AC or DC for Hipot Testing 7

[BobM2]

I'd like to check our motors with a Hipot test. The field wires are a bit close to the metal housing. The motors are universal and we run them on 115 vac 60 hz. I assume I should use an AC Hipot. When would one use a DC Hipot test?

 

[sreid]

Motors often fail an AC Hi-Pot test due to the capacitance fron the winding to case. UL allows a DC test with a higher voltage.

 

[aolalde]

Hi-Pot test is a kind of “Go no Go” test of the insulation and it could prematurely age it.
New manufactured electrical machines are required to pass a proof Hi-Pot test of;
Vp-ac = 2*Vnom+1000 AC Volts for one minute.
Repetitive test is not recommended, however when necessary 60% of the voltage applied to a new insulation is recommended and never should exceed 85% of the initial test.

DC Voltage Hi-Pot is accepted as equivalent but the value must be increased by 1.7.
Vp-dc = 1.7 x Vp-ac

DC is used to reduce the KVA requirements and allow light high voltage supply of the test equipment. For large machines the circuit has a high capacitive current component, demanding more current and proportionally more kVA during the test.

To verify the insulation condition of a machine an evaluation could be done by measurement of the insulation resistance ( IR ) and the polarization index ( PI ), these taken with a Megohms meter ( Megger). These are not destructive tests.

 

[mpparent]

A megger test is prudent here. In the NETA standard, a 250V rated motor is tested at 500V, and a minimum resistance measurement is 25Mohms. I'm not sure what the standard for 125V rated equipment is. If you get a low reading, you sure don't want to do an AC HiPot test.

Mike

 

[dbaird]

I agree with the meggar method. Large size companies that use many motors will have, as part of their preventive maintenance programs, a yearly schedule setup for each motor to be Lubrication / Cleaning / Inspections / Electrical Tests and results documented. Check out:

http://www.coleparmer.com/techinfo/...e=Understanding+Insulation+Resistance+Testing

David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[BobM2]

What's the difference between a Hipot test and a meggar test? Don't they both apply a high voltage and measure (and limit) the current?

 

[eemotor]

A Megger test generally uses DC voltage at a level that's close to the motor's rated voltage. For example, 500V for a 480V motor. It is meant to be used as an indicator of insulation resistance measured in ohms. It is not destructive. Hi-Pot test may use AC or DC and will usually indicate leakage current. AC hi-pot testing is destructive at levels of 2xVnominal + 1000V. However, I've seen some equipment (DC) that has one analog display with several scales that show Megohms and DC current at the same time.

 

[mpparent]

A megger test is an insulation resistance test, ie measuring the resistance between two points. A HiPot test is a "strength" test, if you will, and can identify weaknesses, etc. Megger tests cannot do that.

Mike

 

[SteveKW]

Universal motors like to coat the inside with carbon dust from the brushes. Make sure the motor is cleaned first so the carbon doesn't bias your insulation test!

 

[sreid]

Generally I think of a Hi-Pot test as a safety test for new or rebuilt electrical equipment.