3phase power for VAXen [was Re: VAX 780 on eBay]

Matt Burke matt at 9track.net
Tue Jan 4 17:18:07 CST 2022

As the owner of a VAX-11/780 I can confirm what Guy says about the 866
power controller. This is the only component that needs 3-phase power.

There is a small 3-phase transformer inside that provides 24V DC to the
control circuitry. The power controller not only interfaces to the DEC
power control bus to switch on other cabinets (with a delayed output)
but also connects to airflow sensors above each blower. If the airflow
fails then it cuts power to the system. The blowers themselves are all
single phase.

I converted my power controller to run on single phase by replacing the
transformer (18VAC, 20VA). It's also important to note that the neutral
wire from the line filter to the output rail is undersized for running
the system on single phase. If the system were running on 3-phase then
the neutral current would be minimal but running from single phase means
ALL the current is going through neutral wire. I replaced this wire with
one of a suitable rating.

I am in the UK so the figures I give are for running a system which
would normally be on 415V 3-phase from a 240V single phase supply. For
110V you can probably just double the current figures. I have the FP780
option as well as 2x DW780 and RH780 controllers. To power everything in
the main cabinet takes about 16.5A. I run this all off a 20A circuit
breaker (B-Curve RCBO). I have to turn on the H7100 power supplies one
at a time. If I try to turn them all on at once the inrush will trip the

There are two different versions on the H7100 in this system. The older
version uses a 3 ohm resistor, which is shorted by a relay after power
up, to limit the inrush. These have quite a high inrush current. The
later version uses a thermistor in place of the resistor which greatly
improves the situation.

Once you have the main cabinet powered then to get a usable system you
then need about another 4A to run the two BA11K Unibus boxes and
whatever power is required for a disk drive. In my case that's usually
an RA70 or RA72 so fairly minimal. I don't have any Massbus disks but I
do have a TU77 connected to the system.

The heating effect is noticeable after the system has been running for a
while so you may also have to consider air conditioning power requirements.


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