The permanent split-capacitor (PSC) motor uses only a run capacitor
to provide the phase shift required to start the motor. Fig. 1 shows
examples of PSC motors, and Fig. 2 shows two diagrams of the PSC
motor. In the diagrams note that the run capacitor is connected between
the run and start windings and no disconnecting switch or relay is required
to dc-energize the start winding from the applied voltage when the motor
has started. Also note that the run capacitor is oval shaped. The run
capacitor has a metal case, which allows it to dissipate extra heat that
is built up in the capacitor since it remains in the circuit at all times.
Above: Fig. 1: Examples of permanent split-capacitor (PSC) motors. Notice the
run capacitor mounted on the second motor.
Above: Fig. 2: (a) Electrical diagram of a PSC motor. (b) Electrical diagram
of a multi-speed PSC motor.
When voltage is applied to the motor, current will flow through the run winding to the common terminal. At this same time current will flow through the run capacitor to the start winding. When the current flows through the run capacitor. it will provide a phase shift that is large enough to start the motor. As the rotors speed increases, a counter EMF will he produced in the start winding that will limit current through it to less than 1 A when the motor reaches full speed. The small amount of current in the start winding when the motor is operating at full speed is small enough so that it won't cause the start winding to overheat.
When the motor shaft sees an increase in its load, it will slow down slightly. The decrease in the rotor’s rpm causes a decrease in the counter EMF. which makes a larger potential difference between it and the applied voltage. The larger potential difference causes an increase in the current in the start winding, which will cause an increase in rotor torque that increases the rotor’s rpm.
This characteristic allows the PSC motor to operate with a constant speed under varying load conditions without using any mechanical devices. The PSC motor is generally used for applications such as small hermetic compressors. blade fan loads, and other loads that require constant speed.
