Mini & Mainframe Power Requirements

From: William Donzelli <>
Date: Sun Jan 18 18:19:44 1998

So, trying to scare me, ehh?

> Having an IBM mainframe would be great provided you have a direct
> connect to the nearest power station! Here's several messages from
> knowledgeable folks that posted to alt.folklore.computers awhile back:

<lots of useful info snipped>

OK, running a big mini or a mainframe class machine is not for everyone.
However, running a processor plus a single disk and perhaps a tape may not
be all that bad. Water cooled machines, however, would be too much to

For those that only want to plug things into the wall, turn back now!

A typical mainframe machine from the past is going to want 3-phase power.
While most people do not have access to it, it can be produced or the
equipment modified to run off single phase. 3-phase can be made from a
phase shift network, a motor-generator, or a Scott connected transformer
fed by a second phase. Most of these solutions are a bit costly, not very
efficient, and probably a general pain in the butt. The better solution is
to look at the power supplies in the computer. Often the three phase is
only split up amongst the supplies, and maybe for some fans (or in the
case of some IBM tape drives _fractional_ horsepower motors to move the
glass doors up and down - expensive, but clever). A simple way to go would
be to rewire the supplies (in a reversable fashion) to work off one phase,
and maybe get a small convertor for the fans - no big deal. PDP-10s can be
run this way - its been done. Most others probably can as well. If,
however, the mains go into a 3 phase transformer, some thinking is
required. Many 3-phase transformer _can_ be run on a single phase nicely,
but the correct windings must be left open or shorted (if I can dig out a
past issue of *Electric Radio* magazine, I can come up with specifics.
Some hams need to do this trick to get vintage broadcast transmitters
operating from thier shacks.).

Some big old IBM systems want to have 400 Hz power. It is used so the
power supplies can be made smaller, especially in the filtering circuits.
This is probably more of a problem that needing 3-phase (a double whammy
if 400 Hz 3-phase power is needed!), because 400 Hz supplies are
EXPENSIVE, often taking the shape of big motor-generators.

Anyway, how much power do these computers require? Lots, but perhaps not
as much as one might think. A fully decked out 3081 CPU only needs about 7
kVA. I am sure something like a smaller S/360 or 370 would be less - my
guess is at 4 to 5 kVA. As long as you do not run the thing all day, it
will not be that expensive! For example, maybe running a small mainframe,
plus a disk and tape, for a night of hacking might cost $10 in
electricity. Running something smaller like a big PDP-11 system might cost
a couple of bucks - large deal. Compared to some hobbies dealing with older
technology (airplanes, cars, trains, boats, etc.) - that is not much.

One last point I would like to make is that before you go and try out your
newly aquired Big Blue Box, check out the circuits that you will be using.
The startup currents for some of the things can be nasty, and a jammed
circuit breaker is certainly _not_ a good thing.

William Donzelli
Received on Sun Jan 18 1998 - 18:19:44 GMT

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