In a split-phase motor, the running winding must have _____________________ to ensure ________________ .

Question

Examoogle · Accepted Answer

Option B: Low resistance and high inductance, High operating Efficiency

Explanation & Concepts:
In a split-phase single-phase induction motor, the running (main) winding is designed with low resistance and high inductance compared to the starting (auxiliary) winding. This high inductance-to-resistance ratio ensures that the current in the running winding lags the supply voltage by a large phase angle (nearly $90^\circ$). Low resistance minimizes $I^2R$ copper losses during continuous operation, thereby maximizing operating efficiency.

Working Principle: To create a rotating magnetic field in a single-phase induction motor, two phase currents with a significant phase difference are required. The starting winding has high resistance and low inductance (current nearly in phase with voltage), whereas the running winding has low resistance and high inductance (current lags voltage by a large angle). This creates a spatial and temporal phase shift of around $30^\circ$ to $40^\circ$, producing the starting torque. Once the motor reaches ~75% rated speed, a centrifugal switch disconnects the starting winding, leaving the low-resistance main winding to run at high efficiency.

Key Formulas: $\phi = 	heta_a - 	heta_m$ — Phase angle difference between auxiliary and main winding currents; 	an(	heta_m) = \frac{\omega L_m}{R_m} — Phase lag angle of the main winding current; P_{cu} = I_m^2 R_m — Copper loss in running winding (minimized when $R_m$ is low)

Key Points: The main winding remains connected to the supply permanently during normal operation.; Low resistance in the running winding reduces continuous $I^2R$ heat loss, thereby ensuring maximum efficiency.; High reactance (inductance) causes main winding current $I_m$ to lag supply voltage $V$ by nearly $70^\circ - 80^\circ$.; The starting winding has high resistance and low reactance so its current $I_a$ is almost in phase with supply voltage $V$.

Advantages: High operating efficiency during steady-state run condition due to low main winding resistance.; Simple, rugged construction with low cost compared to capacitor-start motors.

Disadvantages: Moderate starting torque (150% to 200% of full load torque).; Requires a centrifugal switch to disconnect the auxiliary winding at ~75% synchronous speed.

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

Additional Info: The centrifugal switch disconnects the starting winding when the rotor reaches approximately 75% to 80% of synchronous speed.. Option A is incorrect because low inductance would decrease the phase shift necessary for starting torque.. Option C and D describe characteristics closer to auxiliary windings rather than the main running winding.

Answer: B is correct — Low resistance minimizes running copper losses to maximize efficiency, while high inductance provides the required reactive magnetic field and phase lag.

Exam Tip: Always remember: Auxiliary winding is designed for starting (High R, Low X for max phase split), while Main winding is designed for running (Low R, High X for max efficiency and magnetic flux).

Examoogle · Answer

Option A: Low resistance and Low inductance, High operating PF

Examoogle · Answer

Option C: High resistance and High inductance, High operating Efficiency

Examoogle · Answer

Option D: High resistance and low inductance, High operating PF

Feature	Running Winding	Starting Winding
Resistance (R)	Low	High
Inductive Reactance ( $X_{L}$ )	High	Low
Wire Gauge / Conductor Size	Thick wire (low R)	Thin wire (high R)
Connection Duty	Permanently connected	Temporary (disconnected by switch)

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