Foucault current (or Eddy)

[xo1tech]

Any of you guys ever dealt with an Eddy bouncing around between the pinion and spur with old school electronics installed in an RC10?

I have not done any testing with any other rc vehicle and cannot say this would have ever been an issue while running this car, but I do know this - I've had issues with AM radio systems in the past where changing capacitors and/ or crystals, re-locating antenna wire and/ or electronics, truing comms, adjusting brush-spring tension, replacing receivers, speed controls, motors, or just about anything else you can think of really didn't fix the problem.

In this example the anodized coating where the motor plate mounts to the tub caused an excessive amount of resistance to the ground path and allowed the random current I witnessed to be generated.

I do not know that the Eddy current would have EVER caused ANY problems but, I DO KNOW THIS, removing the anodized coating and minimizing the resistance at the motor plate/ tub connection made the Eddy go away.

The way I look at it, ANYTIME you can remove a variable, the more predictable and consistent things become.

Here is an excerpt from a document I wrote detailing the build of my '94 Worlds Car.


In the FINAL STAGE OF BUILD PROCESS, I was breaking in the motor (running at low rpm on test stand in total darkness), when I noticed an eddy current (greenish-blue ghostly ball of electrical current) bouncing around between the spur and pinion at the exact moment of acceleration.

After trying several different composition spur gears, lubricants, motors, and motor plates, I called Associated Electrics Technical Support and they had never heard of such and could not help me.

I called Mark Lowry with Litespeed Products (motor builder and friend) to see if he had any ideas. He was not aware of this phenomenon ever occurring and could not offer any advice either.

With over 300 hours invested in the build, and not wanting to take a chance of an electrical glitch taking control of the car, I shelved it.

Years later (upon further investigation), I determined the motor plate was not grounded properly. After removing the anodized coating from the motor plate and chassis where they bolt together (not visible at all with car assembled), the problem went away.

The motor (not being grounded properly) was causing the Eddy current build up between the pinion and spur.

As many electrical glitches as I experienced in the days of AM radios, I look back and wonder how many times this could have been an issue and I just did not realize it.

WHEN IN DOUBT – OHM IT OUT!

If you guys have any questions or comment feel free to post them.

[muziyi]

Although this problem I also don't know what is the best method, but is still worth studying.

[jwscab]

it's not something I've ever heard of. I can tell you that there could be a couple of electrical 'circuits' that you might be seeing.

the first is obviously the motor drive system, current flows in the loop between the motor leads and into the motor armature. it's completely isolated from the rest of the car unless you have capacitors tied into the the can from positive and negative. you are much better off running just one cap between positive and negative if you suspect any weird current loops.

the second which could possibly be what the description of 'blue/green ball' was most likely a static build up. if the motor armature and can didn't make a very good electrical connection to the rest of the car, you could build up a static charge as the teeth of the pinion rub across a plastic spur gear. while it's not any current flow, it is could be a high voltage potential(high electric field) and could affect an AM system pretty easily. by making the armature and can (the electrical connection is through the bearing interface) grounded into the motor plate and chassis, you could reduce the electric field. it's possible under the right lighting and atmospheric conditions to see an electric field. that would be my guess of what was being seen.