Conversion of electrical power into mechanical power takes place in the rotating part of an electric motor. In A.C. motors, rotor receives electric power by induction in exactly the same way as the secondary of a two-winding transformer receives its power from the primary. Hence such motors are known as a rotating transformer i.e. one in which primary winding is stationary but the secondary is free to rotate.
3-phase induction motor has two main parts
Stator and rotor
- The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power of the motor.
- As the name suggests , is the stationary part of the motor this part is made of silicon steel stampings (i.e. thin sheets)the stampings are slotted.
- When complete stator is assembled, the through slots are formed on inner side of the stator.
- The slots may be open type, semi- open type or closed type. In these slots, a 3-phase winding is accommodated.
- This winding may be star or delta connected. The three ends of this windings are brought out into the terminal box where 3-phase A.C. supply con be connected.
Given by the formula:
Ns= synchronous speed
f = Frequency
p = no. of poles
There are two types of induction motors depending upon the construction of the rotor :
- Squirrel cage type
- Wound rotor type
Squirrel cage type
This is the simplest and most rugged construction.The rotor consists of a cylindrical laminated core with skewed rotor slots.The rotor conductors which copper bars ,are placed in these slots and brazed to and rings .The rotor body is made of silicon steel stampings. The constructions of the rotor looks a cage of “squirrel” and hence called as “squirrel cage type motor”.
Wound rotor type
In this type of induction motor. The rotor is wound for the same number of poles as that on the stator.The rotor is made up of laminations with slots on the outer periphery in which a 3-phase rotor winding is placed .The three phases are connected in star connection internally.The remaining three terminals are brought out connected to the slip-rings mounted on the shaft.
Wound rotor and Squirrel cage