how to meg motors?
How important is it to adjust your test voltage for condenser fan motors, compressors and fan motors on 230 or 480 volt equipment? test all at 500 vdc or test at 250 vdc for 230 volt motors and 500 vdc for 480 volt motors.
they recommend never meg at higher voltage than operating voltage. motors have an insulation class like wire ( most wire 600v insulation) who knows what a motor is classed at or who cares. if it is 230 v motor and rated at 600 volt insulation and you meg it at 500v, if you pop that weak spot on the windings , it is harder to get the customer to see the value in your hi tech preventative maintenance than if you meg it at 250 and show a couple megs and will order it and be back before it goes to ground.250 meg on 230 system and 500 meg on 480 is probably exceptable, but i would meg at 250 first on the 480, make sure it shows nothing before going up to 500
WOW, my megger has a 9v battery, I've never seen one like you guys are talking about, even the hand crank ones use a DC voltage. What brand are you using??
true meg ohm test equipment will have multiple voltage test settings, and can be powered by batteries, cranks, and power cords. They have capacitors and transformers inside to build up the output voltage to the level that you have set. A VOM (volt-ohm meter) powered by a 9 volt battery is not a true meg ohm tester. Using VOMs you are just testing that the line has some insulation on it, I guess. It is a quick and easy way to tell if you already have a burn out. It will not give you good test that a motor is about to fail.
The proper method is to test at the voltage that the motor is rated at. Most motors have insulation rated at 600 volts (even if they are 230 volt units) and so you need to set the megger to 500 volts. If you test at a higher voltage than the unit is rated at, you can start to break down the insulation and cause damage to the windings (this is sometimes refered to as a di-electric strenght test).
You must take a reading at about 10 seconds, then keep on going to get a reading at one minute. The value at one minute had better be the same as, or greater than the ten second reading. This tells you that the insulation is good over time.
Medium voltage motors (over 600 volts to 6000 volts) need to be tested at 5000 volts or so. That is why some meters have that much higher setting on them. I don't recommend that anyone that has not had specific med voltage training do anything at this voltage!
Tight is tight, Too tight is broke.
Originally Posted by onetime
yes my supco megohm meter also uses a 9volt battery and has no voltage adjustments!
i been using it for about 10 years when i get a compressor tripping a breaker intermittetly it tells me the condition of the windings!
there will always be a more expensive way and better way to do things but i got some good service with my 9volt supco!
if your testing ultra expensive equipment for documented performance data you should probably get the best megohm meter available!
remember do not meg in a vacuum!
I use a Supco just like Airmechanical and it works great at least to the extent of my knowledge, thats why I asked for a model and mfg of yours so I could do some research as the Supco is what I was taught with and have never used or seen another style.
Originally Posted by techtalker
What exactly are you looking to test ? How does it serve your customers ? I have done PMs for years on all makes or RTUs and I think I am missing something here.
It can give you a good idea of the condition of the motor winding. For instance, a reading of infinity to 1000 megohm to ground would show the insulation is in fine condition. Fifty to 20 megohm......you got some problems. More than likely moisture and are headed to possible burnout. Change drier core and test oil for acid, change if it shows acidic. A reading of zero shows that somewhere the insulation is compromised and is shorting to ground. Time for another compressor. And as stated before, never meg a compressor in a vacuum. To do so could damage the winding due to arcing.
"Nearly all men can stand adversity, but if you want to test a man's character, give him power."
Most of the megometers sold in supply houses are rated to 500v you can find it somewhere in the paper work, you also can buy same type of Meg with 9V battery rated for 1000V.
what is the purpose? I have several meggers, including, supco,fluke and annie.
I used to meg stuff and I was told over 500 was marginal.
a guy from carlyle told me 100!
I megged an old 06E and had 25 megohms.
That was about 3 years ago, it still megs at 25!
true knowledge exists in knowing that you know nothing.
I have not seen any one talk about peak voltage that motors deal with every day. If you meg a 480 volt motor with 500 vdc then you are not really checking the windings any different then with your VOM. 480 volt peak voltage is almost 700 volts... 698 with out going to get my calculator out.
Originally Posted by berg2666
A VOM isn't as sensitive as the megohmer and according to Fluke the Nanoseimens test is not a replacement nor should it be used as a megohm test. The megohm test is a bleed test used to check insulation bleed to ground.