Induction Machines: Their Types and Working Principle

Introduction to 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 is a hollow cylindrical structure on which the stator windings are mounted, through which AC supply is given to the induction machine that helps to establish a magnetic field in the air gap.

The rotor is a solid cylindrical structure placed in the cavity of the stator with an optimal air gap, allowing it to rotate freely. There are two types of Rotor used in induction machines: Squirrel Cage Rotor and Slip Ring or Wound Rotor. In the squirrel cage rotor, conductive bars are mounted whereas the wound rotor uses windings.

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 discussed previously, an induction machine works on the principle of Faraday's Law of Electromagnetic Induction, similar to the transformer. According to this law, when AC supply is given to stator winding, it produces a rotating magnetic field (flux) in the air gap, which rotates at synchronous speed. This magnetic field links with the rotor conductors or windings and induces EMF in it, and simultaneously, current is also induced. The induced current in the rotor conductors produces its own magnetic field, which interacts with the stator’s magnetic field. The interaction between these two fields generates torque in the machine, causing the rotor to rotate since it is set free to rotate.

In induction machines, the rotor never rotates at synchronous speed, therefore, these are commonly known as asynchronous machines. The rotor in induction machines always rotates at a speed slightly less than the synchronous speed. The difference between these two speeds is called slip speed, and the percentage of this speed relative to synchronous speed is called slip. Slip in induction machine is a crucial parameter in the analysis of an induction machine.

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

An induction motor is a type of induction machine that converts electrical energy into mechanical energy, commonly used to derive heavy industrial as well as light commercial loads. Induction motors are the most popularly used motors among other electric motors because of their simple construction, less maintenance and good speed regulation. As discussed previously induction machines are most famous as motors.

The working principle of the induction motor is the same as that of induction machines, discussed above. An induction motor is either single-phase or three-phase, according to the nature of AC supply. Although both types of motor work on the same working principle, their behavior differs when powered by single phase and three phase AC supply. Therefore, these two motors studied separately.

Single-Phase Induction Motor

As the name implies, these motors are fed by single phase AC supply. They are commonly used for low power commercial and domestic applications such as fans, washing machines, kitchen equipment, low power tools and other low power appliances used in homes and offices.

These motors are also known as fractional kilowatt motors and are not self-starting therefore, they require different starting methods to start. So, on the basis of starting techniques, a single-phase induction motor is 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 suggests, these motors are powered by three phase AC supply, and are commonly used for high power industrial applications such as cranes, conveyer belt, large exhaust fans, lathe machines, elevators, drillers etc. These types of motor offer a wide range of speed control and have several different methods of speed control which make these motors best fit for various industrial applications.

These motors are self-starting motors and draw heavy current in the starting, therefore, in order to minimize the starting current different starting methods are required.

Related Topics:-

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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