case grounded compressor?

Paul S_3

Hey everyone....I went on a service call yesterday and found the breaker tripping to the condensing unit....I condemned a faulty compressor ...I was not getting continuity from any terminal to ground...but I was getting resistance from any terminal to ground (I was getting 357 ohms)...my question is what exactly is going on in the compressor? Why want I getting the beeping from my meter on the continuity setting....and why was there resistance? Thanks Paul S

Jamie Hall

Somewhere in there...

there is a moderate resistance short to ground, probably in a motor winding.  There should never -- like never! -- be a resistance less than 1 meg and preferably 10 meg from any terminal except neutral (if it's present -- it sometimes isn't, depending on the wiring) to ground.  Neutral should show at most a quarter ohm or so to ground.



Different multimeters interpret "continuity" differently, but commonly they mean less than a few ohms.  Most would not interpret several hundred ohms as continuity.

Paul S_3

Meg

So should I get a megaohmeter....would that be helpful in this situation...Paul S

RJ_4

meter

what setting is your meter on,   I recommend getting a megohm meter and re checking.   always check the elec circut.  for loose wiring or burnt comp. contactor contacts before condeming compressor. if it is single phase it could be a bad capacitor.    If your not getting contiuity to ground it may not be grounded

Jamie Hall

Yes indeed

burnt contacts, loose contacts -- bad cap. in single phase.  All possibilities.



I do hope, though, that the case and frame are all properly grounded!  There must be a ground wire back to the switchboard (green or bare); this is, so far as I know, without exception a code requirement.  Neutral (white) if it's present, should show low or no resistance to ground.  Hot (black or red or both) should show one megohm plus to ground or neutral (if present) and to each other if there are both black and red when the unit is off and the fuses/breakers are pulled (don't play with this stuff unless they are!) -- but there is an exception to that: some more modern units have a computer board in them which draws power at all times, whether the unit is off or on.  That may call for a little more creativity in checking!

Paul S_3

compressor

The meter was on continuity and I wasn't getting a beep to ground....I always check contractor for voltage drop...the capacitor was a 40/5...I was getting that exactly...the breaker kept tripping...so I pulled the wires off the compressor and reset breaker it did not trip and I had voltage at the wires...them I set my meter to ohms...I got a reading from each terminal to ground of 357 ohms...so I condemned the compressor...Paul S...(condenser is six months old....what could of caused that?)

Techman

grounded compressor

Multi-meter that have a "continuity" checker ,that is for very low resistances. It is not for any kind of "hi resistance" readings. Many meters have two "ohm" scales ,lo & hi . The continuity setting will not work for testing for "ohms" . Then depending on the type of wire/winding problem in the comp motor the "short to ground" may be lo ohms or hi ohms, depending. Having a tripping CircuitBreaker w/ the comp wired in , and the CB not tripping with the comp wires removed sure indicates a bad comp motor.



A Meg0meter is not an average tool for an average mechanic. So I would shy away from that meter for now.



Jamie Hall, having a neutral  wire indicates a 115v system or a sophisticated system. Most residential comp motors are 1 phase ,220v , not using a neutral. On 115v comp motors , the Common, Start, and Run terminals are isolated from the rest of the wiring when testing ,so , the "neutral" feed of  the comp should not be grounded or shorted . The C ,S , & R terminals and the two windings ( S & R ) are isolated. Respectfully, Terry.

Jamie Hall

Just trying to cover all the bases

" Most residential comp motors are 1 phase ,220v , not using a neutral.

On 115v comp motors , the Common, Start, and Run terminals are isolated

from the rest of the wiring when testing ,so , the "neutral" feed of

 the comp should not be grounded or shorted . The C ,S , & R

terminals and the two windings ( S & R ) are isolated"



Quite true.  I was just trying to cover all the bases... and perhaps confused the issue.  Sorry...

Techman

Grounded comp

Jamie, or  I mistook your explanation . I read a lot of your stuff on Steam Heat. You are quite knowledgeable.

unclejohn

You weren't

Getting a reading to ground because your compressor wasn't grounded. It was shorted. There is a difference.

Unknown

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