Mini & Mainframe Power Requirements

From: Don Maslin <>
Date: Mon Jan 19 00:03:57 1998

On Sun, 18 Jan 1998, William Donzelli wrote:

> 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
> handle.
> 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.).

One scheme that I have seen used to power a 3-phase motor (on a lathe or
such) is to connect a 3-phase motor of comparable rating or higher to the
single-phase mains and also connect all three phases to the motor to be
powered. The motor connected to mains is powered up - it may require a
twist to get it spinning - and when up to a steady speed, the lathe (or
whatever) motor may be powered up. I'm sure that there are some
inefficiencies, but I have seen it work.

                                                 - don

> 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
    Don Maslin - Keeper of the Dina-SIG CP/M System Disk Archives
         Chairman, Dina-SIG of the San Diego Computer Society
       Clinging tenaciously to the trailing edge of technology.
         Sysop - Elephant's Graveyard (CP/M) - 619-454-8412
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Received on Mon Jan 19 1998 - 00:03:57 GMT

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