Which of the following motor has the highest starting torque?

Question

Examoogle · Accepted Answer

Option B: Capacitor Start motor

Explanation & Concepts:
Among single-phase induction motors, the capacitor-start motor provides the highest starting torque. It achieves this high starting torque—typically 300% to 400% of the full-load torque—by placing a high-value electrolytic capacitor in series with the auxiliary (starting) winding. This creates a Phase Shift ($ \alpha$) close to $90^\circ$ between the currents in the main and auxiliary windings during starting.

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 magnetic field. By inserting a capacitor in series with the starting winding, the auxiliary winding current ($I_{aux}$) leads the supply voltage ($V$), while the main winding current ($I_{main}$) lags the voltage due to its higher inductance. This yields a quadrature phase difference ($ \alpha \approx 90^\circ$), generating a strong, uniform rotating magnetic field at standstill. The starting torque $T_{s}$ is directly proportional to $ \sin \alpha$, maximizing torque when $ \alpha = 90^\circ$. Once the motor reaches approximately 75% to 80% of rated speed, a centrifugal switch disconnects the auxiliary winding and capacitor.

Key Formulas: $T_s = k \cdot I_m \cdot I_a \cdot \sin \alpha$ — Starting torque equation where $\alpha$ is the phase angle between main ($I_m$) and auxiliary ($I_a$) winding currents; $\alpha \approx 90^\circ \implies \sin \alpha \approx 1$ — Phase displacement condition in Capacitor Start motor for maximum starting torque

Key Points: Capacitor start motors deliver a high starting torque ranging between 300% to 400% of rated full-load torque.; A short-duty electrolytic capacitor is used in series with the starting winding to provide high phase shift at low cost.; The centrifugal switch disconnects the starting winding and capacitor at approximately 75%-80% of synchronous speed.; Starting torque comparison: Capacitor Start > Capacitor Start Capacitor Run > Resistance Split Phase > Shaded Pole.

Advantages: Extremely high starting torque compared to other single-phase motors; Simple construction and dependable operation; Better starting power factor

Disadvantages: High initial cost due to centrifugal switch and electrolytic capacitor; Cannot handle frequent starting cycles due to short-duty rating of the starting capacitor; Centrifugal switch introduces a potential mechanical point of failure

Applications: Heavy-load equipment such as air compressors, pumps, and refrigerators; Conveyors and machine tools requiring high breakaway torque

Additional Info: Option A (Resistance Start motor): Has moderate starting torque (~150% of full load torque) because the phase angle $\alpha$ is relatively small (around $30^\circ$).. Option C (Capacitor Run motor): Has low starting torque (~50%–100% of full load torque) because it uses a small continuous-duty capacitor optimized for running efficiency rather than starting performance.. Option D (Shaded pole motor): Has the lowest starting torque (~50% or less of full load torque) due to poor magnetic shifting and low efficiency.

Answer: B is correct — Capacitor Start motors offer the highest starting torque (300%-400% of full-load torque) among single-phase induction motors due to the $90^\circ$ phase shift provided by the starting capacitor.

Exam Tip: For competitive exams, remember the torque hierarchy order: Capacitor Start Capacitor Run (High Running Efficiency & Good Starting Torque) vs Capacitor Start (Highest Starting Torque) > Resistance Split Phase > Shaded Pole (Lowest Efficiency & Torque).

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Option A: Resistance Start motor

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Option C: Capacitor Run Motor

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Option D: Shaded pole motor

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