Construction of three phase Induction motor consist of 2 important parts namely Stator and rotor. Always the stator and rotor is separated with a small air gap ranging from 0.4-4 mm, which is depends on the size and capacity of the motor.
Construction of Three Phase Induction Motor
It contains a steel frame that surrounds a hollow cylindrical shaped core made of skinny laminations of silicon steel to cut back hysteresis loss and eddy current loss. A number of equally spaced slots are providing on the inner bound of the laminations as shown in the fig above. The insulation connected to arrange a balanced three phase star connected circuit or delta connected circuit. The three phase stator coil is wound for a particular no. of poles as per the necessity of speed. If the number of poles are more, then the motor speed is less and vice-versa. A rotating magnetic field with perpetual magnitude is developed once a three phase supply if supplied to the stator winding. This rotating magnetic field makes currents within the rotor by due to electro-magnetic induction.
The rotor is fixed on a shaft which a hollow laminated cylindrical is shaped core having parallel slots on the outside boundary. The winding is placed on these slots is named as rotor winding. The rotor winding can be categorized into two types they are Squirrel cage type and Wound Type rotor.
Squirrel Cage Rotor Type:
It contains a laminated cylindrical is shaped core having parallel slots on the outside boundary. Either an aluminum bar or copper bar is fixed in each slot. At one each end all these bars are joined with metal rings is termed as end ring. This arrangement makes permanently short circuited winding which is everlasting. There will not any electrical supply to the rotor but the current induced in it because of transformer action from stator.
Thus the induction motors which uses squirrel cage rotor is named as squirrel cage induction motor. Because of its advantage of simple and robust construction enable it to operate in the utmost adverse situations. On the other hand it has disadvantage of a low starting torque. This is due to permanently short circuited rotor bars. To have a high starting torque it is not possible to add resistance externally.
Wound Rotor Type
This type of rotor contains a laminated cylindrical shaped core and it carries a three phase winding like on the stator. The rotor windings are equally distributed in all the slots and it’s usually connected in star. The rotor windings open ends all are brought together and connected to three insulated slip rings fixed on the shaft of rotor with a brush resting on every slip ring. Therefore all the three brushes are connected to the three phase supply rheostat shown in the fig below. Thus to have a high starting torque external resistance is connected in the circuit of rotor. By varying the external resistances gradually to zero the motor runs to its rated speed.
At starting period the external resistances are engaged to have a high starting torque. Once the motor reaches its rated speed, the wound rotor runs similar to squirrel cage rotor by short circuiting the three brushes.