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Rotor resistance method of speed control is used for
Squirrel cage I.M.
Slip ring I.M.
Both of above
None of above
Slip ring I.M.
The rotor resistance control method is specifically applicable to slip ring (wound rotor) induction motors because they provide physical access to the rotor windings via slip rings and brushes. By connecting an external variable resistance to the rotor circuit, the effective resistance of the rotor is increased, which directly influences the torque-speed characteristics.
The rotor resistance control method is specifically applicable to slip ring (wound rotor) induction motors because they provide physical access to the rotor windings via slip rings and brushes. By connecting an external variable resistance to the rotor circuit, the effective resistance of the rotor is increased, which directly influences the torque-speed characteristics.
TmaxтАЛтИЭX2тАЛ1тАЛ тАФ Maximum torque is independent of rotor resistance
smтАЛ=X2тАЛR2тАЛ+RextтАЛтАЛ тАФ Slip at maximum torque is proportional to total rotor resistance
In an induction motor, the starting torque and the slip at which maximum torque occurs are dependent on the rotor resistance. By increasing the rotor resistance (R2тАЛ), the slip at maximum torque increases, resulting in a reduction of motor speed for a given load torque. Since the power dissipated in the external resistors is lost as heat, this method is typically used for speed regulation over limited ranges rather than continuous high-efficiency operation.
Only possible in wound rotor (slip ring) motors due to external access to rotor windings.
Increasing rotor resistance decreases the speed of the motor for a constant load torque.
Provides a high starting torque with low starting current.
Inefficient at lower speeds due to heavy I2R copper losses in the external resistors.
Simple and effective speed control method.
High starting torque.
Low starting current.
Poor speed regulation at low speeds.
High power loss due to heat dissipation in external resistors.
Not suitable for squirrel cage induction motors.
Cranes and hoists.
Elevators and lifts.
Loads requiring high starting torque.
| Feature | Squirrel Cage | Slip Ring |
|---|---|---|
Access to Rotor | Internal (Short-circuited) | External (Slip rings) |
Squirrel cage motors have permanently short-circuited rotors, making it impossible to insert external resistance.
This method is a form of variable frequency/slip control where the slip is artificially increased.
B is correct тАФ The rotor resistance method requires access to the rotor windings via slip rings to insert external resistors.
Always remember that any speed control method involving rotor resistance is limited to Slip Ring motors; for Squirrel Cage motors, one must use Stator Voltage or Frequency control methods (V/f).