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Old 11-26-2018, 07:13 PM   #21
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Originally Posted by TheDude_Abides View Post
So We broke the bus bars and I ohm'd everything out. Since the bus is watercooled there are hose clamps on the Neg that were touching the Pos. We've buttoned everything up and are going to try again after break. Previously I was getting a reading bus to bus and then a reading to ground. Now I have neither. Next question. When I megger these coils what kind of reading can I expect to get in respect to ground. I understand from positive to negative is a dead short. It should read completely open to infinity right? Im meggering at 1000v and it will only build to 70v with around 0.048M ohms. That to me doesnt seem right. If you have a completely open circuit then their should be no resistance. Yes the coils are isolated.
Generally over 100 megaohms is acceptable for wire, medium voltage, etc. On under 1000 V motors it's 5 megaohms or more though because the coil insulation is thin for power density on AC. It's not unusual though for rotor coils and similar thin, high current coils to be 1 megaohm per 1000 V which is more likely in your case. The standards for under 1000 V is to use 500 V whether AC or DC according to IEEE and NFPA. Only NETA who don't usually have any rhyme or reason for what they do recommends going that high. This is an insulation test, NOT a conductivity test. Check conductivity with standard ohm meter. Better yet if you can test inductance which can be used like a turns ratio test to detect shorted turns.

You still have something shorting somewhere or the coil is burned up.

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Old 11-26-2018, 07:16 PM   #22
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Also megger tests are HIGHLY sensitive to contamination. It's almost really a contamination test. And they are temperature sensitive.

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Old 11-27-2018, 08:08 AM   #23
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So I was poking around our store room and found a 5kv hipot. Seems to be older but well taken care of. Assuming its calibrated, can I hipot the coils? I megged the capacitor this morning and all is well but I know I can hipot that as well. Anything else? Do I need to isolate anything is the power supply itself?
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Old 11-27-2018, 01:12 PM   #24
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So I hipotted the coils to ground. The ones that are on the heater right now will not build voltage past 700vDC. The DC mA spike at 5 and the breaker trips the tester. I checked some of the others in the coil room and everything on the front of racks are doing the samething. However the ones torward the back of the racks built to 1250vdc with minimal rise in current. I believe I may have found the red hering.
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Old 11-27-2018, 01:13 PM   #25
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I did isolate the coils from the power supply.
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Old 11-27-2018, 01:36 PM   #26
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I have never Hipot tested a coil; or anything for that mater...

My question are the coils damp / full of cooling water? I would drain the coils and heat them up with an external source to dry them out and then see what you get.

Having said that, if the coils are shorting out, it could be for a number of reasons, the primary reason being there is a pinhole and the cooling water is migrating through the refractory. There also could be a gap in the refractory or there could be carbon building up somewhere on the inside if the refractory is not smooth. This happens when product is too small for the coil or the rails are not setup properly and the billets drag along the bottom of the coil. Carbon dust builds up in the crevices and eventually works it way to the coil and blows it out.

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Old 11-27-2018, 08:23 PM   #27
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