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Permanent-split Capacitor Induction Motor

  • A permanent-split capacitor motor (also called single-value capacitor motor) is a single-phase induction motor which has the same capacitor in series with the starting (or auxiliary) winding for both starting and running. Because it runs continuously as a permanent split-phase motor, no centrifugal switch is required.
  • The motor starts and runs by virtue of the quadrature phase-splitting produced by the two identical windings. As a result, it does not possess the high running torque produced by either the resistance-start or the capacitor-start motor.

Furthermore, the capacitor used is designed for continuous duty and is of an oil-filled type. The value of the capacitor is based on its optimum running rather than its starting characteristics. The result is that this motor has a very poor starting torque, about 50 to 100 per cent of rated torque.

Fig. 5 shows the connection diagram and phase relations of a permanent split-phase motor.

  • Because of the fairly uniform rotating magnetic field created by equal windings whose currents arc displaced by almost 90°, the torque is fairly uniform and the motor does not exhibit the characteristic pulsating hum developed by most single-phase motors when loaded.

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This motor possesses the following merits:

(i)               Higher power factor at full-load

(ii)            Lower full-load line current

(iii)          Higher full-load efficiency

(iv)          Increased pull-out torque.

  • The permanent-split capacitor motor is more expensive than the equivalent split-phase or capacitor-start induction motor. This is primarily due to the fact that the auxiliary (or starting) winding is now also a running winding. It must therefore have a continuous duty rating and as such is heavier than if it were were short-time-rated.

Uses. Because of its instant response as a reversing motor, its quieter operation, and the possibility of speed control, the permanent-split capacitor motor is used for exhaust and intake fans and blowers, unit heaters and office machines.