Split-Phase Motors
Because three-phase power is not available to small business and residential locations, the air conditioning equipment for these areas is powered by single- phase electric motors. The single-phase motors used in air conditioning systems are generally one of two types. These are the split-phase and the shaded-pole induction motor.
SPLIT-PHASE MOTORS
Split-phase motors fall into three general classifications. These are:
1. The resistance-start induction-run motor,
2. The capacitor-start induction-run motor,
3. The permanent-split capacitor motor (PSC). It should be noted that although all permanent-split capacitor motors contain a run capacitor, some are equipped with a separate starting capacitor to improve starting torque. The motors with the separate starting capacitor are often referred to as capacitor- start capacitor-run motors.
Although all of these motors have different operating characteristics, they are similar in construction. Split-phase motors get their name from the manner in which they operate. These motors operate on the principle of a rotating magnetic field. A rotating magnetic field, however, cannot be produced with only one phase. Split-phase motors liter- ally split single-phase power in order to imitate a two-phase power system. A rotating magnetic field can be produced with two separate phases.
THE TWO-PHASE SYSTEM
In some parts of the world, two-phase power is produced. A two-phase system is produced by having an alternator with two sets of coils wound 90° out of phase with each other, Figure 11–1. The voltages of a two-phase system are, therefore, 90° out of phase with each other, Figure 11–2. The two out-of-phase voltages can be used to produce a rotating magnetic field. Because there have to
be two voltages or currents out of phase with each other to produce a rotating magnetic field, single- phase motors use two separate windings and create a phase difference between the currents in each of these windings. These motors literally “split” one phase and produce a second phase, hence the name split-phase motor.