Construction of 3-phase Induction Motor

CONSTRUCTION OF 3‒PHASE INDUCTION MOTOR

A 3‒phase induction motor consists of two main parts:

1. Stator

2. Rotor

Stator

The stator is the stationary part. It is built up of high‒grade alloy steel laminations to reduce eddy‒current losses. The laminations are slotted on periphery and are insulated and from each other. These laminations are supported in a stator frame of cast iron or fabricated steel plates. The insulated conductors are placed in these slots. These stator conductors are connected to form a three‒phase star or delta connected winding and is fed from a 3‒phase supply. It is wound for a definite number of poles, the exact number of poles being determined by the requirement of speed. Greater the number of poles, lesser the speed and vice versa. When 3‒phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating magnetic field induces current in the rotor by electro‒magnetic induction. [Ref Fig. 5.1]

Rotor

The rotor is also build up of thin laminations of the same material as stator. The laminated core is mounted directly on the shaft. These laminations are slotted on their outer periphery to receive the rotor conductors. There are two different types of induction motor rotors.

1. Squirrel cage rotor - Motors using this type of rotor are known as squirrel cage induction motor.

2. Phase wound or wound rotor - Motors using this type of rotor are known as slip ring motor (or) phase wound motor.

1. Squirrel Cage Rotor

The rotor consists of a cylindrical laminated core with parallel slots or skewed type of slots. One copper or aluminium bar is placed in each slot. At each end of the rotor, the rotor bar conductors are short‒circuited by heavy end‒rings of the same material, thus resemling a squirrel cage and hence the name. A squirrel cage rotor is shown in Fig. 5.2.

The skewing of cage rotor conductors offers the following advantages:

(i) More uniform torque is produced and the noise is reduced during operation.

(ii) The locking tendency of rotor is reduced. During locking, the rotor and stator teeth attract each other due to magnetic action.

2. Phase‒wound rotor

It consists of a slotted armature Insulated conductors are put in the slots and connected to form a three‒phase double layer distributed winding similar to the stator winding. The rotor winding is uniformly distributed in the slots and is usually star‒connected. The open ends of the stator circuit are brought out side the rotor and connected to three insulated slip ring arrangement. The slip rings are mounted on the shaft with brushes resting on them. The brushes are connected to three variable resistors connected in star. At starting, the external resistances are included in the rotor circuit to give a large starting torque. These resistances are gradually reduced to zero as the motor runs up to rated speed.

The external resistances are used during starting period only. When the motor attains normal speed, the three brushes are short‒circuited so that the wound rotor runs like a squirrel cage rotor. A slip ring motor is shown in Fig. 5.4.