How does a wound rotor induction motor achieve variable speed operation?
1 Answer
A wound rotor induction motor, also known as a slip ring induction motor, achieves variable speed operation by controlling the resistance connected to its rotor windings. Unlike a squirrel cage induction motor, where the rotor windings are short-circuited, the wound rotor induction motor has external connections for its rotor windings, which are brought out through slip rings and brushes.
Here's how the variable speed operation is achieved in a wound rotor induction motor:
Starting at Low Speed: When the motor is initially started, the rotor windings have high resistance connected to them. This high resistance reduces the starting current, which helps to limit the inrush of current during motor start-up. As a result, the motor starts with a relatively low speed.
Speed Control: Once the motor is running, the resistance connected to the rotor windings can be adjusted. By varying this resistance, the effective rotor impedance is changed. According to the slip principle, the difference between the synchronous speed (speed of the rotating magnetic field) and the rotor speed is called slip. The slip determines the torque and hence the speed of the motor.
Lowering Rotor Resistance: To increase the motor's speed, the resistance in the rotor circuit is gradually reduced. As the resistance decreases, the rotor current increases, which results in a higher torque and an increase in the motor's speed.
Higher Speeds: By further reducing the rotor resistance, the motor's speed can be further increased, allowing for a wide range of variable speeds.
Full Speed Operation: When the rotor resistance is minimized (usually close to zero), the motor operates at its full speed and full load capacity, just like a standard squirrel cage induction motor.
The variable speed operation of the wound rotor induction motor makes it suitable for various industrial applications where precise control of speed is required, such as in cranes, hoists, conveyors, rolling mills, and large fans. Additionally, the wound rotor induction motor also offers better starting torque compared to squirrel cage induction motors, making it beneficial in applications where high starting torque is necessary.
Here's how the variable speed operation is achieved in a wound rotor induction motor:
Starting at Low Speed: When the motor is initially started, the rotor windings have high resistance connected to them. This high resistance reduces the starting current, which helps to limit the inrush of current during motor start-up. As a result, the motor starts with a relatively low speed.
Speed Control: Once the motor is running, the resistance connected to the rotor windings can be adjusted. By varying this resistance, the effective rotor impedance is changed. According to the slip principle, the difference between the synchronous speed (speed of the rotating magnetic field) and the rotor speed is called slip. The slip determines the torque and hence the speed of the motor.
Lowering Rotor Resistance: To increase the motor's speed, the resistance in the rotor circuit is gradually reduced. As the resistance decreases, the rotor current increases, which results in a higher torque and an increase in the motor's speed.
Higher Speeds: By further reducing the rotor resistance, the motor's speed can be further increased, allowing for a wide range of variable speeds.
Full Speed Operation: When the rotor resistance is minimized (usually close to zero), the motor operates at its full speed and full load capacity, just like a standard squirrel cage induction motor.
The variable speed operation of the wound rotor induction motor makes it suitable for various industrial applications where precise control of speed is required, such as in cranes, hoists, conveyors, rolling mills, and large fans. Additionally, the wound rotor induction motor also offers better starting torque compared to squirrel cage induction motors, making it beneficial in applications where high starting torque is necessary.