Slipring Motor

A slip-ring motor, also known as a wound-rotor induction motor, is a type of three-phase induction motor distinguished by its rotor design. Unlike the common squirrel-cage motor, the slip-ring motor has a rotor with insulated windings connected to external resistors via slip rings and brushes. This unique feature allows for the adjustment of rotor resistance during operation, offering significant advantages, especially during motor startup.

Here's a breakdown of the key aspects of slip-ring motors:

Construction:

• Stator: Similar to a squirrel-cage induction motor, the stator has three-phase windings.
• Rotor: The rotor consists of a cylindrical laminated steel core with slots housing insulated three-phase windings. These windings are connected to three slip rings mounted on the rotor shaft but insulated from it.
• Slip Rings and Brushes: Carbon brushes press against the slip rings, allowing for an external electrical connection to the rotor windings.

Working Principle:

The working principle is based on Faraday's law of electromagnetic induction, similar to other induction motors. When AC power is supplied to the stator windings, a rotating magnetic field is produced. This field induces voltage and current in the rotor windings, creating their own magnetic field. The interaction between the stator and rotor magnetic fields produces torque, causing the rotor to rotate.

The key difference lies in the ability to add external resistance to the rotor circuit through the slip rings and brushes.

Advantages:

• High Starting Torque with Low Starting Current: By adding high resistance to the rotor circuit at startup, the motor can produce a high starting torque while keeping the starting current relatively low. This is crucial for applications with heavy loads or high inertia.
• Adjustable Speed Control: The external resistance connected to the rotor can be varied during operation, allowing for speed control of the motor. Lowering the resistance increases the speed, and vice versa.
• Smooth Acceleration under Heavy Loads: The ability to control the rotor resistance facilitates smooth acceleration even with significant loads.
• High Overload Capacity: Slip-ring motors can typically handle higher overload conditions compared to squirrel-cage motors.