Insulation Testing

Discussion in 'General Electronics Chat' started by DeeBee, Aug 14, 2008.

  1. DeeBee

    Thread Starter New Member

    Aug 14, 2008
    Could any industrial electrical types explain how you would do an insulation test on three phase equipment such as motors or trafos? Do you measure the insulation to ground? If so, what sort of values would you expect? And if you were to measure between the coils, what sort of values would you expect (roughly!), or does it vary wildly according to the type of motor/trafo?

    Any help appreciated!
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    The device used for that is called a megger. I would want to see very low values of leakage in all cases.

    Here is a Wikipedia article -

    Active Member

    Mar 18, 2007
    Hi DB,
    When testing,first ground the starter terminal, frame, and motor shaft. If you're testing a DC motor, lift the brushes out. Discharge the field winding by grounding. Then remove the field winding from ground and connect to the (-) line connection on the megger. Connect the (+) earth terminal to ground. You can also measure the stator winding in a similar manner.
    >the megger is fed by dc voltage.(eg a440v to 550v motor is fed by500vdc to 1000vdc)
    >Apply the voltage for a fixed period of time eg 30sec.Take you readings
    Roughly the values to expect-not less than 1Mohms(But am not sure.

    U can also check this link(
  4. DeeBee

    Thread Starter New Member

    Aug 14, 2008
    Hello chaps,

    Sorry, wasn't very clear. I know what a megger is, have used them many times to measure single phase equipment but it's the three phase AC side that has me a bit stumped. I assume you don't measure across phases as you would have a short anyway on delta/star arrangements, so I assume you would measure to earth? What sort of resistance values would you expect on a healthy piece of equipment? I would imagine it would be similar on a trafo and a motor? What level of voltage would you use compared to the operating voltage?

  5. gerty

    AAC Fanatic!

    Aug 30, 2007
    Where I worked they used a 'hipot' tester. It puts a max of 2,500 volts at a very low current across the wiring/motor being tested. Any leakage to ground would cause a high current to flow setting off the hipot detector.The hipot voltage was referenced to ground and if there were no leakage it passed the test. We used to build LARGE air conditioners, 300 ton, and all the hipot testing was done before any of the electronics was installed into the unit.
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    HiPots are good. The one I used went to 30,000 volts. We checked capacitors with ratings typically about 20,000 volts for leakage.

    You would remove the three phase connections from the load (or transformer). When you evaluate what you have, it should be three coils with two connections each. You can measure phase to phase leakage as it should be isolated from one another. You can also evaluate phase to chassis.

    When the three phase device is connected to the load (or transformer), that is when the coils are connected as wye or delta.

    If all you have is the three phase connections, the only leakage evaluation can be phase to chassis.