SPLIT-PHASE MOTORS
Split-phase motors also referred to as induction-star induction-run (ISIR) motors, have a relatively low starting torque compared with the other single-phase motors but more torque than the shaded-pole motor. They range in size from 1/20 horsepower to abort 1/3 horsepower. Split-phase motors get their name from the fact that a single power supply is split between two individual windings — the run and the start — to produce the necessary torque to start the Motor.
A split phase induction motor is provided with the main winding and auxiliary or start winding placed in space quadrature and are connected in parallel to a single-phase supply. The main winding has a low resistance and high reactance whereas the auxiliary winding has a high resistance and low reactance. The auxiliary winding is effective during the starting of the motor and is disconnected from the supply when the motor attains 75 percent of its synchronous speed. A centrifugal switch, S is used to disconnect the auxiliary winding from the source as shown in Fig. Under the normal running condition, only the main winding is effective.
The run winding is energized whenever the motor is energized. This winding has a lower resistance than the start winding, which is only in the circuit long enough to help the motor start. For the motor to start, both the start and run windings must be energized. When both windings are energized, the current flows through each of the windings at a different rate, creating a phase shift.
The phase shift is measured in electrical degrees and is often referred to as the phase angle. The larger the phase angle, the more starting torque a motor has. For a point of future reference, a split-phase motor, as just described, has a phase angle of about 30° degrees. If the run and start windings were constructed and configured exactly the same, with the same size wire and the same number of turns, the phase angle would be zero, the magnetic field would have no imbalance, and the motor would not start.
Once the split-phase motor has started, the start winding must be removed from the electric circuit. The start winding is designed to be energized for only a short time and can become damaged if it is not de-energized. One commonly used device to remove the start winding from the circuit is the centrifugal switch (Fig.), which opens and closes its contacts depending on the speed of the motor. The electrical contacts on the switch are connected in series with the start winding and are normally closed.
When the voltage is initially applied to the motor, both the start and run windings are energized and the motor begins to turn. Once the motor has reached a speed equal to about 70 percent of its rated speed, the contacts oi the centrifugal switch open, de-energizing the start winding. The run winding, or main winding, is now the only winding energized, and the motor continues to run in this fashion since the turning rotor now creates the imbalance needed to keep the motor running. When the motor is de-energized, it begins to slow down and the centrifugal switch closes in preparation for the next startup. Other components can be added to the split-phase motor to increase its torque, as well as its range of applications such as the capacitor, potential relay etc.
EXAMIANS