Hi-Pot Testing an Inductor

[Time]

Does anyone know if/how this is done?  Surely its done on the powerful DC magnets found in medical equipment and particle accelerators.  

My first thought was to test them with AC high voltage allowing the impedance to limit the current.  Though, when considering the breakdown processes in insulations AC conditions and DC conditions are much different so this is insufficient.

My second thought was to simply limit the current with a series resistor but this would cause all the voltage to drop across the resistance and not the coil - essentially applying no field to the coil windings.

The operating currents are far too high to just allow for a quick simple test under double or more operating voltage.  Due to safety, I need to be confident that there are no insulation faults between windings before normal operation.

What are your thoughts/experiences?  

[Neilm]

You don't give too much detail on what you are testing, or what voltage you need but I 'm not sure you understand the details.

The test you are doing is to test the insulation of the windings to the accessable parts (case). Therefore the test voltage will appear across these points not across the coil.

If you are doing this in a company, then you should already have worked out what voltage is required to test the product, both the test voltage and the test duration.

Companies would buy or higher a dedicated piece of test equipment for testing the insulation.  The coil of the magnet would be connected to one terminal of the insulation tester, the other terminal would be connected to the metal outer case. If the outer case is not conductive then a layer of tin foil would be wrapped around it.

The insulation tester will have a current limit, usuall in the range of 1-3 miliamps. A good one will detect both breakdowns and partial discharges.

If you don't know the test voltage then you would have to figure it out from the application but I would be suprised if it were less than 1kV.

Yours

Neil

[Time]

Sorry, what I am trying to test might be rather unusual.  I am not trying to directly test the insulation on the magnet wire itself.

I want to test for nicks or flaws in the insulation that might have occured during construction.  The coils are being made in house.  They are rather large and high inductance.  They are going to be very strong electromagnets.

The operating voltage is around 400 V in the most extreme case. The coils themselves are going to be subjected to a very powerful constant voltage power supply during operation and will see the full 400 V voltage drop.



edit: I am begining to believe this isn't practical or possible.  I was assured by a colleague it has been done and should be done but I think he has misunderstood what was actually being done in the first place (he wasnt directly responsible for the testing).

[Neilm]

400V not that high a voltage, (my professional background is in High votlage and supply networks where insulation is tested at multiple kVs.). If the insulaiton is damaged, what would the worst affects be? I would assume that the worst would be a shorted turn which could be detected by excessive current draw or incorrect resistance measurement of the winding

I would suggest that you contact specialist application engineers and explain the application in more detail and they may be able to help. You may have to contact the manufacturers of this equipment directly. As i work for one of them designing High voltage testers I can't give an impartial recommendation.

Yours

Neil

[FreeThinker]

When I was a lad.....
we used to check dc motor for a partial short with a device called a growler.Don't knowif it is what you need but wiki has a link
http://en.wikipedia.org/wiki/Growler_(electrical_device)

[Zero999]

400V not that high a voltage

Yes 400V is still low voltage so you don't need any more precautions than dealing with mains.

Even enamelled wire will be abler to withstand that voltage, although it's good practise to keep the voltage between neighbouring turns below a couple of hundred volts so it can withstand the coil can higher voltage transients.

[Time]

I made that argument but the counter argument was there could be nicks and flaws produced by winding process.  I think we just have to be careful winding the coils and trust our craftsmanship.  I can't think of how you would possibly be able to tell if there was a minor flaw after the fact.  Aside from low power excitation and seeing if it behaved abnormally.

[Time]

When I was a lad.....
we used to check dc motor for a partial short with a device called a growler.Don't knowif it is what you need but wiki has a link
http://en.wikipedia.org/wiki/Growler_(electrical_device)

Oh, very interesting in the least.  This might be what we need.

[orbiter]

I didn't know what a growler was untill I watched Aussie50's channel on You Tube

[Zero999]

I made that argument but the counter argument was there could be nicks and flaws produced by winding process.  I think we just have to be careful winding the coils and trust our craftsmanship.  I can't think of how you would possibly be able to tell if there was a minor flaw after the fact.  Aside from low power excitation and seeing if it behaved abnormally.

How many layers is the coil?

What's the highest voltage between neighbouring turns?

You'd have to make a pretty bad job of it to get the full 400V between neighbouring turns. In Europe the standard three phase voltage is 400V RMS which is 566V peak and I'm pretty sure not much attention is paid to how transformers are actually wound. It's only for voltages over 1kV where any care is taken to the winding process.

[Neilm]

400V not that high a voltage

Yes 400V is still low voltage so you don't need any more precautions than dealing with mains.

Even enamelled wire will be abler to withstand that voltage, although it's good practise to keep the voltage between neighbouring turns below a couple of hundred volts so it can withstand the coil can higher voltage transients.

As I recall you can get some enamelled wire that is good to over 1kV.

Neil

[Time]

The polymide on this wire is rate for 3 kV DC.  We ironed out what needs to be done. I kind of feel foolish for even starting a thread about it.