There is a type of AC motor in industry that you may have hidden away in your process plant that has brushes in too. These are known as slip-ring motors. They are often referred to as wound-rotor motors too. That description probably explains more of the construction of the motor then slip-ring does actually.
Instead of having cast rotor bars as you’d expect to see on a standard AC electric motor, the wound-rotor motor has copper windings. very much as you’d expect to see in the stator, except these are in the rotor. You can probably see this construction on the photos.
This makes the construction more complicated, more expensive and it does require more maintenance. So…why make them? Well…standard AC motors don’t have a high starting torque, The slip-ring motor does. They are found in crushers, lifts, cranes…anywhere where a high starting torque is needed.
I’ve purposely skipped a lot of technical stuff here about how they work. Google Kirchoff’s and Lorentz’s Laws if you want to know more about induction motor operation. I will add that the slip-ring motor is controlled by adding a resistance to the rotor circuit. This has to be done external to the motor and that’s where the slip-rings come in handy. They provide the electrical connection from the rotor windings to the external resistance.
The connection is from a stationary brush to the slip-ring. By definition the brush makes physical and electrical contact by the surface of the ring slipping underneath it. Hence…slip-ring.
Now to the point of this post, they do need regular inspection and maintenance. The surface of the slip-ring should be smooth and free from defects. Just look at the state of these!
What we believe happened here is that one of the brushes wore down so much, that what was left disintegrated. The spring that provides the brush pressure then made contact with the slip-ring. Friction then played its part with lots of heat, lots of sparking and as the photos show, a well knackered surface and a broken motor.
I’ll bet the failure of this motor was unexpected, as such, it would have caused production downtime. Downtime that was entirely avoidable if regular brush inspection was done.
So, what can you do? Well, first of all, find out if you’ve got any slip-ring motors on your site. Be thorough though, like this motor they can look like a standard AC motor and be easily missed. They are less common now with improved inverter technology but there are still plenty in service. Then if you have, I’d recommend every three months they are inspected on a Preventative Maintenance task.