The main body of the Induction Motor comprises of two major parts:

Induction Motor

Stator

The stator is made up of a number of stampings in which different slots are cut to receive 3 phase winding circuit which is connected to 3 phase AC supply.

3 Phase Induction Motor

The three-phase windings are arranged in such a manner in the slots, that they produce a rotating magnetic field after AC supply is given to them.

Usually, windings are kept at different pitch circle with 30 % overlap to each other.

motor overhauling

The windings are wound for a definite number of poles depending upon the speed requirement, as speed is inversely proportional to the number of poles, given by the formula:

N_s= 120f/p

Where N_s= synchronous speed

f = Frequency

p = no. of poles

Rotor

The rotor consists of a cylindrical laminated core with parallel slots that carry conductor bars.

Conductors are heavy copper or aluminium bars which fit in each slot. These conductors are brazed to the short-circuiting end rings.

Rotor of 3-phase Induction motor

The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed for the following reasons:

They reduce magnetic hum or noise
They avoid stalling of the motor

Induction Motor

Principle and Working Of 3 Phase Induction Motor

When 3 phase supply is given to the motor, the resulting current generates a magnetic flux “Ø”.

Due to the switching sequence of 3 phase current in R, Y, and B, the generated flux rotates around the rotor conductor.

According to Faraday’s law, which states that –“an emf induced in any closed circuit is due to the rate of change of magnetic flux through the circuit”, Emf is induced in the Copper bar and due to this, current flows in the rotor.

The direction of the rotor can be given by Lenz law which states that – “the direction of induced current will be in the opposite of the motion causing it.”

Here the relative velocity between the rotating flux and static rotor conductor is the cause of current generation; hence the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity, thus rotating the rotor of the induction motor.

Advantages of Induction Motor

The motor construction and the way electric power is supplied give the induction motor several benefits such as:

– They are robust and simple in construction with very few moving parts

– They can efficiently operate in a rugged and harsh environment such as in seagoing vessels

– The maintenance cost of 3 phase induction motor is less and unlike that of DC or synchro motor, they do not have parts like brushes, commuters or slip rings etc.

– An induction motor can operate in an intrinsic environment as they do not have brushes which can cause the spark and can be dangerous to such an atmosphere