The main winding of Split-phase Single-Phase Induction Motor has

Question

Examoogle · Accepted Answer

Option B: Low Resistance and High Reactance

Explanation & Concepts:
The main winding (running winding) of a split-phase single-phase induction motor is designed to have low resistance and high inductive reactance. This is achieved by making it with thick wire and placing it deep into the stator slots. Consequently, the current in the main winding lags the supply voltage by a large phase angle (nearly 90°).

Working Principle: To produce a rotating magnetic field in a single-phase induction motor, a phase difference is required between the currents of two stator windings. The auxiliary (starting) winding has high resistance and low reactance, causing its current to be nearly in phase with the voltage. The main winding has low resistance and high reactance, causing its current to lag significantly. This creates a phase difference (approx. 30° to 40°) between the two currents, producing a revolving magnetic field to develop starting torque.

Key Formulas: $\phi_m = 	an^{-1}\left(\frac{X_m}{R_m}ight)$ — Phase lag angle of main winding current; $\alpha = \phi_m - \phi_a$ — Phase difference between main and auxiliary winding currents; $T_s \propto I_m I_a \sin(\alpha)$ — Starting torque proportional to current phase difference

Key Points: Main winding is made of thick copper wire and placed deep into the stator slots to maximize inductance.; Auxiliary winding is made of fine (thin) wire placed shallow in slots, giving high resistance and low reactance.; A centrifugal switch disconnects the auxiliary winding once the motor reaches 75% to 80% of synchronous speed.

Advantages: Simple construction and low initial cost; Reliable operation with no complex electronic controls

Disadvantages: Low starting torque (around 150% to 200% of full-load torque); High starting current with low starting power factor

Applications: Small blowers and centrifugal pumps; Washing machines and small fans; Office machinery and machine tools

Additional Info: Option A describes the properties of the Auxiliary (starting) winding, not the main winding.. Option C and Option D are incorrect as the two windings in a split-phase motor must have contrasting R/X ratios to establish the necessary phase difference.

Answer: B is correct — The main winding has low resistance and high reactance so that its current lags the applied voltage, creating a phase difference relative to the auxiliary winding current.

Exam Tip: Remember: Main winding = Thick wire + Deep slots = Low R + High X. Auxiliary winding = Thin wire + Shallow slots = High R + Low X.

Examoogle · Answer

Option A: High Resistance and Low Reactance

Examoogle · Answer

Option C: High Resistance & Reactance

Examoogle · Answer

Option D: Low Resistance & Reactance

Feature	Main Winding	Auxiliary Winding
Wire Gauge/Thickness	Thick wire (low resistance)	Thin wire (high resistance)
Slot Location	Placed deep in stator slots (high $X_L$ )	Placed near top of slots (low $X_L$ )
Impedance Characteristics	Low $R$ , High $X_L$	High $R$ , Low $X_L$
Current Phase Angle	Current lags supply voltage heavily	Current nearly in phase with voltage

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