Resistance to ground (megger test)
Hipot test
Coil resistance
Magnetic field
No single test 'positively determines the health'. It is possible to pass one of these tests yet not be safe to operate.
A bit more information would be reasonable and useful. That information would include and not be limited to
AC or DC design
Operating voltage
Operating current
Application
Air cooled or oil filled
Human contact or accessibilty
Physical size
Duty cycle
Thank you for your responses. Some more information:
This electromagnet is wired in parallel with another electromagnet, and I noticed that it seems to be operating at higher current, relative to the other magnet, than it used to (using 89% of the total current vs. 78% previously, or 14% higher). It is difficult to replace so I need to be sure it is broken (internally shorted?) first. It operates DC, 100% duty cycle at up to about 50 amps. It is 8G square wire, 306 turns, 4.8" long by 3.2" in diameter. It is encapsulated in boron nitride containing epoxy and surrounded in circulating cooling water. It is part of a ion thin films deposition source. I have calculated its wire length and resistance based on the manufacturers data; and the current is about 14% high compared to calculated. What test do you recommend? What is "megger test"? Thank you.
The megger test can be done by most maintanence departments. Particurly those that must maintain motors. It is often a small hand cranked generator that measures the resistance to ground at an elevated voltage. It may not suggest shorted turns. It would probably only suggest 'something' is wrong with the coil. You think that now.
The coil resistance should be measured of both coils when disconected from the system. It will need to be done with a much more accurate ohm meter than typically found in most multimeters. The coils should be left off say maybe over a weekend for this test. It is most important they be the same temperature.
A encapsulated coil is probably imposible to repair.
If the coils are not being cooled the same, the warmer one will draw less current.
The manufacturer should be able to tell you the coil resistence when original. Thst is the only way to know for sure.
That has to be a very low DC resistance coil. You will definitely need an accurately calibrated 4 wire meter. Still probably won't be able to tell if you have a couple of shorted turns. There are meters that can detect shorted turns by impulse testing but you need a known good coil to compare to.
Other than the increased power draw, is it causing any problems?
If you use a megger or hipot don't go to real high voltages, you can damage insulation.
In the future you may also want to measure inductance and capacitance. Measuring such low resistance is difficult. Some motor shops will have high precision meters for this.
