The Principle of Operation of a 3 –Phase Induction Motor

            CONSTRUCTION

         An Induction motor has basically two parts – Stator and Rotor

 Stator:

•    The part of an AC induction motor's magnetic structure, which does not rotate.

•     It usually contains the primary winding. The stator is made up of  laminations  with a large hole in the center in which the rotor can turn; there are slots in the stator in which the windings for the coils are inserted.

•    The Stator is made up of a number of stampings with slots to carry three phase windings. It is wound for definite number of poles.

•     The windings are geometrically spaced 120 degrees apart.

 Rotor:

•    Two types of rotors are used in Induction motors.

•    A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots.

•    These bars are short-circuited at both ends by means of short-circuiting rings.













•    The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew.
    
•    It helps to make the motor run quietly by reducing the magnetic hum.

•    It helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic                   attraction between













¾ WOUND ROTOR

•    A wound rotor has three-phase, double-layer, distributed winding. It is wound for as many poles as the stator.

•     It is always wound 3-phase even when the stator is wound two- phase. The three phases are starred internally. The other three winding terminals are brought out and connected to three insulated slip rings mounted on the shaft with brushes resting on them.

•    These three brushes are further externally connected to a 3-phase star connected rheostat

•    This makes possible the introduction of additional resistance in the rotor circuit during the Starting period for increasing the starting torque of the motor.















•    When running under normal conditions, the slip rings are automatically short-circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together.

•    The brushes are automatically lifted from the slip rings to reduce the frictional losses and the wear and tear.

•    Hence, it is seen that under normal running conditions, the wound rotor is short-circuited on itself just like the squirrel-case rotor.















1. Frame:
  
Made of close-grained alloy cast iron.

2. Stator and Rotor Core:
  
Built from high-quality low-loss silicon steel laminations and flash enameled on both sides.

3. Stator and Rotor Windings:
  
Have moisture proof tropical insulation embodying mica and high quality varnishes. Are carefully spaced foremost effective air circulation and are rigidly braced to withstand centrifugal forces and any short-circuit stresses.

4. Air-gap:
  
The stator rabbets and bore are machined carefully to ensure uniformity of air - gap.

5. Shafts and Bearings:
  
 Ball and roller bearings are used to suit heavy duty, trouble-free running and for enhanced service life.

6. Fans:

Light Aluminium fans are used for adequate circulation of cooling air and are securely keyed onto the rotor shaft.

7. Slip rings and Slip-ring Enclosures

•    Slip rings are made of high quality phosphor bronze and are of molded construction.

•    The three phases are connected internally and the other ends are connected to slip rings mounted on shaft with brushes resting on them.

•    The brushes are connected to an external resistance that does not rotate with the rotor and can be varied to change the N-T characteristics.



         In fact an Induction motor can be compared with a transformer because of the fact that just like a transformer it is a singly energized device which involves changing flux linkages with respect to a primary (stator) winding and Secondary (rotor) winding.






 









                                                                        

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