Phase loss on single phase 480 transformer

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jimbob843

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Ok guys so I’ve been trying to find the answer to a problem I’ve recently come across. Here’s the scoop. A dock transformer has lost power. It is fed from an MDP at the head of said dock. High winds caused a fault and caused the breaker to trip. It wouldn’t reset. So after troubleshooting I found a leg had been pulled apart from its splice inside a hand hole in the dock. Full of water, caused the fault. No big deal. Pull more in from the strain relief and made a better splice. Now breaker holds. Go to test the primary at transformer and get 277V on phase A and 140V on phase B. This is a single phase 480 to single phase 240 transformer. Now I know the phase B is most likely damaged and not supplying adequate voltage. But wouldn’t the A phase side of the transformer still proved an output given it has 277v on the primary? It’s not putting out anything and just doesn’t seem to be working at all. No signs of damage inside of transformer and I’m kind of stumped. Any suggestions at this point before further investigating? Does the transformer need both phases adequate in order to work?
 
Yes, the transformer needs both supply conductors to work. It looks like B phase is open from the voltages you gave.
 
jimbob843 said:
Ok guys so I’ve been trying to find the answer to a problem I’ve recently come across. Here’s the scoop. A dock transformer has lost power. It is fed from an MDP at the head of said dock. High winds caused a fault and caused the breaker to trip. It wouldn’t reset. So after troubleshooting I found a leg had been pulled apart from its splice inside a hand hole in the dock. Full of water, caused the fault. No big deal. Pull more in from the strain relief and made a better splice. Now breaker holds. Go to test the primary at transformer and get 277V on phase A and 140V on phase B. This is a single phase 480 to single phase 240 transformer. Now I know the phase B is most likely damaged and not supplying adequate voltage. But wouldn’t the A phase side of the transformer still proved an output given it has 277v on the primary? It’s not putting out anything and just doesn’t seem to be working at all. No signs of damage inside of transformer and I’m kind of stumped. Any suggestions at this point before further investigating? Does the transformer need both phases adequate in order to work?
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Yes it should still put out some voltage if there is voltage on the primary. It's a 2:1 ratio so it should be half.

When you say you tested the primary, what were you testing it to? Ground?

What was the voltage phase to phase on the primary? I bet it is zero and that is why you don't get any voltage on the secondary
 
See that’s what doesn’t make sense to me. I pulled the wires from the primary side lugs. Energized them. Just wire I found A phase to ground is 277 and B phase to ground is 140. Even with 277 on A phase and landed on the primary, there was still no voltage on its secondary side. I’m aware of an issue with the wire for B phase. But given its input voltage to primary, shouldn’t there be voltage on the secondary?
 
jimbob843 said:
See that’s what doesn’t make sense to me. I pulled the wires from the primary side lugs. Energized them. Just wire I found A phase to ground is 277 and B phase to ground is 140. Even with 277 on A phase and landed on the primary, there was still no voltage on its secondary side. I’m aware of an issue with the wire for B phase. But given its input voltage to primary, shouldn’t there be voltage on the secondary?
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Your measuring one thing and connecting another. If you connected “A” phase and ground to the primary then yes, it would have put out voltage on the secondary. Your connecting “A” and “B” to the primary which have no potential because there is an open on “B”. If you measured “A” to “B” there would be no voltage so of course the transformer would not put out anything on the secondary.



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Eddy_Current said:
Your measuring one thing and connecting another. If you connected “A” phase and ground to the primary then yes, it would have put out voltage on the secondary. Your connecting “A” and “B” to the primary which have no potential because there is an open on “B”. If you measured “A” to “B” there would be no voltage so of course the transformer would not put out anything on the secondary.



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ahhhh I see. Thank you sir
 
jimbob843 said:
Thanks I just wasn’t sure. I’ve never encountered this particular situation. It still doesn’t seem right that it just wouldn’t put out anything. No humm buzz or nothing.
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To get output voltage on the secondary you must have current through the primary. Line to ground voltage is mostly meaningless, line to line volts is what will drive current through the primary, and you never mentioned what that value may be.

Next step other than measuring line to line volts (still fairly certain something is wrong with your phase B conductor) is to make sure you have 480 volts line to line at the feeder device, if not the problem is upstream from that device.
 
jimbob843 said:
See that’s what doesn’t make sense to me. I pulled the wires from the primary side lugs. Energized them. Just wire I found A phase to ground is 277 and B phase to ground is 140. Even with 277 on A phase and landed on the primary, there was still no voltage on its secondary side. I’m aware of an issue with the wire for B phase. But given its input voltage to primary, shouldn’t there be voltage on the secondary?
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I expect that you are measuring the voltage with a high impedance input meter and you really don't have any voltage on B phase. You are just reading capacitive coupled or ghost voltage. You need to have power available between A and B to have any secondary voltage. I don't think you have that. What is the voltage across the transformer primary with the conductors connected to it?
 
To possibly clarify what I said earlier, your 480 volt primary on your transformer has no connection to neutral. If you only hook up H1 lead and nothing else you have no current through the primary, no curent on primary means no magnetism is produced by primary to excite the secondary coil and produce secondary voltage.

Your low volts you did read on the other line is possibly capacitively coupled and can only be read with a high impedance meter as suggested. Connect it to a real load and that "capacitor" is shorted across and there is no sufficient power behind what was charging that capacitor to supply any significant voltage/current, and there is indeed an open circuit condition somewhere between that second primary transformer terminal and the source.
 
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