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.
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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