A wound rotor induction motor, also known as a slip ring induction motor, is a type of asynchronous electric motor that is widely used in various industrial applications. It shares many similarities with the more common squirrel cage induction motor but has a unique rotor construction and operational characteristics.
Here's how a wound rotor induction motor operates:
Stator: Like all induction motors, the wound rotor induction motor has a stationary part called the stator. The stator consists of a laminated iron core with evenly spaced windings that are connected to a three-phase AC power supply. When AC voltage is applied to these windings, a rotating magnetic field is created.
Rotor: The key difference in the wound rotor motor lies in its rotor construction. Instead of the simple squirrel cage structure found in the rotor of a squirrel cage induction motor, the rotor of a wound rotor motor has a set of insulated winding coils (phases) that are wound around the rotor core. The ends of these winding coils are brought out through slip rings and are accessible externally.
Slip Rings and Brushes: The wound rotor induction motor features slip rings, which are conductive rings mounted on the rotor shaft. These slip rings provide electrical connections to the winding coils on the rotor. Carbon brushes press against the slip rings, allowing external circuit connections to the rotor windings. The slip rings and brushes enable external control of the rotor circuit.
Starting and Operation: When power is applied to the stator windings, a rotating magnetic field is established, inducing a voltage in the rotor windings. Initially, the rotor is stationary, and the relative speed between the rotating magnetic field and the rotor windings results in an induced voltage. The rotor current flows through the slip rings and brushes, allowing external resistance to be added to the rotor circuit. This external resistance is gradually reduced as the motor accelerates, increasing the rotor current and torque.
Control and Speed Regulation: The use of external resistance in the rotor circuit allows for better control of the motor's starting torque and acceleration. As the motor accelerates, the relative speed between the rotor and the magnetic field decreases, causing the "slip" to reduce. This reduction in slip leads to a decrease in the rotor current and an increase in the motor's speed.
Advantages and Applications: Wound rotor induction motors offer several advantages, including improved starting characteristics, better speed control, and the ability to handle high-inertia loads. They are commonly used in applications where controlled starting torque, adjustable speed, and smooth acceleration are important, such as in cranes, hoists, conveyor systems, and large fans.
In summary, a wound rotor induction motor operates by using a set of wound rotor windings connected through slip rings and external resistances. This design allows for enhanced control over starting characteristics and speed regulation, making it suitable for various industrial applications.