This method is only applicable for slip ring or wound rotor induction motor wrim.
Slip ring rotor resistance.
Whereas in squirrel cage motor it is not possible to add any external circuit because the bar of the motor is permanently slotted at the end of the ring.
The equivalent resistance in the circuit can be adjusted by an electronic control system the slip of the generator can be.
As the motor accelerates.
A motor with a wound type rotor is known as a slip ring induction motor.
The working principles and stator construction of the slip ring induction motor are exactly the same as that of the squirrel cage motor.
Figure 7 17 shows a wound rotor induction generator with rotor resistance control dynamic slip control.
The slip ring motor has an external resistance circuit for controlling the speed of the motor.
What is a slip ring induction motor.
Instead of bars in this motor three windings are used.
Each slip ring is connected to one of the three phases of rotor windings.
These motors are usually started with full line voltage applied across its terminals.
Rotor resistance control is also one of the methods by which we can control the speed of the induction motor.
As these motors have external resistance connected to its rotor circuit the value of starting current is adjusted or kept minimum by increasing the resistance of the rotor circuit.
As rotor winding terminals are not available in squirrel cage induction motor.
In this scheme the rotor windings are connected through slip rings and brushes with an external variable resistor.
In rotor resistance control method the rotor circuit resistance is varied by connecting a variable external resistance for speed control of induction motor.
This method is not applicable to cage rotor induction motor.
The rotor circuit resistance has significant impact on starting torque the speed at which the breakdown torque occurs and the slip during the normal running operation.
However the rotor construction of the slip ring motor is quite interesting.
The motor which employing the wound rotor is known as a slip ring induction motor or phase.
A wound rotor motor also known as slip ring rotor motor is a type of induction motor where the rotor windings are connected through slip rings to external resistance.
The speed of the wound induction motor can be controlled by connecting an external resistance in the rotor circuit through slip rings.
As the slip rings turn with the rotor the brushes maintain constant contact with the rings and transfer the resistance to the rotor windings.
Also rotor windings are connected through slip rings to external resistance.
The slip ring brushes made of graphite are connected to a resistive device such as a rheostat.
To overcome this problem the slip ring induction motor comes into the picture.
The disadvantages are high copper losses and the low torque.
Adjusting the resistance allows control of the speed torque characteristic of the motor.
7 14 shows how the motor torque speed curve would change if rotor resistance is increased and all other parameters in the motor equivalent circuit stay the same.
