Induction Motor: Working Principle, Types & Definition

What is an Induction Motor?

An induction motor, also known as an asynchronous motor, is a type of AC electric motor that is widely used. In this motor, the electric current required to produce torque in the rotor is obtained through electromagnetic induction from the rotating magnetic field of the stator winding. The rotor of the induction motors can be of two types: a squirrel cage rotor or a wound-type rotor.

Induction motors are called asynchronous motors because they operate at a speed lower than their synchronous speed. Therefore, it’s essential to know what synchronous speed is.


Synchronous Speed

Synchronous speed refers to the speed at which the magnetic field rotates in a rotary machine and depends on the machine’s frequency and number of poles. An induction motor always operates at speed lower than its synchronous speed.

The stator produces a rotating magnetic field, which induces a flux in the rotor, ultimately resulting in the rotation of the rotor. However, since there is a delay between the flux current in the rotor and the flux current in the stator, the rotor will not be able to achieve its synchronous speed. You may know the difference between the motor’s synchronous and actual speed is called the slip.

Two main types of induction motors are classified based on the input supply they receive. The two types are single-phase induction motors and three-phase induction motors. While single-phase induction motors are not self-starting, three-phase induction motors are.

Working Principle of Induction Motor

To make a DC motor rotate, we need to provide double excitation. This means we supply power to the stator and the rotor using a brush arrangement. On the other hand, an induction motor only requires a single supply. This makes it fascinating to understand how an induction motor operates.

The term “induction” in the induction process refers to inducing a current in the stator winding by electromagnetic induction when an electrical supply is given to it. This current flow in the coil generates a rotating magnetic field in the stator. The rotor winding is arranged so that each coil becomes short-circuited.

When the rotating magnetic flux from the stator cuts the short-circuited coil in the rotor, it causes an electric current to flow through the coil of the rotor. This is due to Faraday’s law of electromagnetic induction. As the current flows through the rotor coils, it generates another flux in the rotor.

There are two fluxes in an induction motor: stator flux and rotor flux. The rotor flux lags behind the stator flux, creating a torque that causes the rotor to rotate in the direction of the rotating magnetic field. This principle applies to both single and three-phase induction motors.

Types of Induction Motors

The types of induction motors can be classified as either single-phase or three-phase.

Single Phase Induction Motor

The single-phase induction motors can be classified into the following types.

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

Three-Phase Induction Motor

There are two types of three-phase induction motors. These are.

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

We previously mentioned that single-phase induction motors are not self-starting, while three-phase induction motors are. What is a self-starting motor?

When a motor starts running independently without requiring external assistance or manual intervention to the machine, it is considered ‘self-starting. For instance, the 3-phase induction motor automatically rotates when we turn on the switch. Therefore, it is an example of a self-starting machine.

It’s important to note that the fans used in home appliances are single-phase induction motors that are not self-starting. However, they are designed to start by using a starting coil and a capacitor.

Why is the Three-Phase Induction Motor Self Starting?

The three phases have a 120° phase difference in a three-phase system. Applying three phases to the stator produces a rotating magnetic field with a 120° phase difference, which causes the rotor to rotate.

When considering three phases – R, Y, and B – in a motor, the rotor will move towards the magnetized phase at any moment. For example, the rotor will move towards phase R winding if phase R is magnetized. Phase Y will become magnetized and attract the rotor in the next moment, followed by Phase B. This cycle will continue, causing the rotor to rotate.

Why is the Single Phase Induction Motor not Self-starting?

A single-phase induction motor is an interesting device, as it can rotate the rotor despite having only one phase. However, it is not a self-starting motor and requires a solution to overcome this issue. The AC supply produces a sinusoidal wave that creates a pulsating magnetic field in the stator winding, which is uniformly distributed.

We add a capacitor in series with the auxiliary winding to start the motor. The capacitor creates a phase difference when the current passes through both coils. This phase difference will produce a starting torque in the rotor, causing it to begin rotating.

When the capacitor of a single-phase induction motor is disconnected, we can observe that the fan does not rotate independently. However, if we rotate the fan with our hand, it will start rotating. This is why we use a capacitor in a single-phase AC motor.

Induction motors have several advantages, making them suitable for a wide range of applications. One of their most significant advantages is their high efficiency, reaching up to 97%. However, one major disadvantage of induction motors is that their speed varies with the load applied to them.

The direction of rotation of an induction motor can be changed by altering the phase sequence of the three-phase supply. For instance, if the supply is RYB and the motor rotates forward, changing it to RBY or YRB will cause the motor to rotate in the reverse direction. This method works for three-phase motors. In the case of single-phase motors, reversing the capacitor terminals in the winding can change the direction of rotation.


In this article, you have learned-

  • What is an induction motor?- An induction motor is an AC electric motor that generates torque by electromagnetic induction of the electric current in the rotor from the magnetic field of the stator winding.
  • Working Principle of Induction Motor: The stator generates a rotating magnetic field, which cuts the short-circuited rotor conductor, producing the flux. The stator and rotor magnetic field interaction creates torque and rotates the motor.
  • Types of Induction Motors- Single-phase and three-phase. Three-phase induction motors are of two types: squirrel cage and wound rotor.
  • Why are the 3-phase induction motors self-starting?– The motor’s stator generates a rotating magnetic field, causing the rotor to rotate. That is why it is a self-starting motor.
  • Why are single-phase induction motors not self-starting?– The motor’s stator generates a pulsating revolving field, and the motor cannot start. Another means is used to split the phase to make it self-starting.

Induction motors are known for their efficiency, ruggedness, and relatively low maintenance requirements, making them suitable for various applications in manufacturing, transportation, HVAC (heating, ventilation, air conditioning), and more.

  1. Series wound DC Generator

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