Newsgroups: sci.engr.electrical.compliance
Subject: Re: 3phase power question
References: 
"Dmx Designer"  writes:

> Hi ... I have a 3phase question for you. (Our local theater has 3ph. and 
> only using 2 legs for lighting right now.)
> 
> I just metered it tonight and here's what I got.  A-B=237, A-C=234,
> B-C=236.  To ground and nuetral, the three legs read: A=115, B=207, C=114.
> 
> I took these readings with a decent digital meter right on the mains coming
> in. There was very little load on the system at the time.  I don't see any
> way that I can use the B leg. Do you?
> 
> By the way, I went to another 400 Amp feed that comes into the other part of
> the building and got the exact same voltages.
> 
> Is there something wrong with the B -Leg coming in at 207v and the other 2
> legs at 115? I thought all three legs should be equal voltage 120degrees out
> of phase?

The classical three-phase system have three legs that have equal voltage 
120degrees out of phase. Those classical system can be depending 
on the situation be groudned or not. The three-phase transformer 
(or transformer group) could hve it's output wired in delta or 
star way. In case of star wiring the center point of the transformer 
could be grounded to ground or not (depends on application). 
Other wiring method is delta where there is no center point to ground. 
Many delta wired outputs are run as floating "no ground". 
In some situations there are needs to ground such systems. 
Delta output can be either corner grounded or grounded from the 
center of the one phase generating output coil on transformer. 

In your case you have a system that delta outptu grounded from 
the center of one transformer secondary coil. 

> Another guy locally thought it was either a grounding issue, or a bad tap on
> the transformer.

I quess it is the issue that your power is just grounded in this way. 
Most propably intentionally. 

> The theater is using the 2 115v legs... which are nearly maxed. We'd now
> like to see if we can draw off the 3rd leg, if we can get some answers on
> whether it should be 115v coming in? I'm very much aware that the phases 
> should be
> balanced, and that's the issue right now, in us only using 2 out of the 3 
> legs.

I have read that in USA there are systems that are designed to 
be intentionally used as systems that are not balanced. 
Your system could be one of those. 

The system could be designed to have support lots of power to 
single phase loads (through that center tapped coil) and then 
some three phase power for some small motors. 
Yopu power could be generated instead of one large classical three phase 
tranformer using two separate transformers: 
One large single phase transformer (center tapped and generated those 
two phases) and then second maybe smaller transformer that generates 
the third phase.

> Thanks if you can shed any thoughts on this one. We're located in NY state.
> Bill Osborne L.D.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/