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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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When the rotor reached ___________ (rated speed), the centrifugal switch (CS) connected in series with starting winding will disconnect the ___________ from 1-Φ supply.
20-30 % speed, auxiliary winding
40-50 % speed, main winding
70-80 % speed, auxiliary winding
50 % speed, main winding
70-80 % speed, auxiliary winding
Quick Trick: Remember the 75% rule for single-phase motors: the switch trips near normal operating speed (~70-80%) to disconnect the auxiliary (starting) winding once main torque takes over.
Remember the 75% rule for single-phase motors: the switch trips near normal operating speed (~70-80%) to disconnect the auxiliary (starting) winding once main torque takes over.
Identify the Role of the Auxiliary Winding
A single-phase induction motor is not self-starting. An auxiliary (starting) winding creates the initial rotating magnetic field to produce starting torque.
Identify the Function of the Centrifugal Switch
Once the motor accelerates near rated speed, the auxiliary winding is no longer needed and would overheat if left energized. The centrifugal switch (CS) is connected in series with the auxiliary winding to open its circuit.
Determine the Threshold Speed Range
Standard design practice dictates that the centrifugal switch operates open at approximately 70% to 80% of rated synchronous speed.
A: 20-30% speed is too low to disconnect the starting winding as sufficient main torque hasn't developed; B: Main winding must remain connected for continuous operation; D: Main winding is never disconnected by the centrifugal switch.
C is correct because the centrifugal switch disconnects the auxiliary winding once the rotor reaches 70-80% of rated speed, allowing the motor to run solely on the main winding.
Always remember: Main winding stays connected indefinitely; starting/auxiliary winding gets disconnected by the centrifugal switch between 75% to 80% speed.