Using a megger for hvac/r

[rscamaro]

We have an oldish Biddle Megohmmeter, runs on 110v and produces up to 5,000vdc. Used it today on a big centrifugal motor, comes in handy when you want to know the condition of the insulation. It may even help make you look like you know what your doing.

...Ron

[aikenite]

I recently used mine to find a bad condenser motor that read o k with a meter.I will admit that i don't use it often,but in this case it saved the day!

[hvacrmedic]

I recently used mine to find a bad condenser motor that read o k with a meter.I will admit that i don't use it often,but in this case it saved the day!

Care to elaborate? Was it tripping the breaker?

[aikenite]

On a carrier 50tc there are 4 condenser fans.They are fed by 15 amp fuses The fuses had blown several times and other guys would just replace the fuses and it would work for a day or two.I decided to find and correct the problem so I isolated each motor and megged the motors.1 motor gave odd readings compared to the other 3.We ordered a new motor and replaced it The unit has been working fine for a couple months.Like i said ,I don't do many start-ups so when I break out my megger I am looking for something unusual.By the way, the motor that was bad had a lot of oil inside as if the bearings were bad.

[hvacrmedic]

On a carrier 50tc there are 4 condenser fans.They are fed by 15 amp fuses The fuses had blown several times and other guys would just replace the fuses and it would work for a day or two.I decided to find and correct the problem so I isolated each motor and megged the motors.1 motor gave odd readings compared to the other 3.We ordered a new motor and replaced it The unit has been working fine for a couple months.Like i said ,I don't do many start-ups so when I break out my megger I am looking for something unusual.By the way, the motor that was bad had a lot of oil inside as if the bearings were bad.

Repeated over amping and overheating of the windings could break down the insulation enough to show a difference in leakage current between that and the other identical motors. I wouldn't have thought of testing for bad bearings like that, but in hindsight the idea seems to have some merit despite its lack of conclusiveness. Thanks for sharing that.

[aikenite]

I wasn't looking for bad bearings, that was just something that I did notice about the motor.As far as being conclusive the unit is working fine ,that's good enough for me.There are times when you're good and other times when you're lucky.I bought my megger at a pawn shop for around $40.00,but in this case I wouldn't take a gold quarter for it.When I started out I debated whether or not to get one but I couldn't pass this up.There haven't been many times that I have had to resort to this but I don't like to borrow things and it got the job done.Hope you have a great week!

[maxster]

if the motors are 460v they have a higher rate of going nuke then 208V ones.might want to take a megger reading on a new one just to see if the others change over a season or on spring start up after a blizzard or two melts into the windings and those top bearings..

[Lightning_Boy]

Checked grainger link. No longer in catalogue.
Thinking of spending $600-$800 on a good one.
Is it worth going upto a 5000vdc capable unit or not?? Will the readings be more accurate?

Thinking Mitchell, Megger or aemc at the moment...

[Russ57]

I love my old hand crank Biddle. If buying new today I'd go AEMC.

Match the megger to the voltage classes you work with. If you work on 4,160 motors you need a 5,000 VDC unit.

IMHO top uses for a megger are establishing trends and looking for problems that aren't obvious. Can be a time saver on shorts that come and go or are moisture related. It does require that one have an idea of what a pass/fail reading is. I would not call 100,000 ohms to ground "okay". What is okay on a 120 VAC motor isn't the same as what is okay on a 4160 motor.

Also need to know when not to use one, such as a hermatic in a vacuum.

[kaptnkirk]

I am required to do a Meg/Ohm test of the compressors on my annual inspections. Customer requirement. 2 Ton to 20 Ton. I usually do it at the contactor???

[LU91Tech]

Any rule of thumb on which voltage to do a test. Our megger at work has selectable voltages of 500,1000,2500,and 5000 to test with.

[hvacrmedic]

Any rule of thumb on which voltage to do a test. Our megger at work has selectable voltages of 500,1000,2500,and 5000 to test with.

Typically around twice the supply voltage. For a 230 volt motor use the 500 volt setting, etc.

[Lightning_Boy]

I currently don't work on anything bigger than 460/480v.
Is it worth getting a 5000v capable meg ohmmeter ?
Really liking the AEMC stuff. 1050 model has my attention but not my budget :0(