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?
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.
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.
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:
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.
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.
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!