View Full Version : 60hp Motor runs with grounded winding
01-14-2010, 09:43 PM
Same post in the theory forum, but got zero replies so here it is for you freon jockeys.....
Commercial building fed by a bank of transformers supplying 460V 3 phase - 3 wire to the building main panel - no utility ground or neutral. Up on the roof, 5H60 open drive compressor driven by 60hp 364T frame motor. Motor amps while running are 74A (fla = 77A). Check voltage on each phase to ground with motor off -A 262V, B 280V, C 270V.
Check voltage to ground when motor is running A 0V, B 482V, C 490V - wtf? Check windings to ground with ohmeter - short conditon about 2 ohms to ground. However, motor runs and there is 480VAC across all 3 phases. I'm assuming that this behavior is because there is no utility ground bonded to the building. My concern is with the 60hp drive motor. The owner says "hey it still runs, why change it?" Anyone have any comments as to the potential damage and danger here?
01-14-2010, 09:52 PM
Wow, that is a little scary. My concern would be getting shocked by the motor. That 0 volt from L1 to ground is nutty. I would write it up and wait for a call, it will fail.
01-14-2010, 09:53 PM
Its a corner grounded delta. Easily put, one leg and ground are the same. That is the leg you get 0v on. http://ecatalog.squared.com/pubs/Electrical%20Distribution/Switchboards/QED-S/2700DB0202.pdf
01-14-2010, 10:04 PM
According to DWP it is not a corner grounded delta. This is evident when the motor with the grounded winding is not running - there is about 260VAC to ground on the A phase. When the motor is running there is 0 VAC to ground on the A phase.
01-15-2010, 12:00 AM
I've seen similar symptoms only twice in 30 years. Most people still don't believe me. Both times the compressor was showing a dead short to ground, yet the compressor ran fine during operation. The condenser fan would ran at half speed on the off-cycle because the contactor was a 2 pole and the compressor was grounded. That was the only clue that there was a problem.
Your compressor has a ground on the A phase but it won't trip the CB because there isn't a good ground, so it does not trip. Current still flows through phases A-B-C correctly. No current or very little current flows to ground.
Only problem is that the compressor body is "hot" and somebody can be shocked if they touch the compressor and they are grounded. Be careful!
01-15-2010, 06:16 PM
Is that 5H60 direct drive or belt drive?
You may want to really consider getting the new motor on site before the old one goes. I lost a 60hp direct drive 5H60 and it took 2 weeks for a new one to come in. We had to get a refer truck on site to replace a few coolers we had to shut down to keep the cases going. We now keep a spare just in case...
01-15-2010, 07:17 PM
I've only ever seen something similar one time, though the motor had a distinct growling sound when it was running. This was on a 40 hp built up air handler and you could light a 120 volt trouble light with it. One thing you might try is to find a good earth ground near it and check for voltage between the motor case and that ground while it's running.
Oh and I'd watch my ass working around that thing 'cause it's gonna get somebody before long!
01-15-2010, 07:57 PM
Hold this ground wire and now touch the motor, oh your still alive OK should we change the motor now?:eek2:
Have an electrician get a good ground run to Ur equipment!
01-15-2010, 11:07 PM
I saw a floating 240 delta do some funny things when a single phase fan motor grounded out one time. It wasn't running very happy, but I had over three hundred volts to ground on one leg when the motor was trying to run, and the normal floating delta when it wasn't.
Sounds like your transformer bank is Y connected, and there are only three phases of it coming at you. And the machine you're working on isn't isn't grounded.
This is an illegal hookup, and can get you killed. You don't want to be the one completing the circuit to ground.
Get some body with you who knows what they're talking about before you get killed.
It takes less than a tenth of an amp of current to kill most people.
Don't mess around with this installation, It's a job for the undertaker waiting to happen.
Even if you change out the motor with an internal ground fault, the installation is still deadly. This is serious business, my friend. Somebody at home is counting on you.
01-16-2010, 12:15 AM
Dang copper thieves, they done stole the ground wire comming into the building. :p I agree with the previous posters, get a sparky out there to get this thing up to code before someone gets hurt. Gotta love some of the screwy stuff we see sometimes.
01-19-2010, 04:18 PM
Now the end of this story. After I scared the crap out of the owner with the dire consequences posted on this thread he gave me a PO to change the motor. Found a new motor and was ready to schedule the crane, order a mag, and center cplg. I sent my service mgr to the jobsite to figure out the rigging and asked him to verify the grounded winding at the motor leads. I had previously checked and found the ground condition by checking at the load side of the mag starter.
When he pulled off the peckerhead he found the kearny connector on one of the leads had worn through the insulation tape and was rubbing against the inside of the conduit fitting. After retaping and checking continuity, the short to ground was gone! Fired up the motor and now all is well. I told the owner what we found and he was so happy to have saved a large chunk of change as well as employing an honest contractor.
So, here's the lessons learned,
Checking voltage to ground with a 3 phase motor running can sometimes indicate a grounded condition
When suspecting a grounded winding - always confirm winding condition by checking for continuity by disconnecting the motor leads from the starter load side wiring.
Peckerheads should be checked at regular maintenance intervals.
Thanks everyone for your input,
01-20-2010, 12:33 AM
Question on this situation?
Even if bldg didn't have a bonded ground, this sounds like a heavey piece of equipment. Possibly sitting on some structural steel.
This structural steel wouldn't give you a good enough ground trip it out?
Also how does a motor this size stay on line even with a partial ground?
01-20-2010, 12:48 AM
There was no utility supplied ground to the building from the power origination. The power supply came from a 3 wire 460 (no neutral or ground) power supply - often referred to as a Delta configuration.
While there is an earth ground at the building it is many miles away from the nearest utility ground connection. There is considerable resistance from one stake in the ground to another. Basically there was no path for current to flow back to it's source.
This is probably a good subject for a new thread - the difference between grounding and bonding.
01-20-2010, 05:36 PM
...now all is well. I told the owner what we found and he was so happy to have saved a large chunk of change as well as employing an honest contractor.
...And a ton's worth of gold in customer good will. :angel:
01-20-2010, 05:36 PM
01-20-2010, 06:34 PM
...And a ton's worth of gold in customer good will. :angel:
Ya he's mine for life now. That is until the building gets sold.....
01-20-2010, 08:49 PM
I have never come accross this type of situation before. It would make a excellent thread.
01-20-2010, 11:04 PM
Ive seen this before, it was a very long time ago in S.F. I think a transformer with ungrounded neutral or something. A lot of guys were afraid to work there because you could see voltage at ground on other machinery with a failed motor somewhere else. I never got shocked because of this, but it made troubleshooting a pain. I cant remember the exact reason other than it had to do with the stepdown x-former within the building.
01-21-2010, 02:38 AM
I`ll never forget my first experience w step down transformer w/ grounded tap , such as absbrtek mention , power phase to phase, but one leg 0 to ground at earth potential, quite perplexing. even after several conversations and reading years later still dont fully understand, simpliflied concept , is single phase circuit still has current on neutral , but negative potential. regards
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