Three phase induction motors
Three – Phase induction motor is the most common and popular type of a.c motor used for industrial drives. This motor is used because it is cheap, robust, efficient and reliable. It has good speed regulation and high torque. It requires little maintenance and has a reasonable overload capacity.
Construction
A three-phase induction motor has mainly two parts:
- Stator.
- Rotor.
The part of the motor that is fixed means stationary is called the stator.
The rotating part of the motor is called rotor.
Fig: Induction motor stator with double-layer winding partly wound
The Stator of the motor is built up of high-grade alloy steel laminations, and all the laminations are insulated from each other which are placed on the inner periphery of the system. These laminations are supported on the frame of the cast iron. The conductors of the stator are connected to form three-phase winding either in Star or delta connection.
Types of induction motors:
The induction motors are divided into two parts-
- Squirrel-cage rotor or simply cage rotor
- Phase wound or wound rotor. The motors which use this type of rotor are also known as slip-ring motors.
The principle of operation of a Three-phase induction motor
Consider a conductor on the stationary rotor is subjected to a magnetic field that is produced when three phase supply is connected to the three-phase winding of the stator. If the rotation of the magnetic field is clockwise, then it will have the same effect as the conductor moving anticlockwise in the stationary field. By faradays law of electromagnetic induction, a voltage will be induced in the conductor.
(a)
(d)
Fig: Production of Torque
Speed and Slip:
The difference between the synchronous speed and actual speed of the rotor is called the Slip Speed.
Therefore we can say that the ‘Slip Speed’ shows the speed of the rotor relative to the field.
Perunit (p.u)
Where,
   Ns = synchronous speed in r.p.m    Nr = actual rotor speed in r.p.m Slip Speed = Ns ?N r.p.m