GATE EE 2022 Set 1 — Question 24
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Electrical Machines → Single-Phase Induction Motors → Starting Methods
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Question
The type of single-phase induction motor, expected to have the maximum power factor during steady state running condition, is
Correct answer
(D) capacitor start, capacitor run.
Solution
The question asks to identify the type of single-phase induction motor that has the maximum power factor during steady-state running conditions.Let's analyze the power factor characteristics of each type of single-phase induction motor:
1.Split-phase (Resistance Start) Motor: This motor uses a main winding and an auxiliary (start) winding. The auxiliary winding has a higher resistance-to-reactance ratio than the main winding, creating a phase difference between the currents in the two windings. The auxiliary winding is disconnected once the motor reaches about 75% of synchronous speed. During steady-state operation, only the main winding is active, which is highly inductive, leading to a relatively low power factor.
2.Shaded Pole Motor: This is the simplest and least expensive type of single-phase induction motor. It has very low starting torque, low efficiency, and a very poor power factor due to the inherent design of the shading coils, which primarily provide starting torque but also contribute to losses and reactive power consumption.
3.Capacitor Start Motor: This motor uses a capacitor in series with the auxiliary winding to create a larger phase difference between the main and auxiliary winding currents, resulting in higher starting torque. The capacitor and auxiliary winding are disconnected once the motor reaches a certain speed. Therefore, during steady-state running, it operates similarly to a split-phase motor, with only the main winding active, leading to a low power factor.
4.Capacitor Start, Capacitor Run Motor (Two-Value Capacitor Motor): This motor uses two capacitors. A larger capacitor (starting capacitor) is used in series with the auxiliary winding for starting and is disconnected after the motor starts. A smaller capacitor (running capacitor) remains permanently in series with the auxiliary winding during both starting and running. The running capacitor is chosen to optimize the motor's performance (efficiency and power factor) during steady-state operation by making the main and auxiliary winding currents more balanced and closer to a 90-degree phase shift. This continuous presence of the capacitor significantly improves the power factor and efficiency during steady-state running compared to other single-phase induction motors.
Therefore, the capacitor start, capacitor run motor is expected to have the maximum power factor during steady-state running conditions.The final answer is Continue learning with Success Tracker
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