Electric Motor

The main difference between AC and DC motors is that the magnetic field generated by the stator rotates in the ac case. Three electrical phases are introduced through terminals, each phase energizing an individual field pole. When each phase reaches its maximum current, the magnetic field at that pole reaches a maximum value. As the current decreases, so does the magnetic field. Since each phase reaches its maximum at a different time within a cycle of the current, that field pole whose magnetic field is largest is constantly changing between the three poles, with the effect that the magnetic field seen by the rotor is rotating. The speed of rotation of the magnetic field, known as the synchronous speed, depends on the frequency of the power supply and the number of poles produced by the stator winding. For a standard 60 Hz supply, as used in the United States, the maximum synchronous speed is 3,600 rpm.

In the three phase induction motor, the windings on the rotor are not connected to a power supply, but are essentially short circuits. The most common type of rotor winding, the squirrel cage winding, bears a strong resemblance to the running wheel used in cages for pet gerbils. When the motor is initially switched on and the rotor is stationary, the rotor conductors experience a changing magnetic field sweeping by at the synchronous speed. From Faraday's law, this situation results in the induction of currents round the rotor windings; the magnitude of this current depends on the impedance of the rotor windings. Since the conditions for motor action are now fulfilled, that is, current carrying conductors are found in a magnetic field, the rotor experiences a torque and starts to turn. The rotor can never rotate at the synchronous speed because there would be no relative motion between the magnetic field and the rotor windings and no current could be induced. The induction motor has a high starting torque.

In squirrel cage motors, the motor speed is determined by the load it drives and by the number of poles generating a magnetic field in the stator. If some poles are switched in or out, the motor speed can be controlled by incremental amounts. In wound-rotor motors, the impedance of the rotor windings can be altered externally, which changes the current in the windings and thus affords continuous speed control.

Three-phase synchronous motors are quite different from induction motors. In the synchronous motor, the rotor uses a DC energized coil to generate a constant magnetic field. After the rotor is brought close to the synchronous speed of the motor, the north (south) pole of the rotor magnet locks to the south (north) pole of the rotating stator field and the rotor rotates at the synchronous speed. The rotor of a synchronous motor will usually include a squirrel cage winding which is used to start the motor rotation before the DC coil is energized. The squirrel cage has no effect at synchronous speeds for the reason explained above.

Single phase induction and synchronous motors, used in most domestic situations, operate on principles similar to those explained for three phase motors. However, various modifications have to be made in order to generate starting torques, since the single phase will not generate a rotating magnetic field alone. Consequently, split phase, capacitor start, or shaded pole designs are used in induction motors. Synchronous single phase motors, used for timers, clocks, tape recorders etc., rely on the reluctance or hysteresis designs.