Which of the following single-phase induction motor provides highest starting torque?

Question

Examoogle · Accepted Answer

Option B: Capacitor start

Explanation & Concepts:
The capacitor-start induction motor provides the highest starting torque among single-phase induction motors because it utilizes a large starting capacitor in series with the auxiliary winding. This creates a large phase angle (approaching 90°) between the starting winding current and the main winding current, generating a high rotating magnetic field torque.

Working Principle: When the motor is energized, the capacitor shifts the auxiliary winding current leading the main winding current by a significant phase angle. The resulting torque is given by $T = k \cdot I_m \cdot I_a \cdot \sin(\alpha)$, where $\alpha$ is the phase difference between main and auxiliary currents. Since $\alpha$ is high (close to 90°), the starting torque is maximized. A centrifugal switch disconnects the capacitor once the motor reaches 70-80% of its rated speed.

Key Formulas: $T_s = k \cdot I_m \cdot I_a \cdot \sin(\alpha)$ — Starting torque depends on the current in main ($I_m$) and auxiliary ($I_a$) windings and their phase difference $\alpha$.

Key Points: Capacitor-start motors can produce starting torque up to 300-400% of full-load torque.; The capacitor used is typically an electrolytic type, designed for intermittent service only.; The auxiliary winding is disconnected by a centrifugal switch after startup to prevent overheating.

Advantages: Very high starting torque; Suitable for high-inertia loads like compressors and conveyors

Disadvantages: Presence of centrifugal switch reduces reliability; Higher maintenance compared to permanent capacitor motors

Applications: Refrigerators and air-conditioning compressors; Industrial pumps and heavy-duty blowers

Additional Info: Resistance split-phase motors have a lower phase displacement, typically around 30°, leading to lower torque.. Shaded pole motors provide the lowest starting torque, generally limited to small fan applications.. Capacitor-run motors offer higher running efficiency but lower starting torque than capacitor-start motors.

Answer: B is correct — Capacitor-start motors provide the highest starting torque by achieving a large phase displacement between the auxiliary and main winding currents.

Exam Tip: For high-torque requirements in single-phase systems, always look for capacitor-start or capacitor-start-capacitor-run configurations.

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Option A: Resistance split phase

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Option C: Capacitor run

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Option D: Shaded pole

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