Shading coil material?

[rd400guy]

We have an OLD CH motor controller on a Tocco motor-generator for induction furnace. Nameplate shows: 500HP 60Hz 440V 520A 3ph.

From time to time, the shading coil on one of the contactors burns out. We have some spare coils on hand, and also have some "new" ones that were made in house a few years ago. Yesterday when one failed (which is how I became involved in the issue), we replaced the failed coil with one of the "new" in-house produced coils. It appeared to be nickel-plated copper wire, and was very ductile. After installing and switching the motor-generator on, one section of the shading coil began to glow cherry red. We immediately shut down, and replaced it with one of the older ones, and the problem went away.

The older ones are much stiffer (not easily bent like the "new" ones), and have a copper color. One of the techs here said that many years ago he called the Cutler Hammer service folks and they said we could make replacement shading coils out of brazing wire.

So finally to the question: What material should shading coils be made of?

 

[aolalde]

Heat is the result of R*i^2.
The higher the conductivity, the lower the resistance and generated heat. I will recommend Copper No C11000 which is Electrolytic Tough Pitch Copper with electrical conductivity of 101% IACS.

Any brazing wire is a Copper alloy (bronze or brass), the added elements in the alloy result in low electric conductivity, as low as 20% or 10% IACS (even silver alloys). The result is 5 to 10 times the amount of heat generated for a given circulating current.

 

[aolalde]

Above recommendation is valid only when the current in the short circuited turn is limited mainly by the inductive reactance Xl. Xl >>R


Z = R + jXl and i = e / Z

 

[Maui]

If you have an older unit that was not manufactured in-house, and you do not intend to re-use it, I suggest that you cut a section of the shading coil wire from the unit and have it analyzed with an optical emission spectrometer. If you don't have access to one, I'm confident that you will be able to find a local company that could provide this service for you at a minimal cost.


Maui

 

[rd400guy]

Maui, the contactors on this motor controller are the only ones we have (no replacements). The shading coil itself is not of a sufficient size or weight to allow for OE, X-ray, or ICP analysis. It is only one turn of ~16-14 AWG solid wire. We could possibly melt a few of the replacement coils to make a solid enough piece, but then we'd have no replacements left!

But thanks for the suggestion.