Three phase induction machine can work as an induction generator and induction motor. The construction of induction motor and induction generator also same. But for most of the application, its performance as an induction generator is unsatisfactory. In this article we will study about construction of a three phase induction motor.
The induction motor has following parts
Stator is stationary part of the induction motor.
The function of the frame is to support the inner parts of the motor such as core and winding.
The frame of the motor may be casted or fabricated. For small and medium size machine the frame is made of cast iron or casted. For large machine fabricated steel is used.
The stator frame provided only mechanical support to the inner parts and are not designed to carry the stator flux.
The slots may be open, semi closed or closed.
The stator core is a stack of cylindrical steel laminations (which are usually 0.03 cm to 0.05 cm thick) which are slotted their inner periphery. The three phase winding is housed in these slot. The laminations are used to reduce the eddy current losses.
The stator core carries a three phase winding. The six terminals of the winding are connected in the terminal box of the machine.
In general, the same material is used for the rotor and the stator, but on e rotor has thicker laminations because the lower frequencies of rotor flux.
The winding may be star connected or delta connected. The winding’s are designed to be delta connected for normal running.
It is a rotating part of the motor. There are two types of rotors, which are employed in three phase induction motors.
Squirrel Cage Rotor
The motors employing in this type of rotors are known as squirrel cage induction motors. Most of induction motors are employed squirrel cage rotor. It is simple, cheap rugged construction of rotor. It consists of a laminated core with conductors lying. In each slot copper brass, aluminum bar used as rotor conductors. The rotor winding is permanently short circuited and it is not possible to add any external resistance in the rotor circuit.
Phase Wound Rotor or Slip Ring Rotor
Phase wound rotor is also called slip ring rotor. Phase wound rotor carries a three phase wound winding wound for same number of poles as the stator winding. The three finish terminals are connected together forming star point and the three star terminals are connected to three copper slip rings mounted on the shaft. The brush is used to collect the current from the slip rings. These brushes are externally connected to a three phase star connected resistance for the purpose of starting and speed control of motor.