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08-19-2013 11:17 PM

Hipot testing of Electric Machines

What is your view regarding Hipot testing of Electric Machines with DC and AC Hipot equipment. Some school of taught favored DC Hipot Testing while another favored AC.
08-19-2013 11:18 PM
Top #2
08-19-2013 11:18 PM
It depends on what type of machine or electrical components you desire to test.

DC high pot testing has many advantages. AC high pot testing has some advantages and also some disadvantages. AC high pot testing used to be a full frequency test (example 60 cycles). Now a days they have another version of AC high pot tester, the VLF (very low frequency). With the VLF they sometimes test it at a frequency level just over 1 cycle.

I have used both the AC and the DC high pot tester but it needs some good analysis and judgement what is the best to use. It depends upon the material that you may test. It may be a cable insulation, cable splice or terminations, motor windings, medium voltage circuit breakers, a medium voltage bus insulators, transformer bushings, assemblies submerged in oil, etc.

There should also be a need for a good interpretation of the test results data.An electrical engineer with good knowledge of materials and electrical test analysis may give you a better prognosis of what the test result data means.
08-19-2013 11:19 PM
Top #3
08-19-2013 11:19 PM
Thanks. An 11KV high Voltage motor newly rewound. Part of quality test to be done among others is Hipot .Hence which of the hipot test process( DC/AC) is most acceptable/applicable globally as industry practice in this kind of project.Few professionals in this field favoured DC while others favoured AC and they all have their genuine reasons. This was why I threw it open for other experienced professionals in this area to review so that we can all learn.
08-19-2013 11:20 PM
Top #4
08-19-2013 11:20 PM
There are 2 types of tests performed on motor windings. One is the high potential test and the other is the insulation resistance test. Both of these tests are normally performed on the motor windings. Since your motor is a newly rewound motor there are some factors that has to be ascertained because the insulation of the winding has various types of material and temperature rating. Ambient temperature, relative humidity, and cleanliness of the inside of the motor will influence the test readings that you can get. For motors with originally manufactured components the best resource of information and recommendation of testing is the manufacturer because they know their insulation product installed and they will recommend the appropriate test to use, AC or DC, and the voltage level to apply. The AC uses a lower voltage level and the DC uses a more elevated voltage level. The AC test can be a very low frequency or the conventional 50 or 60 HZ. ANSI standards, IEEE standards has lot of info that you can read about the test, but in my opinion the best resource is the manufacturer who developed the product. You will find a lot arguments about the best testing instrument to use by different instrument manufacturers but I rely on the motor manufacturer's recommendation who developed the product. Another factor on the test is the person who will interpret the result. Actually motor maintenance recommends keeping the test results history for use in comparing it to your following scheduled maintenance tests.
08-19-2013 11:20 PM
Top #5
08-19-2013 11:20 PM
My experience is to avoid using DC Hipot testing unless it is definitely required.

If you dont know if its definitely required, then opt for AC hipot tests - typically at 2E + 1KV for new plant during a factory test.

If its an addition test on site, then opt (or argue for) a test at 70% of the factory test value.

To confirm integrity of aged HV electrical plant, use 60% of the original factory test value.
08-19-2013 11:21 PM
Top #6
08-19-2013 11:21 PM
What is the HP rating or the voltage class ?

IEEE 43-2000 Reccommended Practice for Testing Insulation Resistance of Rotating Machines

IEEE 95-2000 Recommended Practice for Insulation Testing of AC Machine 2300V and Over with HVDC


Insulation integrity test are best method perfomed wih the AC power frequency. For very large rotating machines, most portable test set are incapable of delivering the power required at elevated voltage due to the charging current; a variable resonator is normally required to keep the power requirement lower.

Factory tests or production test are done at the power frequency.

Field test due to portability requirements uses HVDC and in conjuction with a polarization index test is good enough.
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