draw excessive current and likely burn out due to continuous operation.
The auxiliary winding is designed for intermittent duty only.
The centrifugal switch is a mechanical device mounted on the motor shaft.
Operating the starting winding continuously leads to thermal damage due to its high resistance and limited heat dissipation capacity.
A capacitor-start motor uses this switch to disconnect the capacitor and auxiliary winding simultaneously.
Improves motor starting torque
Prevents damage to the auxiliary winding
Mechanical wear and tear of centrifugal switch contacts
Risk of failure if the switch gets stuck in the closed position
Domestic appliances like mixers and washing machines
Small compressors and industrial blowers
The 70-80% threshold is an engineering standard to ensure the rotor has sufficient inertia to continue accelerating under the main winding alone.
If the switch is set too low (e.g., 25%), the motor may not have enough torque to reach the required speed.
D is correct — The centrifugal switch opens at 70% to 80% of rated speed to prevent auxiliary winding burnout.
Always remember that the starting winding is purely for starting torque and is not rated for continuous duty; it is a common failure point in single-phase motor maintenance.