Question about Transformer Troubleshooting

[williamj001]

I was doing a job the other day and had some questions about a transformer and its correlation. I will try and explain the situation and would like as much help in understanding what was happening.

What I saw when i got on the job was a 3 phase 240 (what i thought to be a delta since there was no neutral, however there was also no stinger leg initially.) There was 120v to ground on all 3 phases. Between "A" phase and "B" phase was 240v, between "A" phase and "C" Phase was 240v, however between "B" Phase and "C" was no voltage.

There was a 240v to 120v Transformer being fed by a 20amp 2Pole Breaker on the "B" and "C" phases that fed a few 120v receptacles and lights. Because it was on the "B" and "C" phases which was not getting 240v between each other the transformer was not working properly. So I moved the transformer onto the "A" and "B" phases and the transformer now correctly worked.

This is where i got confused. After I moved the breaker feeding the transformer onto the "A" and "B" phases I was reading the following voltages to ground: "A" phase 120v, "B" phase 120v, "C" 206v. between any of the 3 phases i was reading 240v.

Sorry for the long post, but I would like to be thorough with what I do and need a better understanding of what is going on so any help is much appreciated.

 

[realolman]

So now there is NO load at all on the B-C phase?
The transformer was the only load?

question from Captain Obvious:
Line and load fuses all OK?

Are you using a digital multimeter?

 

[Pierre C Belarge]

The second readings sound like a mid-tapped Delta transformer hookup.

 

[winnie]

I am just guessing here, but I think that you have a very poor connection on the C phase; not a totally open circuit, but very high resistance. This could be at a splice or at the supply breaker.

With the load connected between the B and C phase, then phase B essentially 'backfed' through the load, and your 'A to C' voltage measurement was really an A to B voltage measurement. With the load removed, you actually made an 'A to C' measurement, presumably with a high impedance meter.

I suspect that if you were to connect a load from A to C, then your A to C voltage measurement would drop considerably.

-Jon

 

[George Stolz]

I agree with Jon, that would explain the symptoms perfectly.

Your "C" phase is the "stinger" or "high leg." It has a bad connection.

As Jon said, when you first arrived and read 120V from every phase to ground, when you tested between "C" and ground your meter was showing the voltage that "B" was dumping into it, through the load - 120V.

 

[realolman]

The second readings sound like a mid-tapped Delta transformer hookup.

But in order for that there would have to be a grounded neutral.

These readings were taken right at the transformer lugs or somewhere down stream?

 

[williamj001]

I suspect that if you were to connect a load from A to C, then your A to C voltage measurement would drop considerably.

-Jon

Yes! That is exactly right, when i put the breaker on the A and C phase I would get no voltage between the 2 and the transformer would only work when I put it on the A and B phase. That really helped to explain this situation and thank you very much!

 

[williamj001]

These readings were taken right at the transformer lugs or somewhere down stream?

Yes they were taken at the main lugs in the panel.

 

[williamj001]

So now there is NO load at all on the B-C phase?
The transformer was the only load?

question from Captain Obvious:
Line and load fuses all OK?

Are you using a digital multimeter?

Yes the transformer was the only load, however these readings were taken and read the same whether the 2 pole 20amp breaker feeding the transformer was on or off.

Yes the breakers all tested fine no fuses were used.

Yes I was using a fluke digital multimeter.