3 PHASE INDUCTION MOTOR WORKING PRINCIPLE AND CONSTRUCTION

PRINCIPLE OF WORKING OF 3 PHASE INDUCTION MOTOR

• WHEN THE 3 PHASE STATOR WINDINGS ARE FED BY A 3 PHASE SUPPLY, A MAGNETIC FLUX OF CONSTANT MAGNITUDE, BUT ROTATING AT SYNCHRONOUS SPEED IS SET UP.
• THE FLUX PASSES THROUGH THE AIR GAP AND CUTS THE STATIONARY ROTOR CONDUCTOR
• DUE TO RELATIVE SPEED BETWEEN THE ROTATING FLUX AND THE STATIONARY CONDUCTOR, AN EMF IS INDUCED ACCORDING TO THE FARADAY LAW OF ELECTROMAGNETIC INDUCTION
• AS THE ROTOR CONDUCTORS ARE SHORT CIRCUITED BY END RINGS, CURRENT FLOWING IN THE ROTOR CONDUCTORS AND TORQUE PRODUCED BY THE ROTOR DEPENDS UPON THE RELATIVE SPEED BETWEEN ROTOR AND ROTATING MAGNETIC FIELD PRODUCED BY THE STATOR
• HENCE TO REDUCE THE RELATIVE SPEED, THE ROTOR STARTS RUNNING IN THE SAME DIRECTION AS THAT OF ROTTING FLUX

THREE PHASE INDUCTION MOTOR CONSTRUCTION

THE AC INDUCTION MOTOR COMPRISES TWO ELECTROMAGNETIC PARTS

1. STATIONARY PARTS CALLED THE STATOR
2. ROTATING PART CALLED THE ROTOR

STATOR PART IS THE OUTER STATIONARY PART OF THE MOTOR WHICH CONSISTS OF:

• THE OUTER CYLINDRICAL FRAME OF THE MOTOR,WHICH IS MADE OF EITHER OF WELDED SHEET STEEL, CAST IRON OR CAST ALLUMINIUM ALLOY.
• THE MAGNETIC PATH COMPRISES A SET OF SLOTTED STEEL LAMINATIONS PRESSED INTO THE CYLINDRICAL SPACE INSIDE THE OUTER FRAME. THE MAGNETIC PATH IS LAMINATED TO REDUCE EDDY CURRENTS, LOWER LOSSES AND LOWER HEATING
• A SET OF INSULATED ELECTRICAL WINDINGS WHICH ARE PLACED INSIDE THE SLOTS OF THE LAMINATED MAGNETIC PATH

ROTOR :

ROTOR IS A ROTATING PART OF INDUCTION MOTOR. THE ROTOR IS CONNECTED TO THE MECHANICAL LOAD THROUGH THR SHAFT. THE ROTOR OF THREE PHASE INDUCTION MOTOR ARE FURTHER CLASSIFIED AS :

• SQUIRREL CAGE ROTOR
• SLIP RING ROTOR OR WOUND ROTOR OR PHASE WOUND ROTOR.

• SQUIRREL CAGE ROTOR : MOST OF INDUCTION MOTORS (UP TO 90%) ARE OF SQUIRREL CAGE. SQUIRREL CAGE TYPE ROTOR HAS VERY SIMPLE CONSTRUCTION

          THIS TYPE OF ROTOR CONSISTS OF A CYLINDRICAL LAMINATED CORE HAVING             PARALLEL SLOTS ON IT. THESE PARALLEL SLOTS CARRY ROTOR CONDUCTORS.

    

          IN THIS TYPE OF ROTOR, HEAVY BARS OF COPPER, ALUMINIUM OR ALLOYS ARE           USED AS ROTOR CONDUCTORS INSTEAD OF WIRES 

      

         THE ROTOR BARS ARE SHORT CIRCUITED BY END RINGS AT BOTH ENDS, HENCE            IT IS NOT POSSIBLE AT BOTH ENDS. HENCE IT IS NOT POSSIBLE TO ADD ANY                  EXTERNAL RESISTANCE TO ARMATURE CIRCUIT.

    

ADVANTAGES OF SQUIRREL CAGE INDUCTION MOTOR

• THE CONSTRUCTION IS VERY SIMPLE AND RUGGED
• AS THERE ARE NO BRUSHES AND SLIP RING, THESE MOTOR REQUIRE LESS MAINTENANCE

APPLICATIONS

• LATHES
• DRILLING MACHINES
• FAN
• BLOWER
• PRINTING MACHINES

•  SLIP RING OR PHASE WOUND ROTOR :

PHASE WOUND ROTOR IS WOUND WITH 3 PHASE DOUBLE LAYERED WINDING

THE THREE PHASE WINDING IS INTERNALLY STAR CONNECTED.

THE OTHER THREE TERMINALS OF THE WINDING ARE TAKEN OUT VIA 3 INSULATED SLIP RINGS MOUNTED ON THE SHAFT AND THE BRUSHES RESTING ON THEM

THESE THREE BRUSHES ARE CONNECTED TO AN EXTERNAL STAR CONNECTED RHEOSTAT . THIS ARRANGEMENT IS DONE TO INTRODUCE AN EXTERNAL RESISTANCE IN THE ROTOR CIRCUIT.

ADVANTAGES OF SLIP RING INDUCTION MOTOR :

• IT HAS HIGH STARTING TORQUE AND LOW STARTING CURRENT.
• POSSIBILITY OF ADDING ADDITIONAL RESISTANCE TO CONTROL THE SPEED.

APPLICATIONS :

SLIP  RING INDUCTION MOTOR ARE USED WHERE HIGH STARTING TORQUE IS REQUIRED

• HOISTS
• CRANES
• ELEVATORS

DIFFERENCE BETWEEN SLIP RING AND SQUIRREL CAGE INDUCTION MOTOR