Help trouble shooting 3 ph motor circuit - Electrician Talk - Professional Electrical Contractors Forum

baronson · 07-01-2012, 09:04 AM

I was called in to find out why a condenser wasn't working. Turns out the panel (Panel B-120/240 3 ph) feeding the motor controller... A phase to C 240v, B to C 240v, A to B no voltage.

I opened up Panel A (MBP) took out the breaker (which was NOT tripped) feeding Panel B and found major arching signs only on B ph.

Panel B has mostly 1 ph loads and only 2 -3 ph loads- one of which isn't used. So the only 3 ph load in that panel is the condenser. I reset the breaker in Panel A which put my voltages in Panel B normal between phases and then turned on the condenser. Which in turn made my voltages again--nothing between A and B, all the way back to Panel A.

My question are- why didn't any of the breakers trip when the condenser is turned on and why would turning the condenser on only affect my B phase (essentially taking B phase off line).

Thanks for your help I don't have experience troubleshooting 3h motors.

HARRY304E · 07-01-2012, 09:25 AM

Quote:

Originally Posted by baronson

I was called in to find out why a condenser wasn't working. Turns out the panel (Panel B-120/240 3 ph) feeding the motor controller... A phase to C 240v, B to C 240v, A to B no voltage.

I opened up Panel A (MBP) took out the breaker (which was NOT tripped) feeding Panel B and found major arching signs only on B ph.

Panel B has mostly 1 ph loads and only 2 -3 ph loads- one of which isn't used. So the only 3 ph load in that panel is the condenser. I reset the breaker in Panel A which put my voltages in Panel B normal between phases and then turned on the condenser. Which in turn made my voltages again--nothing between A and B, all the way back to Panel A.

My question are- why didn't any of the breakers trip when the condenser is turned on and why would turning the condenser on only affect my B phase (essentially taking B phase off line).

Thanks for your help I don't have experience troubleshooting 3h motors.

Did you install the exact same breaker in Panel A?

If so then that breaker may be the problem.

You said you found major arching signs only on B phase ,If that is on the busing in the panel then you need to fix that as well.

baronson · 07-01-2012, 09:33 AM

Quote:

Originally Posted by HARRY304E

Did you install the exact same breaker in Panel A?

If so then that breaker may be the problem.

You said you found major arching signs only on B phase ,If that is on the busing in the panel then you need to fix that as well.

The breaker in Panel A which feeds Panel B, was replaced a few months ago by another electrician. As it was Sat. morning and no supply houses open nearby until Monday, I took a 3pole 100 amp breaker that was feeding another panel, took those wires off and terminated my wires to Panel B. Doesn't it seem strange that someone had replaced that breaker once before...that makes me think its the load?

Loose Neutral · 07-01-2012, 09:52 AM

What kind of transformer is feeding the 240/120 panel. If it's a closed delta, i can see you having a problem with one of the windings in the load going bad and grounding out that phase giving you those readings.

baronson · 07-01-2012, 10:01 AM

Quote:

Originally Posted by Loose Neutral

What kind of transformer is feeding the 240/120 panel. If it's a closed delta, i can see you having a problem with one of the windings in the load going bad and grounding out that phase giving you those readings.

Is there a quick or easy way to explain why a closed delta xrmr would create that scenario?

hardworkingstiff · 07-01-2012, 10:12 AM

Did you take voltage readings at the MDP when you were missing voltages at panel B? If so, what were they?

Loose Neutral · 07-01-2012, 10:13 AM

I've seen situations that if you have a closed delta and one phase grounds the system doesn't trip you just get 0 volt reading on that phase. That's why you need ground fault detectors on those systems. May or may not be your case just throwing it out.

baronson · 07-01-2012, 10:51 AM

Quote:

Originally Posted by hardworkingstiff

Did you take voltage readings at the MDP when you were missing voltages at panel B? If so, what were they?

I took a reading across the busses and all was good. It was when I tested voltage on the one breaker feeding Panel B- no voltage between A and B. That is when I took the breaker off and saw the arch markings.

hardworkingstiff · 07-01-2012, 11:02 AM

Quote:

Originally Posted by Loose Neutral

I've seen situations that if you have a closed delta and one phase grounds the system doesn't trip you just get 0 volt reading on that phase. That's why you need ground fault detectors on those systems. May or may not be your case just throwing it out.

The OP stated he had a 120/240-V 3-phase service. That would indicated the mid-point of one of secondaries had a ground reference and any phase going to ground would cause sparks.

Now, if the OP misstated the service, we have a different scenario.

InPhase277 · 07-01-2012, 12:01 PM

Sounds like one of the phases is lost. Voltage is appearing on that phase through a load, sometimes.

What about the voltage at the service when everything is off?

Loose Neutral · 07-01-2012, 12:12 PM

Quote:

Originally Posted by hardworkingstiff

The OP stated he had a 120/240-V 3-phase service. That would indicated the mid-point of one of secondaries had a ground reference and any phase going to ground would cause sparks.

Now, if the OP misstated the service, we have a different scenario.

Right, what the hell was i thinking?

baronson · 07-01-2012, 12:24 PM

All voltage was normal across the busses at Panel A. It was when I took the breaker feeding Panel B off, that I saw arching on the B phase. Before I took the breaker off all was good at the panel except for the readings on the wire coming off the breaker feeding Panel B.

hardworkingstiff · 07-01-2012, 12:26 PM

Quote:

Originally Posted by baronson

All voltage was normal across the busses at Panel A. It was when I took the breaker feeding Panel B off, that I saw arching on the B phase. Before I took the breaker off all was good at the panel except for the readings on the wire coming off the breaker feeding Panel B.

So the breaker's bad?

baronson · 07-01-2012, 12:39 PM

No, I took the breaker out and instead took a set of wires off of another 100 amp 3 pole and terminated my Panel B to that new breaker. I had regular readings down the line (at panel B and at the motor controller). I turned on the condenser and now I don't have voltage across A and B all the way back to the breaker in Panel A.

hardworkingstiff · 07-01-2012, 01:26 PM

Quote:

Originally Posted by baronson

No, I took the breaker out and instead took a set of wires off of another 100 amp 3 pole and terminated my Panel B to that new breaker. I had regular readings down the line (at panel B and at the motor controller). I turned on the condenser and now I don't have voltage across A and B all the way back to the breaker in Panel A.

It sounds like you've lost B phase completely and when there is no load on it you see a ghost voltage and then when a load is supplied the voltage goes away. Are you testing with a DMM or do you have a wiggy available?

baronson · 07-01-2012, 01:49 PM

Fluke multimeter. All voltage was what you'd expect once I changed breakers and position in panel -all the way from the MBP to sub panel to motor controller and then once I started the motor--it all went back to nothing between A and B --all the way from motor controller to sub panel to MBP.

A bad winding in the motor making B phase ground out but doesn't trip any breakers?

hardworkingstiff · 07-01-2012, 04:56 PM

You need to have a load across your test points (with your meter) to make sure "ghost" voltages are dissipated and you read actual voltage. One possibility is you have actually lost one leg (probably the high leg) and when you turn on the 3-phase unit the ghost voltage goes away.

noarcflash · 07-02-2012, 11:27 PM

bolt on breaker, or push on ?
the arcing would indicate your problem area of contact.