Induction motors have been ruling
the industrial world for many decades.
In the induction motors used in lifting hoists,
you will see a type of rotor called a slip ring rotor
whereas, in most of the other applications,
you will see a simpler squirrel cage type of rotor.
Why are there are two different designs
of rotor construction for induction motors?
Normal induction motors or squirrel-cage type motors
produce a very low starting torque
and for some applications,
this low starting torque will cause huge problems.
It is in these circumstances that slip ring induction motors
are used as they produce high starting torque.
Let's see this in detail.
First, let's have a look at the way
a squirrel cage induction motor works.
When a three-phase AC supply is connected
to the stator winding, it produces a rotating magnetic field
in the air gap between the stator and rotor.
This RMF cuts the rotor bars.
According to Faraday's law of electromagnetic induction,
an electromotive force is induced in the bars.
Because the rotor bars are short-circuited by the end rings,
this induced EMF generates a current
to flow through the rotor bars.
According to Lorentz's law,
when a current-carrying conductor is placed
in a magnetic field, it will experience force.
You can see the force distribution on the different bars
at a particular moment in time.
These collective forces make the rotor turn.
This explanation of the way an induction motor works
won't be complete without an understanding
of the concept of inductance.
To understand what inductance is,
let's consider a simple circuit.
The circuit is a combination of a resistor
and an inductor in series which is connected
to the AC sinusoidal voltage.
Let us connect a phase angle meter to the circuit
to measure the phase difference
between the applied voltage and the current.
You can see that the current flowing through the circuit
is not in phase with the applied voltage.
This is because of the presence
of inductive reactance in the circuit.
The higher the frequency of the electricity,
the greater will be the inductive reactance
and the phase difference.
A higher resistance value reduces this phase difference.
The same thing is also happening in the rotor.
The rotor is a combination of resistance
and inductive reactance.
Due to the same phase-lag phenomenon,
if the maximum EMF is on one bar,
then the maximum current will be on another bar.
Now, here is one interesting fact about induction motors.
An induction motor produces maximum torque
when the maximum current induced on the rotor
is near to the maximum magnetic flux.
This fact is clear from the comparison of these two visuals.
Let us call it the maximum torque condition.
Throughout this video, please keep this fact in mind.
As the current induced does not meet
the maximum torque condition,
this will reduce the amount
of torque produced by the induction motor.
This phase difference will be high as the motor starts.
Let's see why.
At startup, the rotor speed is zero.
Due to this, the magnetic field will cut through the rotor
at a very high rate and the frequency of the induced EMF
will be high.
This leads to the high phase difference
which causes the very low starting torque
of a normal induction motor.
To overcome this problem,
the slip ring induction motor comes into the picture.
The working principles and stator construction
of the slip ring induction, motor is the same
as that of a squirrel cage motor.
However, the rotor construction of the slip ring motor
is quite interesting.
Instead of bars, in this motor three windings are used.
This construction of the rotor
is aimed at reducing the phase difference.
Let's see how the squirrel cage rotor does it.
For ease of understanding,
instead of the current 24 slots winding,
let's use a 12 slot winding.
Here again, the RMF induces EMF across the terminals
of the windings.
Let's join the winding ends in a star connection
and again assume that the inductive reactance is zero.
The current flow established in the winding
will be as shown.
However, in practice, the current flow
will be lagging behind the induced EMF.
Here again, the maximum torque condition is not met.
In the slip ring induction motor,
there is an option to reduce this EMF
current phase difference by use of external resistance.
The other ends of the coils are connected
to an external resistance via the slip rings.
We saw in the simple circuit that by increasing
the resistance value, we can decrease the phase difference.
As the slip ring induction motor starts,
the external resistance value is increased.
This reduces the phase difference angle
and the current induced
approaches to the maximum torque condition.
This way, slip ring induction motors
will be able to produce high torque
even as they are starting.
These graphs clearly show the higher starting torque
produced by slip ring motors in comparison
to squirrel cage motors.
Apart from the high starting torque,
it also has some other advantages
and although slip ring induction motors
have some disadvantages, they play a very important role
in elevators, cranes, hoists, and industrial uses,
such as printing presses.
Thank you.
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