Construction of Three Phase Induction Motors

CONSTRUCTION OF THREE PHASE INDUCTION MOTORS

The induction motor consists of two main parts (i) Stator (ii) Rotor

Stator

The stator is made up of a number of stampings with alternate slot and tooth. Stampings are insulated from each other. Each stamping is 0.4 to 0.5 mm thick. Number of stampings are stamped together to build the stator core. The stator core is then fitted in a casted or fabricated steel frame. The slots house the three phase winding called stator winding.

It may be connected either in star or delta. The stator winding is made for a fixed no of poles. Fig.3.85 shows the stator lamination.

Rotor

Two types of rotor are

i) Squirrel cage rotor

ii) Slip ring or wound rotor

i) Squirrel cage rotor

Fig.3.86 shows a squirrel cage rotor. This is made up of a cylindrical laminated core with slots to carry the rotor conductors.

The rotor conductors are heavy bars of copper or aluminum short circuited at both ends by end rings. Hence this rotor is called short circuited rotor. The entire rotor resistance is very small. External resistance cannot be connected in the rotor circuit such motors are extremely rugged in construction.

ii) Slip ring or wound rotor

Fig. 3.87 shows a slip ring or wound rotor. In this of rotor windings are similar to the stator winding. The rotor winding may be star of delta connected, distributed winding, wound for as many number of poles as the stator. The three phases are brought out and connected to slip rings mound on the rotor shaft. Variable external resistance can be connected in the rotor circuit with the help of blushes and slip rings arrangements. By varying the external resistance in the rotor circuit, the motor speed and torque can be controlled.