Megging Compressors.

[keith1]

Crossed over from the residential side to the commercial side of the business about 2 years ago.Not much experience using a megger so any input would be greatly appreciated.
Came across a 10 ton trane roof top today,3 phase unit,208/230.Found nothing running-1st check was on secondary side of transformer and no 24vac.Checked primary side of transformer and was only getting one hot leg.Went to fused disconnect and found a bad fuse.Looked for any obvious burnt or shorted wires,ohmed loads to ground-ok.ohmed evap and condensor motor windings-ok.
When I ohmed out compressor with megger set to ohms only I was reading .5 ohms between common/start,common/run and run/start,I switched to megaohms on 500volt scale and put black lead to ground and tested each compressor terminal I got over 100 mega ohms on common and same for start and run.I'm assuming this is fine according to the manual.
I guess what I'm asking is did I do this right and are my readings acceptable.I did later find that on the contactor between L1 and L2 the plastic separator between contacts was cracked,wondering if maybe I was getting any arcing occasionally.I changed out the contactor and ran unit while I went over to another roof top issue and about 2 hours later unit still running.Fla on compressor was rated at 26 amps,when checked I was getting anywhere between 22 and 24 amps.
This was long,I apologize.Thanks so much in advance.Oh yea,if you get good readings on mega ohms-is there any need to go further with that compressor as for tesing?

[sonc]

"is there any need to go further with that compressor as for tesing?"

Sounds like you covered all the bases, you went further than I would have for just a blown fuse. I might have thrown a totaline test tube on it to make sure the gas was still good (if I liked the account). You probably tightened all the lugs?
Good!

BTW, Don't refer to the posts of a 3-phase compressor as CSR,..(It's a dead giveaway for a rezi guy.)

Check contactors and relays for pitting, burnt wires, etc.

Voltage ok, current ok,
Welcome to the Commercial side..never look back...

No more stepping in litter boxes or arguing over 10 minutes travel time with Mr. Wilson.. :)

[fatboy]

When I megohm compressors on pm or service work, I use the data to establish a trend and then compare these readings from pm to pm to see if the megohm value is consistent or dropping.

other factors that can influence your reading can be motor temp(if comp hasn't run in a while/is cold)your readings could be misdiagnosed.

I usually do a 1 min and 5 min reading and record

fatboy

[keith1]

Thanks for the replies,I'll try to break that habit as far as calling three phase posts on compressor CSR,20 years residential experience,work with me-LOL.I truely dont miss working on residential.Since over in the commercial/industrial side its been very rewarding in many ways.Thanks again.

[maintmankc]

BTW, Don't refer to the posts of a 3-phase compressor as CSR,..(It's a dead giveaway for a rezi guy.)

I am going to argue that as 99% of three phase units have CSR stamped on the housing and it makes for a convenient reference for PM cards and service slips.

[caosesvida]

keith1 if you do a search for meggers on this site you will find very deep discussions on the subject.