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The invention of Induction Motors permanently altered the course of human civilization
This hundred-Year-old motor invented by the great scientist Nikola Tesla is the most common motor type even today
In fact about 50% of global electric power consumption is due to induction motors
Let's get into the workings of induction motors or more specifically into Nikola Tesla's genius thinking
The induction Motor has two main parts the stator and rotor
The stator is basically a three coil winding and three-phase AC power input is given to it
The winding passes through the slots of the stator
which are made by stacking thin highly permeable steel laminations
Inside a steel or cast iron frame
When a three-phase current passes through this winding something very interesting happens
It produces a rotating magnetic field
This RMF is what causes the rotor to turn
To understand how the rotating magnetic field is generated
as well as its properties
let's consider a simplified stator winding
Here the three coils are connected 120º apart
A wire carrying current produces a magnetic field around it
When a three-phase power is applied to this special arrangement
the magnetic field produced will be as shown at a particular instant
With variations in AC current the magnetic field takes different orientations
If you compare these three instances you can see that it is like a magnetic field of uniform strength rotating
The rotational speed of the magnetic field is known as the synchronous speed
Assume that you are putting a closed conductor inside it
According to Faraday's law because the Magnetic field is varying
an EMF will be induced in the loop
The EMF will produce a current in the loop
thus, the situation has become like a current carrying loop situated in the magnetic field
according to the Lorentz Force law an electromagnetic force will be produced on the loop
and the loop will start to rotate
The same phenomenon occurs inside an induction motor as well
Here instead of a simple loop something very similar to a squirrel cage is used
The three-phase AC current passing through the stator produces a rotating magnetic field
So as in the previous case current will be induced in the bars of the squirrel cage
Which is shorted by end rings, so the rotor will start to rotate
That's why the motor is called an induction motor
Electricity is induced on the rotor with help of electromagnetic induction rather than direct connection
To aid such electromagnetic induction
insulated iron core lamina are packed inside the rotor
Such small sizes of iron make sure the Eddy current losses are minimum
You can see that the induction motor has a big advantage. It is inherently self-starting
As you can see both the magnetic field and rotor are rotating
But at what speed will the rotor rotate?
To obtain the answer to this question let's consider different cases
Consider a case where the rotor speed is the same as that of the magnetic field
Due to the fact that both are rotating at the same speed
The magnetic field will never cut the loop
Thus there will not be any induced EMF and current
This translates to zero force on the rotor bar and the rotor will gradually slow down
As it slows down the magnetic field will cut the rotor loop
So the induced current and force will rise again
The rotor will then speed up
In short the rotor will never be able to catch up to the speed of the magnetic field
It rotates at a specific speed which is slightly less than the synchronous speed
The difference between the synchronous and rotor speeds is known as slip
Now let's understand why induction motors rule both the industrial and domestic worlds
You can note that induction motors do not require a permanent magnet
They do not even have brushes commutator rings or position sensor
like other electrical machine counterparts
Induction motors are also self started
The most important advantage is that induction motor speed
can be controlled easily by controlling the input power frequency
To understand it properly let's once again consider the simple coil arrangement
We learned that a rotating magnetic field is produced due to the three-phase input power
It is quite clear that the speed of the RMF is proportional to the frequency of the input power
Because the rotor always tries to catch up with the RMF
the rotor speed is also proportional to frequency of the AC power
Thus by using a variable frequency drive one can control the speed of the induction motor very easily
This property of the induction motor makes them an attractive choice
for elevators, cranes even in electric cars
Due to the high-speed band of induction motors electric cars are capable to run with a single speed transmission
another interesting property of the induction motor is that
when the rotor is moved by a prime mover it can also act like a generator
In this case you have to make sure that the RMF speed is always less than the rotor speed
We believe that you have now developed a clear understanding
of the ingenious operation principles behind an induction motor
as well as why it is still ruling the domestic and industrial worlds
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How does an Induction Motor work ?

101 Folder Collection
q32348479 published on September 19, 2019
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