Rotor as the name suggests it is a rotating part of an electrical machine, in which current is induced by transformer action from rotating magnetic field.Induction motor rotor is of two types: Wound Type Rotor or Slip Ring Type Rotor and Squirrel Cage Rotor.
(1) Wound Type Rotor or Slip Ring Type Rotor
This type of rotor is also made up of laminated cold rolled grain oriented silicon steel to reduce eddy current loss and hysteresis loss. The rotor winding is distributed as well short pitched to get a sinusoidal emf output.
The operation of an induction motor is not possible with an unequal number of stator and rotor poles, and this type of rotor does not respond automatically to change in a number of stator poles. Hence the number of rotor poles must be made equal to the number of stator poles.If the rotor is provided with 3 – phase windings; the rotor windings must be connected in star connection irrespective of whether the stator winding is connected in star or delta.
Features of Wound Type Rotor or Slip Ring Type Rotor
The major difference between squirrel cage rotor and wound type rotor is the presence of a slip ring in wound type rotor hence it is also called a slip ring rotor. The three terminals of star connected rotor windings are brought out and are connected to the external resistors through the slip ring.
Slip rings are made up of a high resistance material such as phosphorous bronze or brass. Brush contacts are used for making the connection of the rotor winding with the external circuit, Brushes are made up of carbon or copper material, but carbon is preferred due to its self-lubricating property. So friction losses are less with carbon brushes.
To improve the starting torque, external resistors are used. This external resistor also limits the starting current drawn by the motor at the time of starting. Hence power factor improves.
(2) Squirrel Cage Rotor
In this type of rotor, the rotor winding consists of conductors, in form of copper or aluminum bars embedded in semi-closed slots of a laminated rotor core. To facilitate a closed path in the rotor circuit, both sides of the rotor bars are short-circuited by end rings.
Features of Squirrel Cage Rotor
This type of rotor does not have a definite number of poles, but the same number of stator poles will be induced in the rotor automatically through induction. Hence for a squirrel cage rotor.The squirrel cage rotor has very low leakage reactance as it does contain any winding on the rotor and it results in low starting torque and maximum running torque.
As we know that for increasing the value of stating torque we have to increase the value of rotor resistance and to increase it we have to insert a resistance in series with rotor winding, but in case of squirrel cage rotor we cannot insert it as its rotor bars are short-circuited by end ring from both sides. Thus we can say that the squirrel cage rotor produces good running performance but poor starting performance.
Drawbacks of Squirrel Cage Rotor
Low starting torque
High starting currents
Poor power factor
But starting characteristics of a squirrel cage rotor can be improved partially by modifying circuit design.
Skewed Rotor Bars
One of the purposes of skewing the rotor bars is to increase the value of rotor resistance so that starting torque can be improved. We know that resistance is proportional to length hence by skewing the rotor bars length of bars is increased hence resistance increases so does the torque.
By seeing the above points, we can conclude that due to the provision of an extra resistor, Wound Type Rotor or Slip Ring Type Rotor type of induction motor has good stating performance but poor running performance.
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