Explain the features, characteristics and application of two different types of AC motor

AC motors operate with two rotating magnetic fields on the rotor and stator respectively. Pulling or pushing the poles of the two magnetic fields along, the speed of the stator rotating magnetic field and the speed of the rotor rotating magnetic field, which is relative to the speed of the mechanical shaft, must maintain synchronism for average torque production by satisfying the synchronous speed relation. Otherwise, asynchronously rotating magnetic fields would produce pulsating or non-average torque.

The two main types of AC motors are classified as induction and synchronous.

The induction motor always relies on a small difference in speed between the stator rotating magnetic field and the rotor shaft speed called slip to induce rotor current in the rotor AC winding. As a result, the induction motor cannot produce torque about synchronous speed where induction is irrelevant or ceases to exist.

In contrast, the synchronous motor does not rely on slip-induction for operation and uses either permanent magnets, salient poles, or an independently excited rotor winding. The synchronous motor produces its rated torque at exactly synchronous speed. The brushless wound rotor doubly fed synchronous motor system has an independently excited rotor winding that does not rely on the principles of slip induction of current. The brushless wound rotor doubly fed motor is a synchronous motor that can function exactly at the supply frequency or sub to super multiple of the supply frequency.

Single-phase induction motor

Three-phase motors produce a rotating magnetic field. However, when only single-phase power is available, the rotating magnetic field must be produced using other means.

A common single-phase motor is the shaded-pole motor and is used in devices requiring low starting torque, such as electric fans or the drain pump of washing machines and dishwashers or in other small household appliances. In this motor, small single-turn copper create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil. This causes a time lag in the flux passing through the shading coil, so that the maximum field intensity moves across the pole face on each cycle. This produces a low level rotating magnetic field which is large enough to turn both the rotor and its attached load. As the rotor picks up speed the torque builds up to its full level as the principal magnetic field is rotating relative to the rotating rotor. Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as air conditioners and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque.

 A capacitor start motor is a split-phase induction motor with a starting capacitor inserted in series with the startup winding, creating an LC circuit which produces a greater phase shift and so, a much greater starting torque than both split-phase and shaded pole motors. The capacitor naturally adds expense to such motors.

                           

Poly-phase synchronous motors

If connections to the rotor coils of a three-phase motor are taken out on slip-rings and fed a separate field current to create a continuous magnetic field, the result is called a synchronous motor because the rotor will rotate synchronously with the rotating magnetic field produced by the poly-phase electrical supply. Another synchronous motor system is the brushless wound-rotor doubly-fed synchronous motor system with an independently excited rotor multiphase AC winding set that may experience slip-induction beyond synchronous speeds but like all synchronous motors, does not rely on slip-induction for torque production.

Nowadays, synchronous motors are frequently driven by transistorized variable-frequency drives. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor, aside from stabilizing the motor speed on load changes.

One use for this type of motor is its use in a power factor correction scheme. They are referred to as synchronous condensers. This exploits a feature of the machine where it consumes power at a leading power factor when its rotor is over excited. It thus appears to the supply to be a capacitor, and could thus be used to correct the lagging power factor that is usually presented to the electric supply by inductive loads. The excitation is adjusted until a near unity power factor is obtained. Machines used for this purpose are easily identified as they have no shaft extensions. Synchronous motors are valued in any case because their power factor is much better than that of induction motors, making them preferred for very high power applications.