The main winding & auxiliary winding of Split-phase Single-Phase Induction Motor have spaced ______________ ap

Question

The main winding & auxiliary winding of Split-phase Single-Phase Induction Motor have spaced ______________ apart and are connected in _______________ across the 1-Φ supply.

Examoogle · Accepted Answer

Option B: 90º electrically, parallel

Explanation & Concepts:
In a split-phase single-phase induction motor, the main winding and auxiliary (starting) winding are physically distributed in the stator slots such that they are displaced by 90º electrically from each other. Both windings are connected in parallel across the single-phase AC supply line to operate from the same voltage source.

Working Principle: A single-phase AC supply creates a pulsating magnetic field rather than a rotating one, producing zero starting torque. By placing two windings displaced by 90º electrically and creating a phase difference between their currents (due to differing $R/X$ ratios), a two-phase rotating magnetic field is produced at standstill, generating the necessary starting torque.

Key Formulas: $\alpha = 	heta_m - 	heta_a \approx 30^\circ 	ext{ to } 40^\circ$ — Phase angle difference between main and auxiliary winding currents; $T_s \propto I_m I_a \sin\alpha$ — Starting torque of split-phase induction motor; 	heta_e = \frac{P}{2} \cdot 	heta_m — Relation between electrical angle (	heta_e) and mechanical angle (	heta_m)

Key Points: The auxiliary winding has high resistance and low inductive reactance (thin wire, fewer turns).; The main winding has low resistance and high inductive reactance (thick wire, more turns).; A centrifugal switch disconnects the auxiliary winding when the motor reaches about 75% to 80% of synchronous speed.

Advantages: Low cost and simple construction.; Easily reversible by reversing connections of either main or auxiliary winding.

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

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

Additional Info: For a 2-pole machine, $90^\circ$ electrical displacement is equal to $90^\circ$ mechanical displacement, but for general multipole machines, spatial phase separation in stator winding design is always defined in electrical degrees.. Option A is incorrect because mechanical angle equals electrical angle only for 2-pole machines; space displacement is universally defined in electrical degrees.. Options C and D are incorrect because connecting windings in series would prevent creating the parallel current split necessary for generating phase difference, and $180^\circ$ separation would yield zero net rotating magnetic field.

Answer: B is correct — Main and auxiliary windings are space-shifted by 90º electrically in the stator and connected in parallel across the single-phase line to split current phases and create a rotating magnetic field.

Exam Tip: Remember that starting torque $T_s$ is proportional to $\sin\alpha$, where $\alpha$ is the phase difference between currents; maximum torque occurs at $\alpha = 90^\circ$, which capacitor-start motors approximate better than split-phase motors.

Examoogle · Answer

Option A: 90º mechanically, parallel

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Option C: 180º mechanically, series

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Option D: 180º electrically, series

Feature	Main Winding	Auxiliary Winding
Wire Type	Thick wire (Low Resistance)	Thin wire (High Resistance)
Reactance Property	High Inductive Reactance ( $X_m$ )	Low Inductive Reactance ( $X_a$ )
Operation Mode	Remains connected continuously	Disconnected after starting by centrifugal switch

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