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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: comp.misc
Subject: Re: IBM mainframe for sale (Really 400/416 Hz power for Crays)
Message-ID: <2075@kitty.UUCP>
Date: Tue, 6-Oct-87 00:23:53 EDT
Article-I.D.: kitty.2075
Posted: Tue Oct  6 00:23:53 1987
Date-Received: Fri, 9-Oct-87 00:47:57 EDT
References: <4673@nsc.nsc.com> <2944@phri.UUCP> <365@nuchat.UUCP> <2952@phri.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 45
Summary: Use of high-frequency power supplies...

In article <2952@phri.UUCP>, roy@phri.UUCP (Roy Smith) writes:
> > Have you considered just *how much* a Cray [...]  would weigh if they
> > used 60 Hz power instead? Not to mention the hum from the massive
> > transformer cores vibrating at 60 Hz, and the energy loss...
> 
> 	Still doesn't make sense.  Surely the weight you get to save in the
> cores is more than offset by the M-G set you need to turn 60 Hz into 400
> (or, as Jay says, 416) Hz.  Ditto for the noise and energy loss.  There is
> nothing inherently wrong with big 60 Hz transformers; just walk out to your
> nearest power substation and take a look.
> 	Aircraft use 400 Hz because saving weight is one of the most
> important aspects of aircraft design.  They generate their own power, so
> it's also no big deal to use a non-standard line frequency; they don't have
> to interface to the power grid.  There must be something more than just
> weight savings involved in the decision to use 400 Hz in a Cray.  For
> Earth-bound machines, weight is just not that important; you can always
> make the floor stronger.

	I too, am puzzled as to why Cray would use an M-G set and 400 Hz
power supplies.  With the advent of quality switching power supplies in
the past several years, and the use of other hybrid power supply designs
which use DC-DC converters, large AC transformers as used in purely
linear power supplies are becoming a thing of the past.
	There is a ready supply :-) of off-the-shelf OEM transformers and
power supplies intended for military applications that operate at 400 Hz.
These devices are, in some cases, an order of magnitude smaller than
a 60 Hz counterpart (especially when they use toroidal transformers). 
Perhaps Cray uses an extensive "distributed" power supply concept, and
thereby desires many small power supplies which are located close to
their loads.
	Since solid-state 60->400 Hz inverters are readily available
and more efficient than M-G sets (such inverters have great application
in aircraft ground support), I am puzzled why an M-G set would be used.
However, I can think of one good reason: the M-G set may have an associated
flywheel which provides some immunity from AC power-line disturbances.
There could also be a tandem motor that is supplied from batteries to
achieve a true UPS capability.
	This is, of course, pure speculation on my part.  I have never
even seen a Cray "in the flesh".  I do wish, however, that someone in
the know solves the mystery!

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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