Both single-phase and three-phase motors are used in residential and light commercial system compressors. Permanent split capacitor (P SC ) and capacitor start-capacitor run (C SR ) motors are commonly used in single-phase hermetic compressors because of their good running characteristics and high efficiency.
As shown in Figures SP-9-7 and SP-9-8, both the PSC and CSR motors have at least three external terminals leading to two internal windings. The main or run winding (R) contains relatively few turns of heavy wire. The start winding (S) contains a greater number o f turns o f lighter wire. The point where the two windings meet internally is called common (C). As shown, the arrangement o f the motor windings used in both the PSC and C SR motors is the same.
The designation of a motor as a PSC or CSR motor is determined by the run and/or start circuit components used with the motor. As shown in Figure SP-9-7, the PSC motor has a run capacitor (RC) permanently connected across the run and start windings. Since the run capacitor remains in the circuit during operation, it helps improve the efficiency of the motor. The PSC motor will sometimes use an extra capacitor to aid in starting. When this is done, the motor is designated as a C SR motor. As shown in Figure SP-9-8, the C SR motor has a start capacitor (SC ) and the contacts of a start relay (SR ) connected in parallel with the run capacitor. When the motor is turned on and reaches about 75% of full speed, these contacts open and remove the start capacitor from the circuit. The operation of the start relay circuit is discussed in detail later in this section.
Three-phase motors are generally used when high starting torque is needed or when the motor requirements are greater than 7 HP. All have at least three internal windings, with each winding having an equal resistance and the same number of wire turns. Three-phase motors have good starting and running characteristics and high efficiency. As shown in Figure SP-9-9, three-phase motors require no external starting relays or capacitors.
