Everything you’ll ever need to know about Slip Ring Motors

Whether you know them as Slip Ring Motor or Wound Rotor Motor, these motors can be total life-savers.

Still, you may wonder, how can slip ring motors help you solve your design requirements?

In this guide, you will discover everything you need to know about the working of slip ring motors. It will also give you a comparative between slip ring and squirrel cage induction motors.

Without further ado, let’s dive in.


Let’s start at the beginning.

An induction motor has two main parts: the stator and the rotor. The stator is the static part of an induction motor, while the rotor is the part that rotates.

Simple, right?

The difference between a squirrel cage and slip ring induction motor lies in the rotor.

In a three-phase slip ring induction motor, the rotor has a three-phase winding on the wound.

Each terminal of the winding connects to separate slip rings. The slip rings allow the insertion of impedance (resistance) into the circuit.

This is how a slip ring induction motor diagram looks like:


As a result of the added resistance, the rotor’s current is more in phase with the stator’s current. This creates a bigger torque from the starting.

The science behind the slip ring motor

The slip ring motor construction allows the slip ring to control the output of the motor.

When you start a 3 phase slip ring induction motor, the poles of the stator create a rotating magnetic field, just as in any induction motor.

It is the slip ring rotor with the added external resistance what makes the difference.

When the currents of both the rotor and the stator are more in phase, we obtain a bigger torque. Also, the external resistance, added by the slip rings, lowers the starting current demand.

Since they are so important, whatis a slip ring?

A slip ring is a device that can transmit power, electrical signals or data from a static source to a rotating one.

In a motor, a slip ring commutator maintains the connection between the rotor and the stator

Slip rings are typically made of conductive materials, such as cooper or gold alloys. Sometimes they are manufactured with silver or coin silver.

In this case, slip rings for induction motor are made of copper or copper alloys.

Slip rings are fundamental for many industries.

The machines used to bottle your milk? They use slip rings. And the International Space Station? It has slip rings to help it control the solar panels that power the station.

At Moflon, we pride ourselves in providing top-quality slip rings and innovative technology in the field.


There are plenty of places where you can find slip ring motors.

Some of the most common are cranes, mills, hoists, and conveyors. All these enjoy the slip ring advantages such as:

  • Speed control- it’s easy for you to get smooth acceleration, even under heavy loads. Slip ring motors help to control the speed of lifts, railway traction, elevators, etc.
  • High-inertia loads- if you need to move big loads then your best bet is a slip ring motor. Also, you will be saving some money because of the starting current efficiency.

Okay, so if slip ring motors are so cool, then why even bother with squirrel cages?

Well, there are some things to consider, of course.

One of the major issues with slip ring motors is that they are more costly to maintain than squirrel cage. This is mainly due to the wearing of the brushes of the slip rings.

A slip ring motor is also less efficient with light loads compared to the squirrel cage motor, and are sensitive to voltage fluctuations.

How do I know if a slip ring motor is good for my project?

The most important thing to look at is the size of your load. If you have a high-inertia load, your best bet is to use the slip ring motor.

Slip rings motors are also the absolute go-to if you need to be able to control the speed of the motor.


Each motor has its own application. We can still compare some of its features to help you understand the difference between them a little bit better.

  • Construction – the construction of the slip ring motor is more complicated than the squirrel cage. This has to do with the windings on the rotor and the use of slip rings.
  • Rotor – The rotor of a squirrel cage motor is a set of bars which are short-circuited by end rings. As for the slip ring motor, the rotor has the same winding as the stator.
  • Speed Control- As we’ve mentioned, slip ring motors are ideal for easy speed control. We achieve this by adding external resistance to the rotor, which a squirrel cage can’t do.
  • Torque – Squirrel cage motors have low starting torque which is not a good match with high-inertia loads. This doesn’t happen with slip ring motors, which have high starting torque.
  • Efficiency – Due to high-copper losses, slip ring motors are less efficient than the squirrel cage. This is true for light loads but not for high-inertia loads.
  • Application – Slip ring motors are not as used by many industries due to lower efficiency compared to squirrel cages. Industries use slip ring motors when they need high torque and/or speed control.
  • Maintenance – Compared to slip ring motors, squirrel cage motors have a low maintenance cost. This is due to the phase winding and the slip rings used in the slip ring motors.

The most important thing is to keep in mind your project’s needs.

Take in consideration everything you’ve learned today and decide what’s best for you and your project.


Even when they are not the best fit for every project, slip ring motors are an amazing tool. They power gigantic machines and solve problems that the alternative simply can’t.

There you have it, all there is to know about slip rings. Or all you’ll need to make a good decision, anyway.

To wrap it up:

  • A slip ring motor is an induction motor.
  • It has a three-phase winding on the phase wound rotor.
  • It’s connected to slip rings that can add resistance.
  • It creates a bigger torque while starting and requires less starting current.
  • It’s useful for speed control and high-inertia loads.
  • Maintenance can be high-cost compared to squirrel cage motors.

What are your thoughts on the advantages of using slip ring motors? Did we miss anything?

Let us know in the comments.

Do you still have questions about the benefits of slip ring motors? Contact us to talk with one of our experts!