Another method of splitting the single-phase supply into two phases to be applied to the stator windings is placing a capacitor in series with the starting auxiliary winding. In this manner, the current in the starting winding may be made to lead the line voltage. Since the running winding current lags the line voltage, the phase displacement between the two currents be made to approximately 90° on starting. The circuit of capacitor-start motor is shown in Fig. 4 (a), while the vector diagram of the currents and voltage is shown in Fig. 4 (b). The values of the angles shown are
fairly representative, and are rounded off for convenience. One of the factors upon which the starting torque depends is the sine of the angle between the currents in the two windings. The value of series capacitor may therefore be reduced, while maintaining a phase-shift angle of about 90°.
- The increase in phase angle between starting and running winding currents is not the only difference between the split-phase and capacitor-start motors. The split phase motor must keep the number of starting-winding turns low, so that the current may be nearly ill phase with the line voltage. This, however, is unnecessary in a capacitor-start motor, since the capacitor can overcome the inductance of the winding while still providing the propel
phase shift. There are thus more auxiliary starting turns in the capacitor-start motor than in the comparable split-phase motor. This provides a, greater number of ampere-turns, hence a larger rotating flux, and therefore a further increase in the starting torque.
Also it is seen that for the same magnitudes of field currents, the current Ilr is less ill
capacitor-start motor, because of the greater angle between the two field currents. In addition, the starting power factor is also better. For a given line current, the starting torque is thus much higher for a capacitor-start motor than for a split-phase induction motor. The starting torque of capacitor-start motor is from 3 to 4.5 times the full-load torque, while that of split-phase resistance-start induction motor rarely exceeds twice the full-load torque.
- These motors are manufactured in ratings ranging from 1/10 kW to 3/4 kW, but larger sizes are also available.
- The capacitor-start motor may be reversed by changing the connections of one of the windings, but it is subject to the same limitations as the resistance-start induction motor.
Uses. By virtue of their higher starting torque, capacitor-start split-phase motors are used for pumps, compressors, refrigeration units, air-conditioners, and large washing machines, where a split-phase motor is required that will develop high starting torque under load.