A three-phase induction motor consists of two main parts namely,

• Stator

• Rotor

There is a small air gap between the stator and rotor which ranges from 0.4 mm to 4 mm depending on the power rating of the motor.

Stator

The stator of a three-phase induction motor is a stationary part, and it consists of a cylindrical-shaped frame made up of fabricated steel. This steel frame encloses a hollow cylindrical core made up of thin laminations of silicon steel. On the inner periphery of the core, a number of evenly spaced slots are provided to place the stator winding. The silicon-steel laminations are used to reduce the hysteresis and eddy current losses.

Three windings are placed in the stator slots and are suitably connected to form a balanced three-phase delta or star connected circuit. As per the requirement of motor speed, these three-phase windings are wound for a definite number of poles. Where, greater is the number of poles, lesser is the speed of the induction motor and vice-versa.

When we fed the three-phase stator winding from a balanced three-phase supply, a rotating magnetic field of constant magnitude is produced. This rotating magnetic induces EMF in the rotor circuit by electromagnetic induction.

Rotor

The rotor is a rotating or moving part of the three-phase induction motor. It consists of a rotor core made up of thin laminations of high grade silicon steel to reduce the hysteresis and eddy-current losses. The rotor core is a hollow cylinder, mounted on a shaft. On outer periphery of the rotor core, slots are provided to place the rotor winding.

Based on the construction, the rotor of a three-phase induction motor can be of the following two types −

• Squirrel-cage rotor

• Wound rotor

Let”s discuss these two types of rotors in detail.

Squirrel Cage Rotor

The squirrel-cage rotor consists of a laminated cylindrical core having parallel slots on its outer periphery. In case of squirrel-cage rotor, the rotor winding is made up of metal (copper or aluminium) bars. These metal bars are placed in the rotor slots and are short-circuited at each end by metal rings called end-rings as shown in Figure-2.

From Figure-2, it can be observed that the construction of this rotor resembles a squirrel cage and hence the name. Here, it is also to be noted that the rotor is not connected electrically to the supply, but it derives its voltage and power by the electromagnetic induction from the stator.

The three-phase induction motors that employ squirrel-cage rotor are known as squirrel-cage induction motors. Almost 70% to 80% three-induction motors used in industrial applications are squirrel-cage induction motors because of their simple and robust construction which enable them to operate in most adverse circumstances. Although, the induction motors that use squirrel-cage rotor have a low starting torque.

Wound Rotor

The wound rotor consists of a laminated cylindrical core made up of silicon steel. It carries a 3-phase rotor winding similar to the stator winding as shown in Figure-3.

The rotor winding of the wound rotor is uniformly distributed in the slots and is connected in star fashion. The open ends of the star-connected rotor winding are brought out and connected to three slip rings mounted in the rotor shaft. A carbon brush is resting on each slip ring, and through these brushes, external resistances can be added to the rotor circuit.

At starting, suitable values of external resistances are added into phases of the rotor winding to obtain a high starting torque. These external resistances are gradually removed from the circuit as the motor runs up to speed. The use of external resistances considerably reduces the starting current and increases the starting torque of the motor. Once the motor attains normal speed, the three carbon brushes are short-circuited so that the wound motor runs like a squirrel cage induction motor.