Three phase induction motor, principle of operation, advantages and disadvantages

         

                             Three Phase Induction Motor

Three  phase induction motor can be used for different applications it is

widely used in industries with various speed and load requirements. Three phase induction motor run as constant speed from no load to full load .Speed of three phase induction motor depends  upon frequency .

Construction of three phase induction motor :-

Three phase induction motor 85% of the installed capacity of the industrial purposes because of good operating characteristics, low maintenance, easy construction , good speed regulation and absence of commutator.

Three phase induction motor consist of two main parts--

1     1.  Rotor

       2.  Stator

Rotor -> Rotor of  three phase induction motor are two different type one is  squirrel cage rotor and other is wound rotor or slip ring rotor . wound rotor made up of aluminium or copper bars and the shaft of three phase motor supported on two bearings each ends. A slip ring or wound rotor is made up of three phase winding are  jinxed at one end and other end connected to  slip rings mounted on the rotor shaft.

Squirrel cage rotor consist of cylinder of steel lamination where copper and aluminium conductors embedded  in the surface of rotor.squirrel cage rotor is skewed to avoid cogging.

Stator:-  as the name suggested stator is the stationary part of three phase induction motor and the three phase supply deliver to it. Stator of three phase induction motor consist of stator frame, stator core, stator winding or field winding. Stator winding connected star or delta, it depends what type  of starting method is used. When winding is excited by three phase supply it produces a rotating magnetic field. Three phase winding phases placed at 120 degree apart from each other.

A small distance between the rotor and stator is called air gap. Distance between the stator and rotor varies from 0.4 mm to 4 mm.

Formulas :-

N_s =120f/p

N_s - Synchronous speed

f - Frequency

p - no. of poles

Synchronous speed is inversely propotional to the no. of poles

Slip :- it is the difference between the synchronous speed and operating speed

S = N_s  - N_r/N_s  

N_{s -} stator speed

N_{r -} rotor speed

Percentage slip :-

N_s – N_r/N_s  X  100

Principle of operation :-

When three phase supply provided to the stator winding, it produces a rotating magnetic field and  flux generates due to induced rotor current. According to the  Faraday’s laws of electromagnetic induction. As these rotor conductors are shorted, the current starts to flow through in these conductors. direction of the rotor can be given by Lenz law - the direction of induced current will be in the opposite of the motion, This result the rotor starts rotating in the same direction of the stator rotating magnetic field.

Advantages of three phase induction motor :-

•        Simple construction
•        Low cost
•       Easy to maintain
•        Characteristics to suit most industrial requirements
•        Rugged   

Disadvantages of three phase induction motor :-

•         At heavy load it draws high initial starting current
•         Three phase induction motor speed difficult as compare to dc machine
•      It operates lagging power factor which result to increase I²R losses