Hello Gents,

We historically use AC to flash test our newly wound motors and generators, but we did have one go down in final test and the question has been raised as to whether we should consider DC HiPot first ?

DC testing, other than with a meggar, is not something I am familiar with and would welcome thoughts as whether DC might be better or combination of both is the way to go.

For reference the motor that failed was 440V but do do machines up to 13.8kV,

many thanks
Tony

What do you mean by "flash" test?

Here are my thoughts:

Pro's of AC:
1 - AC provides dielectric stress which is more representative of operating stress.
2 - AC provides better opportunity to localize leakage if lights-out test is done

Pro's of DC:
1 - Whereas ac hi-pot is "pass/fail" (trip or didn't trip), dc hi-pot condutoed in step-voltage manner per IEEE95 can provide a curve of current vs voltage, which provides some means to judge condition of the winding and sometimes vaguely project voltage of failure. It can also give an opportunity (not necessarily a guarantee) that the test can be stopped before failure occurs.
2 - Less destructive for several reasons:
2A - we may have an opportunity to stop the test (as above)
2B - the current trip can be set much lower since it doesn't have to accomodate capacitve current
2C - There is gernally only one discharge of a void. Whereas ac can discharge 60 times per second

Hi Pete,

Flash test is a colloquial term we use for a Hi-pot test.

The dc pro’s are very good, well worth looking into.

Thanks
Pete

jkt,
Let us know where the failure occurred. That is important for you to know, since it could point to a badly wound coil or your vpi process, etc.
The AC Hipot test is the most stressful of all tests and should only be performed on a brand new winding and, IMO, never performed on a winding that has been in service.
I like the term Flash Test. Though there usually isn't a 'flash' (the breaker in the AC Hipot tester usually just pops), it is a real sinking feeling when it happens, because in a 'flash' you just bought a whole 'nother rewind!
Regards,
Ron Brook

Hi Ron,

This is a new winding. It failed just inside the slot section where one of the semi-conductive wedges nicked the insulation.

Certainly AC is a demanding test, the rotor dynamics of the 20% overspeed - both of which do nothing for my blood pressure

tony

Tony,
Unfortunately in this case, it pointed to an assembly problem.
Overspeed testing! Yes, another winner! You don't have anyone looking for that on a two pole motor, do you???!!!
Ron

Unfortunately, in my illustrious position I get to not only watch the 2 pole (60Hz overspeed in our test facility) but also 2 pole generator overspeed; Retaining rings sitting a small fit area and insulations and a HV test at the end.

My last one was really dreadful, I was a bundle of nerves – even the client had to ask at one point if that high speed thumping noise was my heart going or a machine fault

Tony,
The only 2 pole rotors that I have witnessed at over speed test where in bomb proof bunkers. I hope yours is the same. If not, I have a great big life insurance policy for you with me as the primary!!!!!

oh yes, 2500tonnes of concrete !

Note: The voltage level for DC Hipot testing is higher than an AC test. If my memory is correct, the difference is:

DC Test Voltage = AC Test Voltage x 1.4

1.7 is the ratio, according to IEEE 95.

Here is an interesting blurb from IEEE 95-2002:

quote:

The well-compacted slot portions of armature coils appear to have an average electrical strength ratio of
direct voltage to power frequency voltage (rms) between 2 and 3. However, in a machine winding the series
coil or bar connections and leads external to the slots cannot approach the same conditions of mechanical
compaction and electrical strength in their ground insulation. For testing complete armature windings of
rotating machines, therefore, the ratio between direct voltage and power frequency voltage (rms) defined in
this recommended practice is 1.7 for both acceptance and maintenance tests.

Selecting the test voltage is certainly an important part of the process. For maintenance-level testing, the standards specify a range - left to the user to choose (unlike new windings where 2E+1 ac or same thing times 1.7 dc is straightforward). Higher test voltages is of course more probing and also more likely to fail a winding.

The DC Hipot equivalent peak voltage is 1.4 x the AC Hipot RMS voltage.

But, the rule of thumb for equivalent breakdown voltage is x 1.7. DC Hipot requires a higher voltage than AC Hipot to detect the same insulation weakness mainly because DC does not ionize the oxygen and create ozone which is a better conductor of electricity.

NEMA MG-1 requires AC Hipot testing of newly manufactured low and medium voltage motors. It allows the motor manufacturer to substitute DC Hipot testing on high voltage motors (> 6kV) if the x 1.7 multiplier factor is used.

EASA allows DC hipot testing of all remanufactured motors as long as the x 1.7 factor is used. But, in my experience, lots of motor repair shops don't follow the EASA standard.