NCCR Electrical Practice Test

Reducing the capacitance in a capacitor-start motor results in what effect on starting torque?

Increases Decreases No change Reverses

Correct Answer:

Decreases

Explanation:

Starting torque in a capacitor-start motor comes from the phase-shifted current in the start winding created by the capacitor. The capacitor in series with the start winding provides a phase-shifted current that, together with the main winding current, creates a rotating magnetic field and a torque that helps start the motor. When you reduce the capacitance, the capacitive reactance increases, so less current flows through the start winding and the phase difference between the two winding Currents diminishes. With a smaller current and a weaker phase-shifted component, the rotating field is weaker, producing less starting torque. If the capacitance is reduced enough, the start winding may not contribute enough torque for the motor to start at all.

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