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From: bldrnr@apple.UUCP (Brian Hurley)
Newsgroups: comp.misc
Subject: Re: IBM mainframe for sale (Really 400/416 Hz power for Crays)
Message-ID: <6424@apple.UUCP>
Date: Tue, 6-Oct-87 15:17:50 EDT
Article-I.D.: apple.6424
Posted: Tue Oct  6 15:17:50 1987
Date-Received: Fri, 9-Oct-87 22:05:33 EDT
References: <4673@nsc.nsc.com> <2944@phri.UUCP> <365@nuchat.UUCP> <2952@phri.UUCP>
Reply-To: bldrnr@apple.UUCP (Brian Hurley)
Organization: Apple Computer Inc., Cupertino, USA
Lines: 45

In article <2952@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>(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.

    This is not quite true.  The reason for a Cray using 400/416 Hz power
has to do with the source more than weight considerations.  The amout
of power used is very large (I don't know the figures off hand, but it may
fall into the hundreds of KW)  the olny source for this kind of power
is a Sub-station feed.  Which is NOT 60Hz. If you transmitted even 100KW of
power @ 440VAC that would result in a curennt flow of approx 318 AMPS!
Copper ain't that cheap! If I remeber my basic transmission line math, the
higher the AC frequency, the smaller the transmission line needs to be,
but I don't remeber the math that would show a direct corralation. The
end results of high frequency AC power include lighter equipment as well as
lower cost and lower loss transmission.  There are disadvantages to this type
of power as well, the equipment for regulating 440VAC @ 400Hz is not as
inexpensive as 60Hz, but imagine the transformer that could handle 100KW
@110VAC, 60Hz...
>	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.
> - Roy Smith


  Again the weight is not the issue.  The solution to the interface is simple.

  Design a motor that works on 400Hz and have it drive a 60Hz Generator.  If
 the design is mechanicly sound, the losses are comparable to a transformer
of comparable size.


    -Brian
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