Three-Phase Wound-Rotor Induction Motor
3:59 AM
In AC motors, the rotor receives electrical power by Induction exactly the same way as the secondary of a two winding transformer receives its power from the primary. That's why such motors are known as Induction motors.
In fact, an induction motor can be treated as a rotating transformer i.e. one in which primary winding is stationary but secondary is free to rotate. Of all AC motors the poly-phase Induction motor is the one which is extensively used for various kinds of industrial drives.
Types (with reference to rotor construction and operation):
1. Squirrel cage induction motor: The rotor consists of a cylinder laminated core with parallel slots carrying the rotor conductors, which are not wires but consist of heavy bars of copper, aluminum or alloys. The rotor bars are brazed electrically or bolted to two heavy and stout (permanently) short-circuiting end-rings. It is not possible, therefore, to add external resistance in series with rotor circuit for starting purposes. However, the slots are skewed slightly to make motor run quietly without producing magnetic hum and reduces the locking tendency of the rotor so the rotor teeth does not remain in stator teeth due to direct magnetic attraction.
2. Slip-ring induction motor: This type is provided with three phase, double layer, distributed windings consisting of coils used in alternators. The rotor has as many poles as the stator has and is always wound three-phase even with two-phase wound stator. Three phases are starred (Wye-connected) 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 three-phase star-connected rheostat. This introduces possibility for additional resistance in the rotor circuit during the starting period for increasing the starting torque and for changing its speed- torque/current characteristics. 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 wear/tear. Hence, under normal running conditions it is seen that the wound rotor is short-circuited on itself.
Advantages:
1. It has very simple, extremely rugged and almost unbreakable construction especially the squirrel cage type.
2. Its cost is low and is very reliable.
3. It has sufficiently high efficiency. In normal running condition, no brushes are required, hence frictional loses are reduced. It has reasonably good power factor.
4. It requires minimal maintenance.
5. It starts up from rest, needs no extra starting motors and does not need to be syncronized. Its starting arrangement is very simple, especially for the case of Squirrel cage type.
Disadvantages:
1. Its speed cannot be varied without sacrificing efficiency.
2. Just like a DC shunt motor, its speed decreases with increase in load.
3. Its starting torque is somewhat inferior to that of a DC shunt motor.
In fact, an induction motor can be treated as a rotating transformer i.e. one in which primary winding is stationary but secondary is free to rotate. Of all AC motors the poly-phase Induction motor is the one which is extensively used for various kinds of industrial drives.
Types (with reference to rotor construction and operation):
1. Squirrel cage induction motor: The rotor consists of a cylinder laminated core with parallel slots carrying the rotor conductors, which are not wires but consist of heavy bars of copper, aluminum or alloys. The rotor bars are brazed electrically or bolted to two heavy and stout (permanently) short-circuiting end-rings. It is not possible, therefore, to add external resistance in series with rotor circuit for starting purposes. However, the slots are skewed slightly to make motor run quietly without producing magnetic hum and reduces the locking tendency of the rotor so the rotor teeth does not remain in stator teeth due to direct magnetic attraction.
2. Slip-ring induction motor: This type is provided with three phase, double layer, distributed windings consisting of coils used in alternators. The rotor has as many poles as the stator has and is always wound three-phase even with two-phase wound stator. Three phases are starred (Wye-connected) 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 three-phase star-connected rheostat. This introduces possibility for additional resistance in the rotor circuit during the starting period for increasing the starting torque and for changing its speed- torque/current characteristics. 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 wear/tear. Hence, under normal running conditions it is seen that the wound rotor is short-circuited on itself.
Advantages:
1. It has very simple, extremely rugged and almost unbreakable construction especially the squirrel cage type.
2. Its cost is low and is very reliable.
3. It has sufficiently high efficiency. In normal running condition, no brushes are required, hence frictional loses are reduced. It has reasonably good power factor.
4. It requires minimal maintenance.
5. It starts up from rest, needs no extra starting motors and does not need to be syncronized. Its starting arrangement is very simple, especially for the case of Squirrel cage type.
Disadvantages:
1. Its speed cannot be varied without sacrificing efficiency.
2. Just like a DC shunt motor, its speed decreases with increase in load.
3. Its starting torque is somewhat inferior to that of a DC shunt motor.
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