induction motors are the most commonly used electrical machines
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