Wound Rotor Induction Motor: Benefits, Operation & Maintenance

By Mike Switzer, Product Line Manager - Motors

What is a Wound Rotor Induction Motor?

A wound rotor induction motor (WRIM) is a unique type of three-phase induction motor, known for its superior startup capabilities compared to the more common squirrel cage induction motor. This advantage makes it ideal for heavy-duty applications that require high starting torque, such as cranes, hoists, elevators, mills, and machines with large flywheels like punch presses and shears.

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In this guide, we’ll explore the working principles of wound rotor induction motors, their key components, and the advantages and drawbacks that come with using them.

Understanding the Structure

A wound rotor induction motor has a specially designed rotor featuring three-phase wound coils. These coils are installed into a laminated core, similar to the stator winding found in other three-phase induction motors. The rotor windings are connected to three slip rings, each with a carbon brush that provides continuity to an external control circuit.

This unique setup allows the rotor to connect to a rheostat (a variable three-phase resistor), enabling the control of resistance across the rotor windings. This resistance can be adjusted to influence the motor’s starting torque and speed, providing a significant advantage in applications that require controlled startup and variable speed performance.

How Does it Work?

The operation of a wound rotor induction motor can be broken down into four key steps which I have outlined below.

1. Starting with Full Line Voltage: The motor typically starts with full line voltage applied to the stator terminals. Resistance in the rotor circuit, provided by a star-connected rheostat, helps control the starting current and torque.
2. Adjusting Rotor Resistance: As the motor speeds up, the resistance is gradually reduced. This reduction in resistance decreases the current in the rotor, while the starting torque is increased due to an improved power factor. The adjustable resistance allows the motor to achieve a high starting torque with a lower initial current draw.
3. Speed Control via Resistance: During operation, additional resistance can be applied to control the motor speed. When resistance is added, the rotor current and motor speed decrease, while the induced voltage in the rotor windings increases. This relationship allows the motor to produce the necessary torque at a reduced speed, providing smoother acceleration and controlled performance for high-inertia loads.
4. Transition to Squirrel Cage Mode: Once the motor reaches full operating speed, the resistance can be fully removed, allowing the wound rotor induction motor to function similarly to a standard squirrel cage motor.

Advantages & Disadvantages

Advantages

The unique construction of wound rotor induction motors offers several benefits, particularly for industrial applications:

• High Starting Torque: The ability to adjust resistance makes WRIMs ideal for applications that demand high starting torque, such as heavy lifting and machinery with large inertia.
• Speed Control: By adjusting rotor resistance, WRIMs offer variable speed control without requiring additional electronic controls, making them suitable for processes where speed variation is critical.
• Smooth Startup: Unlike squirrel cage motors, WRIMs allow for a smoother, controlled startup with lower initial current draw, reducing strain on the power system and the motor itself.

Disadvantages

Despite their advantages, wound rotor induction motors have some downsides that limit their modern-day use:

• Higher Maintenance: The slip rings and brushes in WRIMs require regular maintenance, increasing the upkeep costs compared to squirrel cage motors.
• Declining Relevance: With the advent of variable frequency drives (VFDs), which allow standard induction motors to achieve variable speed control, the need for WRIMs has decreased. VFDs now dominate industrial applications, offering efficient speed and torque control for traditional squirrel cage induction motors.

Wound Rotor Induction Motors vs. Variable Frequency Drives

Historically, wound rotor induction motors were the preferred choice for applications requiring variable speed control. However, as VFDs became widely available, the need for WRIMs declined. VFDs allow for precise speed and torque control without the need for additional motor components, such as slip rings and brushes, simplifying maintenance and reducing costs.

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Final Thoughts

Wound rotor induction motors remain a valuable option in specific industrial applications, particularly where high starting torque and smooth speed control are required. However, their usage is gradually declining as VFDs continue to dominate the industry. If you are looking for a motor solution for high-inertia loads or specialized variable speed applications, a wound rotor induction motor could still be the ideal choice. However, for most modern installations, a VFD-controlled squirrel cage induction motor is likely to be more efficient and cost-effective.

Looking to learn more about motors and other industry related topics? Click the links below!

• 4 Common AC Motor Issues
• Basic Steps of Electric Motor Repair
• The Basics of Synchronous Motors
• 6 Common DC Motor Issues

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In electric motors, stators are the stationary components through which energy flows from or to the rotor. When energized with alternating current (AC), the stator produces a magnetic field that interacts with the rotor to create rotational motion. The three major parts of a stator are the stator frame, core, and winding.

In electric generators, the stator converts the rotating electromagnetic field into electric current. In fluid-driven devices, the stator guides fluid flow to and from the rotor. Motor stators are generally made from iron, steel, or a printed circuit board (PCB). PCB stators are typically used for low-power applications, as they can be smaller, lighter, and less noisy.

A rotor is the main moving part of an electric motor that spins and interacts with the stator’s magnetic field to convert electrical energy into mechanical motion. Mounted on the shaft of the motor and inside the stator, rotors can be made with wound wire or a series of permanent magnets. The rotor’s rotation is a result of the interaction between the magnetic field and the wound wire, which produces torque. The three main parts of a rotor are the rotor core, shaft, and winding.

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