Classic AC motors: The permanent-capacitor split-phase motor

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.The permanent-capacitor, split-phase motor makes use of two identical stator windings and a capacitor selected for optimum running characteristics. Because no centrifugal switch is employed, the overall reliability of this motor exceeds that of machines dependent on such a switch. Because of electrical and magnetic symmetry, this motor provides quieter operation than the resistance-start or capacitor-start types. The rotor of the permanent-capacitor motor experiences a rotating field that is similar to the field that results from an actual two-phase power source. In contrast, the resistance-start and capacitor-start motors develop a field with a large pulsating component—this contributes to noise and vibration, but not to torque.

FIG. 10 shows the circuit diagram for the permanent-capacitor motor. The equivalent series resistance of the winding is not shown because it’s not an important factor in the basic principle of this motor. Rather, the windings are alike both in inductance and resistance. Both windings are, therefore, “running” windings. Reversal of rotation is very easily accomplished by the use of the single-pole, double-throw switch, as shown in FIG. 10. The characteristics for a typical motor of this type are shown in FIG. 11.


FIG. 10 The permanent-capacitor, split-phase motor. AC source; Reversing switch (external)


FIG. 11 Characteristics of a permanent-capacitor, split-phase motor.

An outstanding characteristic of this motor is its speed controllability. This is readily achieved by varying the applied voltage. When the applied voltage is greater than the rated voltage, the permanent-capacitor motor, like other induction machines, can neither attain synchronous speed nor exceed it. A high applied voltage only increases the starting torque and improves the speed regulation. However, when the applied voltage is lowered, a wide range of speeds below the synchronous speed is available by virtue of the high slip allowed by this motor.

The permanent-capacitor motor operates at a high power factor, and its overall reliability is enhanced by the fact that the capacitor is usually the oil-filled type, rather than the electrolytic type employed in capacitor-start motors. The trade-off for the many excellent characteristics of this motor is its relatively low starting torque. Most motors of this type have starting torques limited to 50 to 100 percent of their rated torque.

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