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If the centrifugal switch does not open at 70 to 80 % of the rated speed of a motor, then it can _______________
Damage to the runing winding
Damage to the starting winding
Damage to the centrifugal switch
Overloading of running winding
Damage to the starting winding
If the centrifugal switch does not open at 70% to 80% of the rated speed, the starting winding (auxiliary winding) remains continuously energized during normal running. Since the starting winding is designed only for short-time duty with high resistance and low reactance, continuous current flow causes excessive heating and ultimately burns out or damages the starting winding.
If the centrifugal switch does not open at 70% to 80% of the rated speed, the starting winding (auxiliary winding) remains continuously energized during normal running. Since the starting winding is designed only for short-time duty with high resistance and low reactance, continuous current flow causes excessive heating and ultimately burns out or damages the starting winding.
PlossтАЛ=Ist2тАЛRstтАЛ тАФ Heating loss in the starting winding
NcutтИТoutтАЛ=(0.70┬аto┬а0.80)├ЧNrтАЛ тАФ Speed threshold at which centrifugal switch opens
In a single-phase resistance split-phase induction motor, the auxiliary winding creates a rotating magnetic field during startup. When the rotor accelerates to about 70-80% of synchronous speed, a centrifugal switch mounted on the shaft disconnects the auxiliary winding. If the switch fails to open due to contact welding or mechanical fault, current keeps flowing through the thin wire of the auxiliary winding, producing I2R heat that destroys its insulation.
Starting winding (auxiliary winding) is designed with high resistance (R) and low reactance (XLтАЛ) using thin wire.
It is short-duty rated and cannot sustain continuous operation without severe overheating.
Running winding (main winding) has low resistance (R) and high reactance (XLтАЛ) using thick wire, designed for continuous rated current.
Centrifugal switch ensures automatic disconnection without manual monitoring.
Provides high initial starting torque by phase shifting starting current.
Mechanical moving parts are prone to wear, spark welding, or failure.
If switch contacts remain open at rest, motor will not start and will hum.
Split-phase induction motors used in bench grinders and small blowers.
Capacitor-start induction motors used in pumps, compressors, and conveyors.
| Feature | Starting Winding | Running Winding |
|---|---|---|
Wire Thickness | Thin gauge wire (High Resistance) | Thick gauge wire (Low Resistance) |
Duty Rating | Short-time duty (Disconnected at ~75% speed) | Continuous duty (Stays energized) |
Reactance (XLтАЛ) | Low inductive reactance | High inductive reactance |
If centrifugal switch contacts fail to CLOSE at rest, the motor cannot produce starting torque and fails to start, drawing heavy stalled rotor current.
Option A & D: Running winding is built with thick wire and designed for continuous duty, so it is not immediately damaged.
Option C: Switch contacts may spark or weld, but the major direct catastrophic damage occurs to the starting winding insulation due to continuous I2R heat.
B is correct тАФ If the centrifugal switch stays closed past 70-80% rated speed, the auxiliary starting winding remains continuously energized and burns out.
In exam questions, remember: Failure of switch to OPEN = Winding Burnout (Starting Winding); Failure of switch to CLOSE = Motor won't start (No starting torque).