We have already studied the basics of an induction machine. In which we studied about the basic principle of working of an induction machine, types of induction machine induction; motor, induction generator and many other things. There we studied that an induction motor can be either single phase or three phase as per nature of supply. In this article we will discuss some basic concepts of 3 phase induction motor like construction of three phase induction motor, operating principle of three phase induction motor, slip in three phase induction motor, effect of slip on rotor parameters and some other basic concepts.

3 Phase Induction Motor

In the induction motor we studied that they work on the working principle of Faraday's law of electromagnetic induction. There we saw that an induction motor can be either a single-phase induction motor or 3 phase induction motor. The three-phase induction motor is employed for the large power applications while single phase induction motors are employed for the low power applications.

3-Phase Induction Motor Construction

Like all electrical motors, the three-phase induction motor consists of two main parts: the stator (which is the stationary part) and rotor (which is the rotating part) which are placed at a least possible air gap. The three phase AC supply is given to the stator winding and rotor is set to be free to rotate to drive the load.

Induction Motor Stator

The stator of a 3-phase induction motor consists of a stator frame, stator core, 3-phase winding as shown in the given figure.

The stator frame is made of cast iron material and it provides the mechanical strength to the induction motor.

The stator core is made of thin laminated silicon steel conductors. There are n number of steel laminated conductors stacked together to form the stator core.

On the stator core stator windings are mounted for these windings, slots are provided on the stator core. Generally, there are three types of slots: Open slots, Closed Slots and Semi closed slots. To get details about these types of slots we have to study the three-phase induction motor construction in detail.

There are three sets of windings, each connected to a separate phase of the AC power supply. These windings are mounted on the stator core for the p number of poles.

These stator windings are distributed in nature and should have 120⁰ space displacement so that these windings produce a sinusoidal rotating magnetic field. As the given supply is a three-phase balanced AC supply which has 120⁰ time displacement.

Induction Motor Rotor

Rotor is the rotatory part of the three-phase induction motor which drives the load. The rotor is placed in the stator with the least possible air gap. Air gap should be as minimum as possible in order to reduce the magnetizing current requirement and power factor improvement. The rotor of an induction motor consists of a shaft and the rotor core. The rotor shaft is a cylindrical bar which on end is coupled to the mechanical load. And the rotor core is the stack of thin laminated steel conductors in which slots are provided for the rotor winding or bars.

Generally two types of rotor construction used in Induction Motor these are

Squirrel Cage Rotor.

Slip Ring Rotor/ Wound Rotor.

Squirrel Cage Rotor

In squirrel cage rotor, rotor winding consists of uninsulated conductors in the form of uninsulated copper or alluminium bars which are placed in semi-closed slots of rotor core. These solid bars are short circuited at both ends by the end rings. These conductor bars are skewed with respect to shaft axis this exercise reduce the crawling effect of motor.

Slip Ring Rotor/ Wound Rotor

In slip ring induction motor rotor contains three phase uniformly distributed winding which is usually connected in star. This three phase winding are connected to slip ring or collector ring which are mounted on shaft. In wound rotor induction motor external resistance are connected in series with rotor winding for high starting torque.

3 Phase Induction Motor working principle

Like all induction machines an induction motor works on the principle of electromagnetic induction. When a 3-phase supply is given to 3-phase winding which is mounted on the stator then it establishes a 3-phase flux in the air gap called stator Rotating magnetic field (SRMF). This stator RMF is rotated at the synchronous speed Ns.

Ns = 120 F / P

F = Frequency of AC Supply

P = Number of poles on stator winding

Ns = Synchronous speed

As the rotor is short circuited by the end rings or slip rings, three phase current and voltage is induced in the rotor winding. The Induced EMF in the rotor produced its own magnetic field in the air gap called rotor rotating magnetic field (RRMF). This Rotor RMF is also rotated at synchronous speed Ns with respect to the stator and is stationary with respect to stator RMF. So, due to the interaction of these two stationary fields a torque is produced which tends to the rotor rotate at the speed N. The direction of the rotation is analyzed according to Lenz’s law. In order to oppose the cause of induced EMF and current, the rotor will rotate in the direction of RMF. Actually, the rotor wants to catch synchronous speed but it slip back by slip speed.

N = Ns (1-s)

N = speed at which rotor rotates

s = Slip

An induction motor never runs at synchronous speed so it is called the asynchronous motor. A well-designed induction motor runs near to synchronous speed on no load. The principle of operation of induction motor i.e Faraday’ Law of Electromagnetic induction is quite similar to transformer so it is also called as rotating transformer with short circuited secondary.

Specifications of three phase induction motor

The three-phase induction motor is employed for the large power application while single phase induction motors are employed for the low power applications.

The three-phase induction motor has good speed regulation.

Three phase induction motors are the self-starting Motors. All though they have drawn a large amount of magnetizing current at starting. So, they required the starting methods of the induction motor to control the high starting current.

Several methods of speed control of three phase induction motor are present to control the speed over a wide range; this property makes it suitable for various industrial applications.