Three phase induction motor is the most popular type of AC motor. It is very commonly used for industrial drives because it is cheap,robust,efficient and reliable. It has good speed regulation and high starting torque and it doesn’t require regular maintenance it has a reasonable capacity.
Three phase induction motor mainly consist two parts :-
The starter and the rotor. The starter is the stationary part and the rotor is a rotating part. The starter is made by high-grade alloy steel laminations to reduce eddy current losses. The laminations are slotted on the inner periphery and insulated from each other. This laminations are supported in a starter frame cast iron or fabricated steel plate. The insulated stator conductors are placed in these slots. The stator conductors are connected to from a 3-phase winding. The phase winding may-be star or delta connected.
The rotor is also made by of thin laminations of the same material as stator. The laminated cylindrical core is mounted directly on the shaft or a spider carried by the shaft.This laminations are slotted on their outer periphery to receive the rotor conductors. There are two types of induction motor rotors:
(a) squirrel cage rotor aur simple rotor
(b) Phase wound or wound. Motors using this type of rotor are also called slip ring motors.
It consists of a cylindrical laminated core with slots nearly parallel to the the shaft axis, or skewed. Each slots contains an uninsulated bar conductor of aluminium or copper. At each end of the rotor, the rotor bar conductors are short-circuited by heavy rnd rings of the same material. The conductors and the end rings from a cage of the tyre which was once commonly used for keeping squirrels; hence its name. A cage rotor.
- More uniform torque is produced and the noise is reduced during operation.
- The locking tendency of the rotor is reduced. During locking, the rotor and stator teeth attract each other due to magnetic action.
WOUND ROTOR OR SLIP RING ROTOR
The rotor 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 are connected in star. The open ends of the star circuit are brought outside the rotor and connected to three insulated slip rings. The slip rings are mounted on the shaft with brushes resting on them. The brushes are connected to three variable resistors connected in star. The purpose of slip rings and brushes is to provide a means for connecting external resistors in the rotor circuit. The resistors enable the variation of each rotor phase resistance to serve two purposes :
(a) to increase the starting torque and decrease the starting current from the supply.
(b) to control the speed of the motor.
COMPARISION BETWEEN CAGE ROTOR AND WOUND ROTOR
(a) Robust construction and cheaper .
(b) absence of brushes reduce the risk sparking.
(c) Lesser maintenance.
(d) Higher efficiency and higher power factor.
(a) High starting torque and low starting current.
(b) Additional resistance can be connected in the rotor circuit to control speed.