Induction Motor: Working Principle, Types, & Definition

What is an Induction Motor?

An induction motor (also known as an asynchronous motor) is an AC electric motor. The electric current in the rotor that produces torque is obtained via electromagnetic induction from the rotating magnetic field of the stator winding. The rotor of an induction motor can be a squirrel cage rotor or wound type rotor.

They are called ‘asynchronous motors’ because, in motoring, they run below the speed of the stator field. That field speed is the synchronous speed.

Synchronous Speed

Synchronous speed is the speed of the magnetic field in a rotary machine. It depends on supply frequency and the number of poles. On a fixed-frequency supply, an induction motor in the motoring mode runs below that speed.

The stator field induces flux and current in the rotor, and the two fields produce torque. That torque exists only while there is slip. If the rotor reached field speed, rotor frequency would be zero and induced rotor voltage would vanish, so motoring torque would collapse.

Induction motors are grouped by supply: single phase and three phase. Single phase induction motors with only the main winding produce no starting torque. three phase induction motors produce a rotating field from the three line currents and are self-starting.

Working Principle of Induction Motor

A wound-field DC motor needs two circuits: a field (or stator) supply and an armature supply through brushes. An induction motor has one AC supply, to the stator only.

Stator current produces magnetic flux that rotates (or, on one phase, pulsates). Cage bars or wound-rotor coils are closed on themselves, so each rotor path is short-circuited.

That stator flux cuts the short-circuited rotor paths. By Faraday’s law of electromagnetic induction, voltage and then current appear in the rotor. Rotor current produces its own flux.

Stator flux and rotor flux then exist together. Rotor current (and its flux) lag the inducing stator field, so a torque appears that pulls the rotor along the rotating field. The same induction process applies to single-phase and three-phase machines once a rotating component of field exists.

Types of Induction Motors

The usual split is by supply (single phase or three phase) and, for three-phase machines, by rotor construction.

Single Phase Induction Motor

The types of single phase induction motors include:

1. Split Phase Induction Motor
2. Capacitor Start Induction Motor
3. Capacitor Start and Capacitor Run Induction Motor
4. Shaded Pole Induction Motor

Three Phase Induction Motor

The types of three phase induction motors include:

1. Squirrel Cage Induction Motor
2. Slip Ring Induction Motor

A single-phase main winding alone is not self-starting. A three-phase motor is. what is a self-starting motor?

A motor is ‘self-starting’ when it develops starting torque from the supply alone, with no hand spin. A ceiling fan that starts when you close the switch is using that kind of arrangement.

A household fan motor is a single-phase induction machine. The main winding alone would not start. An auxiliary winding and capacitor supply the start torque. That arrangement is explained below.

Why is Three Phase Induction Motor Self Starting?

In a three phase system, the three line currents are 120° apart. Those currents in displaced stator windings produce a rotating magnetic field of nearly constant amplitude, and that field starts the rotor.

Label the phases a, b and c. Their mmf axes are 120° apart in space and 120° apart in time, so the resultant field rotates instead of standing still. The rotor is dragged with that rotation.

Working Principle of a Three Phase Induction Motor – Video

Why Single Phase Induction Motor is not Self Starting?

A single-phase stator winding fed from a sine-wave supply produces a pulsating field, not a rotating one. That is why a main winding alone has no starting torque, and why an extra start method is added.

A pulsating field can be treated as two equal fields rotating in opposite directions. At standstill their torques cancel, so the rotor stays put. A hand spin makes one direction win and the motor then runs. The usual electrical fix is a second, auxiliary winding besides the main winding.

A capacitor in series with the auxiliary winding is the common method. The capacitor shifts the auxiliary current relative to the main current. That time shift, plus the space shift of the two windings, produces a rotating field component and starting torque.

Disconnect the capacitor and a typical fan motor stays still. A hand spin will still get it running. The capacitor is what produces the start torque.

Those advantages of an induction motor explain the wide applications of an induction motor. Large high-efficiency industrial machines can reach the mid-to-high 90s (about 97% on some high-kW ratings). A small appliance motor is much less efficient. On a fixed-frequency supply, speed falls a little as load rises (slip). A converter changes the field speed if a wide range is required.

A three-phase induction motor reverses if you swap the phase sequence of the supply, for example RYB to RBY. A single-phase capacitor motor reverses if you reverse the auxiliary winding relative to the main, which is often done at the capacitor leads.