A 3-phase induction motor has two main parts −
The rotor and stator are separated by a small air gap ranges from 0.5 mm to 4 mm depending on the power rating of the motor.
The stator is the stationary part of the motor. It consists of a steel frame which encloses a hollow cylindrical core. The core of the three phase induction motor is made up of thin laminations of silicon steel to reduce the eddy current and hysteresis losses.
A number of equally spaced slots are provided on the inner periphery of the laminated core as shown in the figure. The insulated conductors are placed in these stator slots and are connected in a suitable manner to form a balanced 3-phase star or delta connected stator winding.
The 3-phase stator windings are wound for a definite number of poles depending upon the requirement of speed, i.e., greater the number of poles, lesser is the speed of the motor and vice-versa.
When a balanced 3-phase supply is fed to the stator winding a rotating magnetic field (RMF) of constant magnitude is produced and this RMF induces currents in the rotor circuit by electromagnetic induction.
The rotor of an induction motor is a hollow cylindrical laminated core, having slots on its outer periphery. The rotor windings are placed in these rotor slots.
Depending upon the winding arrangement, the rotor of a 3-phase induction motor is of two types −
The squirrel cage rotor consists of a cylindrical laminated core having slots on its outer periphery which are nearly parallel to the shaft axis or skewed. An uninsulated copper or aluminium bar (rotor conductor) 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 (see the figure). This forms a permanently short circuited winding which is indestructible. This entire arrangement resembles a cage which was once commonly used for keeping squirrels and hence the name.
This rotor is not connected electrically to the supply but has currents induced in it by the electromagnetic induction from the stator.
Those 3-phase induction motors which employed squirrel cage rotor are known as squirrel cage induction motors. Most of the 3-phase induction motors in the industries use squirrel cage rotor because it has simple and robust construction enabling it to operate in the most adverse environment. Although, it suffers from a disadvantage of low starting torque.
The skewing of squirrel cage rotor conductors offers following advantages −
The slip ring rotor consists of a laminated cylindrical armature core. The slots are provided on the outer periphery and insulated conductors are put in the slots. The rotor conductors are connected to form a 3-phase double layer distributed winding similar to the stator winding. The rotor windings are connected in star fashion (see the figure).
The open ends of the star circuit are taken outside the rotor and connected to three insulated slip rings. The slip rings are mounted on the rotor shaft with brushes resting on them. The brushes are connected to three variable resistors which are also connected in star. Here, the slip rings and brushes are used to provide a mean for connecting external resistors in the rotor circuit. The equivalent circuit of the wound rotor is shown in the figure below.
The external resistors enable the variation of each rotor phase resistance to serve following two purposes −