THREE PHASE INDUCTION AC MOTOR
Three Phase Induction AC Motor
A motor which is supplied with three
phase AC and rotates on induction principle is called three phase induction
motor. Three phase induction motor basically has two parts. A stator in which
three phase AC supply is given and the other rotor in which thick copper bars
are placed and their two ends are tightly sorted with the help of rings.
Torque
generation process of three phase AC motor: - when three phase AC is supplied
externally to the stator of a three-phase induction motor, current flows
through the coil. As a result, a rotating magnetic field is created in the
stator of the motor. This rotating magnetic field rotates at synchronous speed
(Ns= 120f/p). The rotating magnetic field
creates magnetic flux, which crosses the air-gap and cuts the rotor conductors.
As a result, voltage is generated in the rotor according to Farad’s law. Now
that the rotor is shorted, current also flows in the rotor conductors. As a result,
a magnetic field is also created in the rotor. The interaction between the
stator and rotor magnetic fields generates torque in the rotor and turns the
motor. This is how a three phase induction motor works.
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Three
phase AC motor starting Method: -
There
are several methods for starting a three-phase induction motor among which the
star- delta starter method is widely used. Continued these types of starters
are generally used for motors with a capacity of 15hp and above.
The
star- delta starter system is of two types: - 1) Star- delta manual
system
2)
Star- delta automatic system
Star- Delta Manual System
Star- delta automatic method is discussed below for starting three phase induction motor: - On switching on, the motor starts in star connection first and after the specified time setting in the timer, the motor automatically gets connected in delta at full speed -
Three phase AC motor speed control method:-
1) The amount of supply voltage increases or
decreases. 2) Changes the frequency value. 3) Changes
the number of polls.
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Advantages of three phase AC motor: -
1) overload capacity is of high quality
2) Highly skilled
3) Can generate starting torque on its own
4) DC
excitation is not required from outside.
Disadvantages: -
1)
Maintenance costs are high
2) Large in size
3) The price is high
4) The speed of the motor cannot be easily controlled .
1) Three phase AC motors are used in various applications in large scale industries. 2) Three phase AC motor is used in rice crushing mill. 3) Three phase AC motors are also used in many other things including power conveyors, lathe machines, grinding and pump machines.
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