Portable antenna-based EMI detection of arcing at wound rotor motor slip rings? 1

[electricpete]

We have some noise issues that seem correlated to operation of a particular special purpose crane. During that period we have some wound rotor motors running wound and also some collector rings and brushes associated with the supply on the stator side (to accommodate getting stationary power onto a rotating platform) although that's not my focus (I'm asked to look at the motors). Inspection of brushes and slip rings didn't suggest a problem, but the inspection was not thorough (it is a difficult to access location).

If I wanted an easy way to check for abnormal arcing of brushes by some means, I'm thinking about two alternate ways:

1. Use some kind of antenna. Maybe the SKF Bearing Discharge Detector pen?

2. Clamp high frequency (radio frequency?) CT onto rotor phase or stator ground and record output with o-scope. This is not my preferred approach because it is more difficult to setup and we don't have a lot of experience with high frequency measurements. Also we have some other candidate source of noise and being able to screen with an antenna would have other benefits.

What do you think? Would that skf device work, or is there some other antenna device that is easy to use and well suited to the purpose?

Yes, I realize there is a lot more to EMI troubleshooting that could be done. I'm just involved in the small piece trying to evaluate the motor.

 

[electricpete]

I imagine there is a higher level signal from the wound rotor motor brushes than would come from a rolling bearing fed from vfd.

We do have the luxury of comparing 4 motors that all run at the same time. I imagine if the skf pen is saturated on the low-sensitivity (10 sec) scale, we can measure the distance away from the motor before the pen comes saturated. We can compare among the 4 motors and the one that gets saturated farthest away is presumably creating the worst arcing.

 

[btrueblood]

I can recall Skogsgurra describing chasing bearing EDM in some older posts. Not sure what methods he was using to measure...

 

[FacEngrPE]

I remember seeing a WW 2 vintage MG set from a radio, that had filter capacitors connected to the brush holders. I am not surprised to find electrical RF noise being generated by the sliding contact between a brush and slip ring.

A quick search turned this up.
[ul]
[li]Where to place a capacitor to prevent arcing in a brushed DC motor?[/li]
[li]DC brush motor EMI[/li]
[/ul]
A wound rotor motor with slip rings will not generate the fundamental frequency that arises in a commutator from bar switching, but will have a host of random factors that can cause less than perfect contact between the slipring and the brush.
Rather than making this a science project, look at slipring surface condition.

This link has a discussion of common slip ring problems.

https://www.arslipring.com/technical-article/slip-ring-common-fault.html.

All of these problems can be discovered without resorting to an RF noise test,

 

[3DDave]

An AM radio tuned between stations should easily detect arcing. Determining what is "abnormal" is a separate topic, but a change in the amplitude of the RF noise could indicate the situation is deteriorating.

 

[waross]

Many years ago I was responsible for a number of brushed motors, both wound rotors and dc generators and motors, with a few Amplidynes thrown in.
It was a dusty environment with a lot of airborne fine dust.
Arcing brushes was an ongoing issue.
The dust would find its way into the brush holders and jam the brushes.
We found that snapping the brushes once a week would clear the dust and avoid arcing.
This also let us put eyes (and fingers) on the brushes to detect worn or faulty brushes or defective springs or brush holders.
A good part of every Sunday, someone snapped all of the brushes in our plants.
Respectfully, Pete, consider regular inspection and avoidance rather than reaction.
A weekly pass with the AM radio is also a good plan to catch untimely failures.
If we missed a Sunday round, we would start to see arcing on about next Tuesday or Wednesday.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!