In split-phase motor, the auxiliary winding is made up of

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Examoogle · Accepted Answer

Option B: Thin wire with fewer turns

Explanation & Concepts:
In a split-phase single-phase induction motor, the auxiliary (starting) winding is made up of thin wire with fewer turns compared to the main winding. This design gives the auxiliary winding high resistance and low inductive reactance, resulting in a high $R_A / X_A$ ratio. Consequently, the current in the auxiliary winding leads the current in the main winding, creating the phase difference necessary to generate a rotating magnetic field for starting.

Working Principle: Single-phase induction motors are not self-starting because a single-phase AC current produces a pulsating magnetic field rather than a rotating one. By splitting the single phase into two parallel circuits (main and auxiliary windings) with different $R/X$ ratios, a phase shift $\alpha$ (typically $30^\circ$ to $40^\circ$) is produced between their respective currents, $I_m$ and $I_a$. This phase difference creates a revolving magnetic field that generates the torque needed to start the motor.

Key Formulas: $	an	heta_a = \frac{X_a}{R_a}$ — Phase angle of Auxiliary Winding current; $	an	heta_m = \frac{X_m}{R_m}$ — Phase angle of Main Winding current; $\alpha = 	heta_m - 	heta_a$ — Phase difference between Main and Auxiliary currents; $T_s \propto I_a I_m \sin\alpha$ — Starting torque equation

Key Points: The auxiliary winding has high resistance ($R_a$) and low inductance ($X_a$) due to its thin wire and fewer turns.; The main winding consists of thick wire with many turns, giving it low resistance ($R_m$) and high inductance ($X_m$).; The centrifugal switch disconnects the auxiliary winding once the motor reaches approximately 75% to 80% of synchronous speed.; The phase difference $\alpha$ achieved in a resistance split-phase motor is typically around $30^\circ$ to $40^\circ$.

Advantages: Low cost and simple construction.; No capacitors required for starting.; Reliable operational performance for low-torque applications.

Disadvantages: Low starting torque (typically 1.5 to 2 times full-load torque).; High starting current.; Poor power factor during starting.

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

Additional Info: Option A (Thick wire with fewer turns) is incorrect because thick wire reduces resistance, which would lower the $R/X$ ratio needed for the auxiliary winding.. Option C (Thin wire with many turns) is incorrect because adding many turns increases inductance ($X_a \propto N^2$), preventing $I_a$ from leading $I_m$.. Option D (Thick wire with many turns) describes the Main Winding, which requires low resistance and high reactance to produce a lagging current.

Answer: B is correct — Thin wire provides high resistance while fewer turns keep the reactance low, creating the high R/X ratio required for phase splitting.

Exam Tip: Remember that starting torque is proportional to $\sin\alpha$. Resistance split-phase motors give $\alpha \approx 30^\circ-40^\circ$, whereas capacitor-start motors achieve $\alpha \approx 90^\circ$, yielding much higher starting torque.

Examoogle · Answer

Option A: Thick wire with fewer turns

Examoogle · Answer

Option C: Thin wire with many turns

Examoogle · Answer

Option D: Thick wire with many turns

Feature	Auxiliary Winding	Main Winding
Wire Gauge/Thickness	Thin wire (high gauge)	Thick wire (low gauge)
Number of Turns	Fewer turns	Many turns
Resistance (R)	High ( $R_a$ )	Low ( $R_m$ )
Inductive Reactance (X)	Low ( $X_a$ )	High ( $X_m$ )
Current Phase Relationship	Current $I_a$ is nearly in phase with voltage	Current $I_m$ lags voltage by a large angle

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