19 Jan
2009
19 Jan
'09
10:28 a.m.
Um, let's clarify something: there are NO three-phase components in a VAX-11. The
switchers, the fans, ALL of it are single phase. Oh, there is one component that's
"three phase": the power supply in the PDU itself, which generates the voltage
to run the relay that switches the switched outlets. That's fused separately.
And yes, the neutral leg has a breaker, too.
And yes, this IS a "full config", with RH780, DW780, CI780 and (lucky us!)
FP785.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org] On Behalf
Of Patrick Finnegan
Sent: Sunday, January 18, 2009 9:43 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Running 3 phase 780s on single phase power (was RE: hams on classiccmp)
On Sunday 18 January 2009, Bob Armstrong wrote:
People need to understand the danger in this - in
any multiphase
power system, the neutral current is the DIFFERENCE of the current in
the individual phases. If the load is perfectly balanced, then the
neutral current will be zero.
But, if you run the same load from three separate single phase
circuits and (worst case) the three circuits are all the same phase,
then the neutral current will be the SUM of all three phase currents.
That's 3 times what it would be otherwise and runs a serious danger
of melting the neutral and starting a fire.
This is exactly the point I was trying to get across.
Actually I thought the DEC power controllers had a
circuit breaker
in the neutral leg too just to prevent somebody from doing something
like this. The split phase models do - are the three phase power
controllers different ?
I *think* that it does, but without opening one up or digging through
bitsavers, I can't remember for sure. I do seem to remember though,
that the breaker has the 3 phases, neutral, and a shunt-trip run
through it for thermal overload/lack of airflow.
Anyways, in my experience on my 11/780 (which isn't a full config), the
CPU, memory box, unibus controller, fans, 11/03, and RX01 together draw
no more than a total of 24A at 120V, so it may be possible to run the
system off of a single 120V supply, but I wouldn't recommend it.
Adding anything like a second memory box, DU780, an RH780 or something
else to the CPU (does the LSTTL in the 785 draw more or less power than
the 780's cpu at their respective clock speeds?)
Pat
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