> Gunther,
>
> be careful with the return line: if you are running three different
> phases, these are phase-shifted 120 degree against each other, so the
> current maximum on each phase will occur at a different time, and the
> center ("ground") line will not need to carry any current at all (in a
> perfectly balanced circuit). If balance is not perfect, the ground wire
> will have to bear at most the same load as any of the live wires.
Not quite correct-- the ground wire will only carry current during a fault
(aside from leakage), and the neutral will (try!) to carry the whole load,
until it burns thru the insulation and shorts out to ground-- then the
ground will start carrying part of the load!
>
> If you feed a single phase into the same circuit on all three lines,
> you will have peak current on all three "live" lines at the same time,
> and the "ground" line will need to handle that. If it's not made to
> that spec, you'll overload it by a factor of 3...
Yes.
>
> I don't think this is true for VAX power supplies, but I have heard
> about process automation computers that are fed with three phases just
> to make the period shorter and to use smaller capacitors. With this
> kind of power supply, you'd get a lot of ripple on the DC output.
The VAX 6K works just fine on 240v single-phase (dryer-like), but I can't
speak for long-term reliability yet. Maybe in another decade or so. ^_^
I dunno how the 780's power supply is constructed, but it _is_ DEC, after
all...
ISTR there was a model that had 3-phase blower motors. A three-phase motor
will not operate on single-phase. It *has* to have all three legs to run.
Check the wires going into it. If there are four, or three and one isn't
green and obviously bonded to the grounding system somewhere, take a good,
hard look at things.
>
> Regards,
> Andreas
Bob
>
> Gunther Schadow wrote:
>>
>> Hi,
>>
>> just to be sure, I would simply put all three phases on the
>> same single phase. Are there any problems with that? The
>> VAX 6000 is much pickier, but the VAX 11 and everything
>> having the simple power distribution box should be fine,
>> right?
>>
>> thanks,
>> -Gunther
>>
>> --
>> Gunther Schadow, M.D., Ph.D.
>> gschadow_at_regenstrief.org Medical Information Scientist
>> Regenstrief Institute for Health Care Adjunct Assistant Professor
>> Indiana University School of Medicine tel:1(317)630-7960
>> http://aurora.regenstrief.org
>
> --
> Andreas Freiherr
> Vishay Semiconductor GmbH, Heilbronn, Germany
> http://www.vishay.com
Received on Fri Feb 01 2002 - 07:28:06 GMT