3 (?) Phase Question

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newservice

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Location
Syracuse NY
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Electrician extraordinaire
Hi, I am a limited master license. Recently a customer had me look at a compressor he wanted to install at his car wash, to replace one he said had stopped working. On examination, he had a 415V, 3 phase motor not working but he didnt care, he wanted the new compressor installed so I did. The building was served by a single phase overhead drop from the pole which went to a 120/240v panel. Simple enough, and this is where I installed the compressor as the nameplate on the motor said single phase 240v and showed a line to line connection , no neutral.. All well and good.

Where it got interesting was looking at the 3 phase. The old motor was connected to only 2 of the phases, and one of them had a blown cartrige fuse in the 3 pole disconnect. When checking the voltages, I find 240 line to line on two of the wires and no other line voltages. Wheres the third phase? Seemingly worse yet, the line to ground (didnt have a neutral nearby to measure against so I went to the rigid pipe) ..was 240V for each of the two lines. Next I went back outside to look at the 3 phase drop, and it was only 2 line wires and a ground, just like the single phase! What gives? Im assuming the 3 phase is supposed to be a 240/416, so wheres the other wire from the pole transformer, and why is the line to line same as the line to ground? Am I missing something or is there a big problem? Given the massive size of these motors, I cant believe they would be made for just phase to phase connection and not a full 3 phases, but if they are, would the POCO supply only 2 wires of the 3 phase and this still doesnt explain the neutral to line being the same as line to line, unless, did they use the uninsulated neutral service drop conductor as the line and omit a neutral?
Didnt have a chance to pull the front from the panel as he would have had to shut down the car wash but thinking thats one of the next moves, because even if it is only 2 of 3 phases should still be 416V between them?
Thanks in advance for your thoughts.
 
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Yes, both single and 3 phase overhead drops from a very congested pole out front, having 3 transformers at the top. And yes the new single phase compressor works fine. The three phase panel has all 3 pole CH breakers, and is fed from a massive old disconnect switch. Havent been inside that panel to see whats up yet.
 
thats what I was thinking but ..with the two services in the same building, how could they do that..ok i have to do some more thinking...and the old motor appeared to have a wire that read 0 volts to ground and two that had 240 between them and 240 to ground
 
newservice said:
thats what I was thinking but ..with the two services in the same building, how could they do that..ok i have to do some more thinking...and the old motor appeared to have a wire that read 0 volts to ground and two that had 240 between them and 240 to ground
Click to expand...

So there was 3 wires brought to the motor and 3 wires were connected, I think kwired has

it right.
 
Except that there was no reading between the first wire and the other two. And there was no 416 between the two, only 240. I think I need to get in that panel and go from there. Good call on the corner ground system thanks for that.
 
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OK did some reading. Apparently a corner ground delta system would have 240v all the way around, no 416v. Yet this guys motor nameplate says 415. I think Im seeing why the motor doesn't work now. :)

Also found that the grounded service drop conductor should be insulated, it is not. Be interesting to see what his other motors are rated at.

Should the single and 3ph have isolated grounds from each other at the premisis? Is there current flowing in that grounded conductor, or just the unbalance, ..i need some more reading.
 
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