AC Motor Connections
A three-phase induction motor is a constant-speed machine. The speed control of induction motors can be achieved by:
• Changing the applied voltage
• Changing the applied frequency
• Changing the number of poles.
The first two methods, however, are rarely used because of the problems associated with reducing the voltage and frequency. The last method is well suited for squirrel-cage motors, as the squirrel-cage rotor adapts itself to any reasonable number of stator poles. The change in the number of stator poles is achieved by providing two or more entirely independent stator windings. Each winding gives a different number of poles and hence a different speed.
Each of the windings is terminated on a different set of terminals, which can be connected up and switched, to connect the winding to the supply. Only one winding is used at a time, the other, being entirely disconnected. This method finds application in elevators, traction, and small motor-driven machine tools.
A dual-voltage motor is a single-phase induction motor. It can be operated from two AC voltages, either 110 or 220 V. Such motors, have two main windings and one starting winding. A suitable number of leads are brought out to permit changeover from one voltage to another.
When the motor is to operate on a lower voltage, the two main windings are connected in parallel. On higher voltage, they are connected in series. The starting winding is always operated on the low voltage mode, for which purpose it is connected across one of the main windings.