Introduction
Three phase induction motor is the most popular type a.c. motor. it is very commonly used for industrial drives. It cheap , robust, efficient and reliable.It has good speed regulation and high starting torque. It requires little maintenance. It has a reasonable over load capacity.

Construction 
A three Phase induction motor essentially consists of two part the Stator and the Rotor. Stator is the stationary part and rotor is the rotating part.

Stator : 
  • It is built up of high-grade alloy steel lamination to reduce eddy current losses.the lamination slotted on the inner periphery and are insulated from each other.
  • These Laminations are supported in a stator frame of cast iron or fabrication steel plate.the insulated conductor placed in these slots.
  • the stator conductors are connected to form a three phase winding.the winding may be either star or delta.

Rotor: The rotor is also built up of thin laminations of the same material as stator. the laminated cylindrical core is mounted directly on the shaft or spinder carried by the shaft.these laminations are slotted on their outer periphery to receive the rotor conductors. there are two types of induction motor rotors:
  1. Squirrel cage rotor or simply cage rotor 
  2. Phase wound or wound rotor .Motors using this type of the rotor are also called slip ring motor. 

Cage Rotor
Shape of Cage Rotor is cylindrical show in figure. Laminated core with slots nearly parallel to the shaft axis or skewed. Each slot contains uninsulated bar conductor of aluminium or copper.At each end of the rotor , the rotor bar conductors are short circuited by heavy end rings of the same material. A cage rotor is show in figure.


The skewing of cage rotor conductors offers the following advantages.
  1. More uniform torque is produce and noise is reduced during operation.
  2. The locking tendency of the rotor is reduced. During the locking , the rotor and stator teeth attract each other due to magnetic action. 
The following advantages of the cage rotor are given below.
  • The cage rotor is cheaper, and the construction is robust.
  • The absence of the brushes reduces the risk of sparking.
  • Its Maintenance is less.
  • The power factor is higher
  • The efficiency of the cage rotor is higher.

Wound Rotor or Slip Ring Rotor  
The wound rotor consists of a slotted armature. Insulated conductor 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 slip rings are mounted on the shaft with brushes resting on them. The brushes are connected to the variable resistor. The function of the slip rings and the brushes is to provide a means of connecting external resistors in the rotor circuit. The resistor enables the variation of each rotor phase resistance to serve the following purposes given below.

1. To increases the starting torque and decreases the starting current from the            supply
2. To control the speed of the motor.

The wound rotors have the following merits
1. High starting torque and low starting current.
2. Additional resistance can be connected in the rotor circuit to  control the                   speed.
   

12 Comments

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