Induction Machine

Induction machines commonly known as “asynchronous machines” are the important class of rotating electric machines which works on the principle of Faraday’s Law of electromagnetic induction. The name induction derives from their working principle.

Induction Machines are AC fed electrical machines. The AC supply is given to the stator only, unlike the synchronous machine which requires AC supply for stator and DC supply for rotor for their operation. As AC supply is given to stator only, induction machines are also called singly excited machines.

Like other rotating electrical machines, the operation of induction machines is reversible; they act as a motor as well as a generator. Induction machines as a motor are the most popularly used in industry because of their compatibility of universal supply (as universal supply is AC in nature), simple construction, less maintenance and good speed regulation.

Construction of Induction Machine

Like all electrical machines, the induction machine consists of two main parts: the stator (which is the stationary part) and rotor (which is the rotatory part). These two parts are placed with an optimum air gap, as shown in the picture.

The stator consists of stator winding through which AC supply is given to the induction machine that helps to establish a magnetic field in the air gap.

Rotor of the induction machine is placed in the cavity of the stator and is set to be free to rotate. There are two types of rotors used in induction machines: Squirrel Cage Rotor and Slip Ring or Wound Rotor.

The construction of an induction machine is common for both induction motors and induction generators. We already discussed the construction of the induction motor in detail. The concept discussed there is applicable as it is here in this article.

Working Principle of induction Machines

As the name implies, an induction machine works on the principle of Faraday's law of electromagnetic induction, same as the working principle of a transformer. According to this law when a supply is given to stator winding the stator winding produces a rotating magnetic field (flux) which rotates at synchronous speed.

This flux links with the rotor's conductor and induces EMF in it. Simultaneously current induced in rotor conductor because rotor conductors are short circuited.

This induced current in the rotor conductors produces its own magnetic flux. This rotor’s flux interacts with flux produced by stator windings. Due to the interaction of these two fluxes torque is produced in the rotor. And the rotor wants to catch the synchronous speed but it slips back by a small speed called slip speed and continues to run at a speed near to the synchronous speed.

The slip in induction machines is an important parameter in the further study of an induction machine.

As an Induction machine never runs at synchronous speed that’s why they are also called asynchronous machines.

Types of Induction Machine

On the basis of electromechanical energy conversion there are two type of induction machines

Induction Motor
Induction Generator

An induction machine can be operated as either an induction motor or an induction generator. There are different criteria or conditions for the induction machine operation as an induction motor or an induction generator. We have to keep these conditions in mind during the operation of the induction machine.

Induction Motor

Induction motor is the type of induction machine which converts electrical energy and mechanical energy. Working principle of the induction motor is the same as that of induction machines. Induction motors are the most widely used motor in industry almost 80% of industrial motor use today are induction motors. Induction motor is further classified in two types according to the nature of supply i.e. single-phase induction motor and three-phase induction motor.

Single-Phase Induction Motor

As the name implies single phase induction motor, these motors are fed by single phase AC supply, and generally used for low voltage, low power applications. These motors are also known as fractional kilowatt motors. Generally single-phase induction motors are used in several domestic and household applications like fans, washing-machine, kitchen appliances, small tools etc. as these applications are supplied with single phase ac due to low power requirements.

Types of Single-Phase Induction Motor

The single-phase induction motor is not a self-starting motor so there they require different starting methods to start. So, on the basis of starting techniques single phase induction motor are classified as follows:

Split phase / Resistance Split Phase Induction Motor

Capacitor Start and Run Induction Motor

Capacitor Start and Capacitor Run / Permanent Capacitor Induction Motor

Shaded Pole Induction Motor

Three-Phase Induction Motor

As the name implies a three-phase induction motor, these motors are fed by three phase AC supply, and generally used for high power applications. Generally, these types of motor are used in industrial applications to drive heavy loads.

These types of motor offer a wide range of speed control by the different speed control methods of induction motor and make it suitable for various industrial applications like cranes, conveyer belt, large exhaust fans, lathe machines, elevators etc.

These motors are self-starting motors and draw heavy current in the starting so different starting methods of induction motor are required to limit the starting current.

Induction Generator

A generator generates electrical energy by converting the mechanical energy or fuel energy into electrical energy. Similarly, an induction generator generates electrical energy and its working principle is based on Faraday's law of electromagnetic induction.

An induction machine works as an induction generator when its slip (s) < 0 (negative slip). Negative slip means the actual speed of the motor is greater than the synchronous speed. Or we can say that the induction machine runs at super synchronous speed. The super synchronous speed of the induction machine is achieved with the help of a prime mover.

Induction generators are not so popularly used because they require a reactive power source all the time to full fill their reactive power demand so that they can generate active power.

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