A wound rotor motor, also known as a slip ring motor, is a type of AC (alternating current) induction motor that provides adjustable speed and torque control through the use of external resistance connected to its rotor windings. It's commonly used in applications where precise control over speed and torque is required, such as in industrial drives, cranes, hoists, and conveyors.
Here's how a wound rotor motor achieves adjustable speed and torque control:
Rotor Windings: Unlike a standard squirrel-cage induction motor, which has short-circuited rotor bars, a wound rotor motor's rotor windings consist of insulated coils. These coils are connected to slip rings, which are metal rings mounted on the motor shaft, allowing external electrical connections.
External Resistance: The key feature of a wound rotor motor is the ability to add external resistance to the rotor circuit. By connecting adjustable resistors to the slip rings, you can control the amount of resistance in the rotor circuit.
Starting and Synchronization: When the motor is started, the rotor windings are connected to the external resistance. Initially, the resistance is set high, limiting the current flow through the rotor windings. This reduces the starting torque and allows the motor to smoothly accelerate without drawing excessive current from the power supply. Once the motor reaches near-synchronous speed, the external resistance can be gradually reduced to increase torque.
Speed Control: By adjusting the amount of external resistance connected to the rotor windings, you can control the rotor current and thus control the speed of the motor. Increasing resistance reduces the rotor current, weakening the magnetic field in the rotor and causing the motor to operate at a lower speed. Decreasing resistance allows more current to flow, resulting in a stronger magnetic field and higher speed.
Torque Control: The torque produced by an induction motor is directly proportional to the product of stator current, rotor current, and power factor. By adjusting the rotor current using external resistance, you can control the torque output. More resistance decreases rotor current and torque, while less resistance increases rotor current and torque.
Smooth Starting and Braking: The use of external resistance in wound rotor motors also allows for smooth starting and braking. During starting, the resistance limits the inrush current, preventing sudden jerks or overloads. During braking, the external resistance can be increased, causing the motor to act as a generator, dissipating energy as heat and slowing down more smoothly than a motor without resistance.
Efficiency Consideration: While wound rotor motors offer excellent speed and torque control, they also have some disadvantages, including energy losses in the external resistance and the added complexity of maintenance due to slip rings and brushes. Modern variable frequency drives (VFDs) have largely replaced wound rotor motors in many applications due to their higher efficiency and ease of control.
In summary, a wound rotor motor provides adjustable speed and torque control through the use of external resistance connected to its rotor windings. By changing the amount of resistance, you can control the motor's speed and torque characteristics, making it suitable for applications requiring precise control over these parameters.