In a split-phase motor

Question

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Option B: The starting winding is connected through a centrifugal switch

Explanation & Concepts:
In a split-phase single-phase induction motor, the starting (auxiliary) winding is connected in series with a centrifugal switch across the AC supply. This switch automatically disconnects the starting winding from the circuit once the motor reaches approximately $75\%$ to $80\%$ of its synchronous speed.

Working Principle: Single-phase induction motors are not self-starting because a single-phase AC supply produces a pulsating magnetic field rather than a rotating one. To create a rotating magnetic field, two windings (main/running and auxiliary/starting) are placed $90^\circ$ apart spatially, with different resistance-to-reactance ratios ($R/X$) to create a phase difference between their currents. Once the motor accelerates and approaches near-rated speed, the centrifugal switch opens due to centrifugal force, disconnecting the starting winding to prevent overheating and power losses.

Key Formulas: $\alpha = 	heta_m - 	heta_s$ — Phase angle difference between main winding current ($I_m$) and starting winding current ($I_s$); $T_s \propto I_m I_s \sin\alpha$ — Starting torque equation of single-phase induction motor

Key Points: The starting winding is made of thin wire (high resistance $R$, low inductance $X$).; The running winding is made of thick wire (low resistance $R$, high inductance $X$).; The centrifugal switch is connected in series ONLY with the starting winding.; The switch opens at $75\% - 80\%$ of synchronous speed.

Advantages: Low initial cost and simple construction.; Reliable operational mechanism without complex electronic controls.

Disadvantages: Low starting torque (typically $1.5$ to $2$ times full-load torque).; Cannot be used for heavy-load starting applications.

Applications: Small fans and blowers; Centrifugal pumps; Washing machines; Small machine tools

Additional Info: If the centrifugal switch fails to open, the starting winding will remain energized, causing it to overheat and burn out due to its high resistance.. If the centrifugal switch fails to close at standstill, the motor will not be self-starting and will merely hum.

Answer: B is correct — The starting winding is connected in series with a centrifugal switch to disconnect it once the motor reaches approximately $75\%$ to $80\%$ of full speed.

Exam Tip: Always remember: in capacitor-start or split-phase motors, the centrifugal switch is exclusively in series with the auxiliary (starting) winding branch, never the main (running) winding.

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Option A: The running winding is connected through a centrifugal switch

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Option C: Centrifugal switch is used to control supply voltage

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Option D: Both starting and running windings are connected through a centrifugal switch

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