There are four general types of single-phase motors. Each has distinctly different characteristics. Compressor motors are designed for specific requirements regarding starting torque and running efficiency. These are two of the reasons why different types of motors are required to meet various demands.
The resistance-start induction-run (RSIR) motor is used on many small hermetic compressors through 1/3 hp. The motor has low starting torque. It must be applied to completely self-equalizing capillary tube systems such as household refrigerators, freezers, small water coolers, and dehumidifiers.
This motor has a high-resistance-start winding that is not designed to remain in the circuit after the motor has come up to speed. A current relay is necessary to disconnect the start winding as the motor comes up to design speed (see Fig. 9-36).
The capacitor-start induction-run (CSIR) motor is similar to the RSIR. However, a start capacitor is included in series with the start winding to produce a higher starting torque. This motor is commonly used on commercial refrigeration systems with a rating through 3/4 hp (see Fig. 9-37).
Capacitor-start and -run
The capacitor-start and -run (CSR) motor arrangement uses a start capacitor and a run capacitor in parallel with each other and in series with the motor start winding. This motor has high starting torque and runs efficiently. It is used on many refrigeration and air-conditioning applications through 5 hp. A potential relay removes the start capacitor from the circuit after the motor is up to speed. Potential relays must be accurately matched to the compressor (see Fig. 9-38). Efficient operation depends on this.
Permanent split capacitor
The permanent split capacitor (PSC) has a run capacitor in series with the start winding. Both run capacitor and start winding remain in the circuit during start and after the motor is up to speed. Motor torque is sufficient for capillary and other self-equalizing systems. No start capacitor or relay is necessary. The PSC motor is basically an air-conditioning compressor motor. It is very common through 3 hp. It is also available in 4 and 5 hp sizes (see Fig. 9-39).