Muliple motors B phase inbalance

[giannistugatti]

I have a 150amp branch circuit, 480v 3phase feeding a 480v 3p 2horse fan motor, a 480v 3p 2horse air compressor motor, and a 3p 480v to 208v 40 kva transformer. The transformer is suppling a 3p 2horse motor for a large commercial iron, heated by natural gas and the other load on the transformer is a 3p motor suppling a large tent washer which is 2horse. When everything begins to run and heat up the overload breakers on the iron's panel trip and when I check the votages on all three phases at the iron's discconect the B and C phase are 90v and 80v respectively. The B phase on the 480v side of the branch circuit drops to 180v with no amprage draw. The 480v compressor motor still runs, the fan motor stops, and of course the iron stops becouse of its overload breakers. The 480v branch circuit is supplied from a 3 wire 480v 3phase bus duct with just a ground. Any ideas as to why the phase inbalance??

 

[Dennis Alwon]

What is the voltage on the load side of the trany with no load on the trany.

 

[giannistugatti]

208v across each phase and 119 to 120 to ground.

 

[ActionDave]

I have a 150amp branch circuit, 480v 3phase feeding a 480v 3p 2horse fan motor, a 480v 3p 2horse air compressor motor, and a 3p 480v to 208v 40 kva transformer.
The transformer is suppling a 3p 2horse motor for a large commercial iron, heated by natural gas and the other load on the transformer is a 3p motor suppling a large tent washer which is 2horse.

When everything begins to run and heat up the overload breakers on the iron's panel trip and when I check the votages on all three phases at the iron's discconect the B and C phase are 90v and 80v respectively. The B phase on the 480v side of the branch circuit drops to 180v with no amprage draw. The 480v compressor motor still runs, the fan motor stops, and of course the iron stops becouse of its overload breakers.

Welcome to the forum. If I may, hitting the "Enter" key, to break up the text and organize information, makes for easier reading.

The 480v branch circuit is supplied from a 3 wire 480v 3phase bus duct with just a ground. Any ideas as to why the phase inbalance??

How old is the installation? Did it ever work correctly? If so, what has changed?
What happens if you isolate parts of the circuits or equipment?

 

[giannistugatti]

The original 480v branch circuit only had the2 horse 480v vent fan motor on it. It was Feb. when I first introduced the old air compressor with the 480v 2horse motor. We did not run the exhaust fun because of the weather. The compressor motor did act up before the transformer and comercial iron was introduced into the branch circuit. The B phase read a low voltage but it was not as low as it reads now when this inbalance happends

 

[Jraef]

The original 480v branch circuit only had the2 horse 480v vent fan motor on it. It was Feb. when I first introduced the old air compressor with the 480v 2horse motor. We did not run the exhaust fun because of the weather. The compressor motor did act up before the transformer and comercial iron was introduced into the branch circuit. The B phase read a low voltage but it was not as low as it reads now when this inbalance happends

That's a nasty voltage drop you've got there, even if you are referencing to Neutral or Ground. But are you? And is it a 4 wire system? Voltage references to ground on 480V ungrounded Delta are not exactly reliable indicators. Shouldn't be that low, but without knowing what you have, it's impossible to draw any conclusions from that yet.

 

[giannistugatti]

It's a 3 wire 480v system. The bus duct is an old SquareD I line. What I found today was the B and C phase (500 mil) cables did not the match up to the B and C phase of the 480 panel. I corrected them. The 480v 3phase compressor started right up. the 40kva 480v to 208v transformer gave the correct output, and the other devices powered from the transformer worked correctly, except for the commercial iron. When the the internal overload tripped a breaker in the iron panel, that's when when the voltages went crazy this time the C phase on the 480v droped votage and had no amp reading. Once the iron was disconnected the 480v 3 phase cleared itself and thus everything went back to normal.

 

[kwired]

That's a nasty voltage drop you've got there, even if you are referencing to Neutral or Ground. But are you? And is it a 4 wire system? Voltage references to ground on 480V ungrounded Delta are not exactly reliable indicators. Shouldn't be that low, but without knowing what you have, it's impossible to draw any conclusions from that yet.

I think he is saying there is a voltage drop on the 208/120 transformer secondary on at least two of the phases to ground,

It would be nice to know what all three phases to ground plus phase to phase voltage is when this happens, and possibly what the voltage is on the 480 volt primary all three phase to phase, while the condition exists.