hipot testing/flash testing

[jammyjim]

Can anyone tell me if these two are the same, when flash testing a motor i would usually use twice the nameplate voltage plus 1kv and add 20%, do the same rules apply on a baker surge test?

 

[UKpete]

No, you may want to use a higher voltage.

I phoned Baker myself a couple of years ago with the same question, because I felt that the manual I had wasn't that clear on the matter. As you probably know, the rule 2xlineV + 1kV applies to a flash test using an AC voltage (at power frequency) and is the RMS voltage to be applied. Incidentally, this flash test is specified in the same way by both NEMA MG-1 and IEC 60034-1.

Where a flash test using a dc test voltage is used (as it was on our Baker tester) the test voltage is calculated by multiplying by 1.41 because that is equivalent to the peak of the RMS voltage (I'm not too happy about that because with DC this voltage is applied continuously whereas with an AC test the peak is only applied twice per cycle). The problem I had was, is the Baker surge test treated as an ac test or a dc test.

What they told me (and I'm reading from the notes I took at the time) is that it is not straightforward because the initial pulse is dc, then it rings at ac; so for the surge test Baker recommend multiplying factors on the 2xlineV + 1kV rule of 1.0, 1.4 and 1.7 and it is up to the user to decide depending on their application, then stick to it as the surge test is basically a comparative test so all testing should be done at identical conditions as far as possible. For a non-inverter rated system I was advised that a 1.4 multiplier would be reasonable, i.e.:
1.4 x (2xlineV + 1kV)
- you start at zero volts and build up to this peak voltage as displayed on the Baker tester.
Pass criteria would be area ratio within 5% and the waveforms are steady.

I believe the US navy use a multiplier of 1.7 as they specify high quality/high reliability insulation materials.

All this applies to testing all new windings, and the test should only be carried out once (1 minute surge test duration). Where testing old winding, a lower voltage should be used. Again practice varies, I have seen figures between 50% and 80% of new voltages quoted.

 

[UKpete]

I might also add that NEMA MG-1 (section 12.5 in my 1998 edition) is a bit sceptical about surge testing and recommends "voltage distribution throught the winding should be investigated because resonant conditions may exist which would cause abnormally high or low stresses at some point in the insulation system of the motor component".

 

[jammyjim]

Thx fot that pete