Many of the low power units used in domestic refrigeration or in small-scale air-conditioning are equipped with single-phase motors (refrigerators, freezers, stand-alone air conditioning units or small heat pumps etc.).

Single phase motors with auxiliary phases, although they are in widespread use are often much less well understood than three-phase motors. The aim of this chapter is not to study how or why these motors operate, but to learn about their electrical connection and to repair this type of equipment and the ancillary equipment needed for their operation (e.g. starting relays and capacitors). Naturally, we will concentrate on compressor motors.

6) Multi- speed Motors : 

The circuit diagram shown opposite represents a motor of this type fitted to the outflow fan of many makes of air- conditioning unit. The principle involves producing a fall in voltage at the motor terminals. This reduces the motor's torque, and therefore its speed.

 A coil (impedance) is connected in series with the PSC type motor. With the switch in position LS, the current passing through the impedance causes a fall in the voltage at the motor terminals. It then rotates at Low Speed. When the switch is in the HS position, the impedance is short-circuited, and the full supply voltage is applied to the motor. It then rotates at High Speed.

EXERCISE : A single- phase 220V motor, fitted with a running capacitor of 3mF, drives an air conditioning unit fan. A 4 terminal switch (C, LS, MS, HS) allows the selection of the fan speed (Common, Low Speed, Medium Speed, High Speed).

There are five wires emerging from the motor. White (W), Red, Black, Yellow, and Green.

 With all wires disconnected, you will find the resistance values measured between each wire in the table opposite. For example, there is 270W between the Yellow wire and the Green.

Make a sketch of the internal wiring of the motor yourself before looking at the solution.

Solution : Refrepair Manual pages 339 to 358)