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What is the difference between slip ring and wound rotor induction motor

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Key Differences Between Slip Ring and Wound Rotor Induction Motors

Understanding the differences between Slip Ring and Wound Rotor Induction Motors is crucial for selecting the right motor for a given application. While both types offer distinct advantages, their suitability varies depending on the specific requirements of the task at hand.

Construction Differences

  • Slip Ring Induction Motor: It features a rotor with wound coils connected to the external circuit through slip rings and brushes. This configuration allows for external resistance to be added to the rotor circuit.
  • Wound Rotor Induction Motor: Similar in construction to the slip ring motor, but the rotor windings are connected in a different manner, which affects its performance characteristics.

Performance Variations

  • Starting Torque and Speed Control: Slip Ring motors are known for their high starting torque and ability to control speed. They are ideal for applications requiring a gentle start with a heavy load. Wound Rotor motors, while also providing high starting torque, offer a more nuanced control over speed, making them suitable for applications where precise speed regulation is necessary.
  • Efficiency and Power Consumption: Both motors exhibit efficient performance, but the ability to add external resistance in Slip Ring motors can lead to more efficient power consumption, especially during startup.


  • Slip Ring Motor: Frequently used in cranes, elevators, and conveyors where variable speed and high starting torque are important.
  • Wound Rotor Motor: Preferred in heavy-duty industrial applications requiring robust performance, such as in hoists and complex machinery in manufacturing industries.

Maintenance Requirements

Maintenance is a crucial aspect for both types of motors due to the presence of slip rings and brushes. However, with proper maintenance, both can provide long service life and reliable performance.

What is a Slip Ring Induction Motor?

A Slip Ring induction motor, also known as a phase wound motor, plays a pivotal role in applications where variable speed and high starting torque are essential. It’s a type of induction motor where the rotor is connected to external resistance through slip rings and brushes. This unique construction differentiates it from other types of motors and contributes to its specific capabilities and applications.

Slip Ring Induction Motor

Construction and Operation: The slip ring induction motor comprises primarily of two main parts: the stator and the rotor. The stator, similar to other induction motors, is a stationary part with windings that receive AC power to generate a rotating magnetic field. The rotor, the heart of this motor, is where things get interesting. Unlike squirrel cage rotors, this motor features a wound rotor connected to the external circuit through slip rings and brushes. These rings and brushes allow for the introduction of external resistance into the rotor circuit, a feature not found in standard induction motors.

Control and Efficiency: The inclusion of external resistance in the rotor circuit is what sets the slip ring motor apart. By varying this resistance, it’s possible to control the motor’s starting current and torque finely. This feature is particularly beneficial in heavy load applications, where a high starting torque is necessary, and smooth acceleration is desired. It also reduces power consumption during startup, enhancing overall efficiency.

Understanding Wound Rotor Induction Motors

The Wound Rotor Induction Motor is a key player in industrial applications where high torque at a low speed is crucial. This motor variant stands out for its unique rotor design, which significantly influences its performance and suitability for certain tasks.

Construction and Working Principle: At first glance, the wound rotor motor shares similarities with the standard induction motor, featuring a stator with windings connected to the AC power supply. The distinction lies in its rotor, designed with a set of windings that are connected to slip rings. These windings are wound similar to the stator, contrasting the squirrel cage rotor found in conventional induction motors. This configuration allows for the connection of external resistors or controllers to the rotor circuit via slip rings, offering a means to control the motor’s performance.

Performance Characteristics: The ability to connect external resistors to the rotor circuit of a wound rotor motor is its most defining feature. This setup enables better control over the starting torque and speed of the motor. As a result, it offers smoother acceleration under heavy load conditions and a higher starting torque compared to standard induction motors. It also helps in reducing the starting current, which can be advantageous in systems where power supply limitations are a concern.

Michal Pukala
Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.