Why is wound rotor motor Better?

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.

For more information, please visit our website.

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.

For more information, please visit CHANGLI ELECTRIC MOTOR.

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

Or, if videos are more your style, subscribe to our YouTube channel to learn more and stay up to date on the latest industry videos!

In need of our services? Find out how Southwest Electric Co. can assist in your business needs! Reach out to us for a quote.

A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.[1]

Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-up a typical rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable power resistor. When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the resistors reduce the field strength at the stator. As a result, the inrush current is reduced. Another important advantage over squirrel-cage motors is higher starting torque.

The speed and torque characteristics of a wound-rotor motor can be adjusted by changing the external resistance, unlike a squirrel cage motor which has a fixed characteristic. This is useful for speed control of the motor.[1]

A wound-rotor motor can be used in several forms of adjustable-speed drive. Common applications include hoists and conveyor systems. Certain types of variable-speed drives recover slip-frequency power from the rotor circuit and feed it back to the supply, allowing wide speed range with high energy efficiency. Doubly-fed electric machines use the slip rings to supply external power to the rotor circuit, allowing wide-range speed control. Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.

Want more information on wound rotor motor? Feel free to contact us.