Nikola Tesla conceived the basic principals of the polyphase induction motor in 1883, and had a half horsepower (400 watt) model by 1888. Tesla sold the manufacturing rights to George Westinghouse for $65,000.
Most large ( > 1 hp or 1 kW) industrial motors are poly-phase induction motors.
By poly-phase, we mean that the stator contains multiple distinct
windings per motor pole, driven by corresponding time shifted sine
waves. In practice, this is two or three phases. Large industrial motors
are 3-phase. While we include numerous illustrations of two-phase
motors for simplicity, we must emphasize that nearly all poly-phase
motors are three-phase. By induction motor, we mean that the
stator windings induce a current flow in the rotor conductors, like a
transformer, unlike a brushed DC commutator motor.
you can also visit this video for more explanation click here
How does an Induction Motor Work ?
Induction motors are the most commonly used electrical machines. They
are cheaper, more rugged and easier to maintain compared to other
alternatives. In this video we will learn the working of a 3 phase squirrel-cage induction motor.
Parts of an Induction Motor
An induction motor has 2 main parts; the Stator and Rotor. The Stator is
the stationary part and the rotor is the rotating part. The Rotor sits
inside the Stator. There will be a small gap between rotor and stator,
known as air-gap. The value of the radial air-gap may vary from 0.5 to 2
mm.
Construction details of a Stator
A Stator is made by stacking thin-slotted highly permeable steel lamination inside a steel or cast iron frame. The way the steel lamination are arranged inside the frame is shown in the following
figure. Here only few of the steel lamination are shown. Winding passes
through slots of the stator.
Effect of 3 Phase Current Passing Through a Stator Winding
When a 3 phase AC current passes through the winding something very interesting happens. It produces a rotating magnetic field (RMF).
As shown in the figure below a magnetic field is produced which is
rotating in nature. RMF is an important concept in electrical machines.
We will see how this is produced in the next section.
The Concept of a Rotating Magnetic Field
To understand the phenomenon of a rotating magnetic field, it is much
better to consider a simplified 3 phase winding with just 3 coils. A
wire carrying current produces a magnetic field around it. Now for this
special arrangement, the magnetic field produced by 3 phase A.C current
will be as shown at a particular instant.
The components of A.C current will vary with time. Two more instances
are shown in the following figure, where due to the variation in the A.C
current, the magnetic field also varies. It is clear that the magnetic
field just takes a different orientation, but its magnitude remains the
same. From these 3 positions it’s clear that it is like a magnetic field
of uniform strength rotating. The speed of rotation of the magnetic
field is known as synchronous speed.
The Effect of RMF on a Closed Conductor
Assume you are putting a closed conductor inside such a rotating
magnetic field. Since the magnetic field is fluctuating an E.M.F will
be induced in the loop according to Faraday’s law. The E.M.F will
produce a current through the loop. So the situation has become as if a
current carrying loop is situated in a magnetic field. This will
produce a magnetic force in the loop according to Lorentz law, So the loop will start to rotate. The Working of an Induction Motor
A similar phenomenon also happens inside an induction motor. Here
instead of a simple loop, something very similar to a squirrel cage is
used. A squirrel cage has got bars which are shorted by end rings.
A 3 phase AC current passing through a Stator winding produces a
rotating magnetic field. So as in the previous case, current will be
induced in the bars of the squirrel cage and it will start to rotate.
You can note variation of the induced current in squirrel cage bars.
This is due to the rate of change of magnetic flux in one squirrel bar
pair which is different from another, due to its different orientation.
This variation of current in the bar will change over time.
That's why the name induction motor is used, electricity is
induced in 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 layers make sure that eddy current losses are
at a minimum. You can note one big advantage of 3 phase induction
motors, as it is inherently self starting.
You can also note that the bars of a squirrel cage are inclined to
the axis of rotation, or it has got a skew. This is to prevent torque
fluctuation. If the bars were straight there would have been a small
time gap for the torque in the rotor bar pair to get transferred to the
next pair. This will cause torque fluctuation and vibration in the
rotor. By providing a skew in the rotor bars, before the torque in one
bar pair dies out, the next pair comes into action. Thus it avoids
torque fluctuation. The Speed of Rotation of a Rotor & the Concept of Slip
You can notice here that the both the magnetic field and rotor are rotating. But at what speed will the rotor rotate?.To obtain an answer for this let's consider different cases.Consider a case where the rotor speed is same as the magnetic field speed. The rotor experiences a magnetic field in a relative reference frame. Since both the magnetic field and the rotor are rotating at same speed, relative to the rotor, the magnetic field is stationary. The rotor will experience a 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 slow down.
But as it slows down, the rotor loops will experience a 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 in synchronous and rotor speed is known as slip.
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