Dielectric stike test

[Inven1]

We are having several large frame, 4160 volt motors being checked for use in Canada. They've never been used, are 4 years old, have had heaters energized and been rotated on a monthly basis. Is a "dielectric strike test" the same thing as a "megger/polarization index test" or a "Hi-pot"?

 

[oldfieldguy]

Inven1-

That's a new one on me... Maybe we can chalk this one up to the differences in colloquial terminology.

Do you have any idea as to what equipment is used and how this test is to be performed?

I'd think you'd want to open them up for a visual inspection for foreign mater and vermin nests, and then do the polarization index and a winding resistance (for baseline data). If your facility can support a test run, that's a prudent thing, also.

old field guy

 

[TheBlacksmith]

Dielectric Strike Test sounds a lot like a Basic Impulse/Lightning Level (BIL) test, much higher voltages and much quicker rise times, like 1.2 milliseconds.

 

[aolalde]

A combination of Insulation Resistance (@2.5kVDC), PI ratio and a step Voltage Test Curve will give you a good picture of the insulation condition. An insulation in good condition for machines 4 kV line to line should have IR>100Mohm @ 40°C, PI>2 and gradual slope curves for step voltage on the 3 phases.
Note that a Hi-Pot Test could be destructive .

 

[oldfieldguy]

Hi-pot testing CAN be destructive. A good operator can head off a lot of problems on a step test, though, if he does a one-minute megger test first. He then compares the megohm value calculated at each step during the step test. If at any time the calculated value of the hi-pot test approaches ore goes below the megger test, the hi-pot test should be terminated.

Sad fact is, though, I have had a 4160-volt motor fail a 2500-volt megger test one minute into the test, so even megger tests can be destructive. Of course, that motor was pretty much gone when we got to it, but it had been running prior to shutting down for "maintenance".

old field guy