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.
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 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.
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 .
Uses:-
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.