they are cheaper rugged

and easier to maintain compared to other alternatives

in this video

we will learn the working of a three-phase squirrel cage induction

motor

it has two main parts

stator

and rotor

stator is a stationary part

and rotor is the rotating part

stator is made by stacking thin slotted

highly permeable steel lamination inside a steel or cast-iron frame

winding passes through slots of stator

when a three-phase AC current passes through it

something very interesting happens

it produces a rotating magnetic field

to understand this phenomenon much better

consider a simplified three-phase winding

with just three coils

A wire carrying current produces magnetic field around it

Now, for this special arrangement magnetic field produced by a three phase

A.C current will be as shown

at a particular instant

with variation in a c current

magnetic field takes a different orientation as shown

From these three positions it's clear that,

it's like a magnetic field of uniform strength rotating

The speed of rotation of a magnetic field is known as synchronous speed

Assume you're putting a closed conductor inside it

since the magnetic field is fluctuating, an E.M.F will be induced in the loop

according to the Faraday's law

The E.M.F will produce a current through the loop

So, the situation has become like a current carrying loop is situated in the

magnetic field

this will produce magnetic force in loop

according to the Lawrence law

so the loop will start to rotate

A similar phenomenon happens inside an induction motor also

Here instead of a simple loop something very similar to a squirrel cage is used

Three-phase A.C current passing through stator winding produces a

rotating magnetic field

so

as in the previous case current will be induced in bars of squirrel cage which

is shorted by the end rings. And will start to start rotate

That's why it's called an induction motor electricity is inducted in the

rotor by magnetic induction rather than direct electric connection

To aid such electromagnetic induction

insulated iron core lamina are packed inside the rotor

such small slices of iron make sure that eddy current losses are minimal

This is another big advantage of a three-phase induction motor

it is inherently self starting

You can see here that both magnetic field and the rotor are rotating

but at what speed the rotor rotate ?

to obtain this answer, let's consider different cases

consider a case where the rotor speed is the same as the magnetic field speed

since both the rotating at the same speed the rotating loop or always

experience constant magnetic field

so there won't be any induced E.M.F and current

this means zero force on the rotor bars

so the rotor will gradually slowdown

but as a slows down rotor loops will experience of varying magnetic field

so induced current and force will rise again

and the rotor will speed up

in short the rotor will never be able to catch up with the speed of the magnetic

field

it rotates at a specific speed which is slightly less than synchronous speed

The difference between synchronous and rotor speed is known as slip

rotational mechanical powers transferred through a powershaft

in short in an induction motor electrical energy is entered via stator

and output from motor, mechanical rotation is received from rotor

energy lost during motor operation is dissipated as heat

a fan at the other end helps in cooling down the motor

hope you've got a good introduction on the working of induction motors