Introduction to Induction Motor

Induction motors are the most widely used electric motors in industrial applications. Almost all induction motors run at essentially constant speed from no-load to full-load conditions. The speed of induction motors depends on the supply frequency and hence these motors are not easily adapted to speed control. Induction motors are simple and rugged in construction, less expensive, easy to maintain, and can be designed and produced with characteristics to suit most industrial requirements.

What is an Induction Motor?

An induction motor is an asynchronous AC electric motor which converts alternating current electricity into the mechanical energy. It is called an induction motor because the electric current in the rotor circuit required to produce the deriving torque is obtained through electromagnetic induction from the rotating magnetic field of the stator winding.

These motors are referred to as asynchronous motors because their rotor rotates at a speed less than the synchronous speed of the rotating magnetic field.

The induction motor is an electromechanical energy conversion device, i.e. it converts electrical energy into mechanical energy in the form of rotation of shaft. Like any electric motor, an induction motor has two main parts namely stator and rotor.

• The stator carries a set of windings called stator winding. The stator winding may be single-phase winding or three-phase winding.

• The rotor carries a short-circuited winding called rotor winding.

In case an induction motor, only the stator winding is fed from an AC supply, while the rotor winding derives its voltage and power from the stator winding through electromagnetic induction.

Working Principle of Induction Motor

The working of an induction motor is based on the principle of electromagnetic induction. In an induction, there are two windings namely, stator winding and rotor winding. The input AC supply is connected to the stator winding, the current flowing in the stator winding produces a magnetic flux. This magnetic flux is usually rotating, hence also called rotating magnetic field. The rotor winding of the induction motor is a short-circuit winding.

The rotating magnetic flux from the stator cuts the short-circuited conductors of the rotor winding. According to Faradays law of electromagnetic induction, an EMF is induced in the rotor circuit which causes a current to flow through it. When the current flows through the rotor winding, another magnetic flux being produced in the machine.

Therefore, there are two magnetic fluxes inside the induction motor, one is stator flux and the other is rotor flux. These two magnetic fluxes interact with each other. Because of that, the rotor will experience a torque which makes the rotor to rotate in the direction of the rotating magnetic field of the stator. In this way, an induction motor runs.

Types of Induction Motors

Depending on the type of input supply, induction motors are classified into the following two types −

• Single-Phase Induction Motors− An induction motor that works on single-phase AC supply is called as a single-phase induction motor.

• Three-Phase Induction Motors− An induction motor which requires three-phase AC supply to operate is called a three-phase induction motor.

Advantages of Induction Motors

The following are some major advantages of induction motors −

• Induction motors have simple and rugged construction.

• Induction motors are relatively less expensive.

• Induction motors have relatively high efficiency.

• Induction motors can be designed to have characteristics to meet the industrial requirements.

• Induction motors need little maintenance.

Disadvantages of Induction Motors

The main disadvantages of induction motors are as follows −

• The speed of induction motors cannot be changed easily because they are essentially constant speed motors.

• Induction motors draw a high inrush current at starting.

• Induction motors always operate at lagging power factor.

• Single-phase induction motors are not self-starting; hence we need provide some extra starting mechanism.