Motor generator

Paul Koning paulkoning at
Wed May 5 07:18:54 CDT 2021

> On May 5, 2021, at 1:07 AM, Chuck Guzis via cctalk <cctalk at> wrote:
> Very large machines, certainly.  There were "site preparation" documents
> that detail this.
> I can quote from the 1975 edition of the STAR-100 hardware manual on
> bitsavers (PDF page 25):
> "Power for the basic computer consists of one 250 kva, 400 Hz motor
> generator set.  The motor-generator set has the capability of providing
> power for the CPU, MCS, I/O and the MCU. The optional memory requires
> the addition of an 80 kva motor-generator set."
> Granted, the STAR was a monster of a machine, but similar setups were
> used for CDC 6000, 7000 and Cyber 70, 170 machines and others.   That
> doesn't include the refrigeration necessary for the chilled water
> supply.  I/O devices such as card readers and tape drives often used a
> "split" supply with the 400Hz supplying the electronics and normal 50/60
> Hz AC supplying power for motors, etc.

That's definitely the case for the CDC 6000 series, the drawings make this clear.  The 6600 mainframe takes 3 phase 400 Hz for power supplies, and 3 phase 50/60 Hz for the compressor motors for the cooling system.  Similarly, the DD60 console display for the 6000 machines takes 3 phase 400 Hz power for its power supplies, but single phase mains power for some auxiliary functions.

An earlier message commented on the whine from power converters.  I don't know how common this practice was, but at the University of Illinois PLATO system which had a pair of 6500 systems, the motor-generator was located near the elevator machinery in a corner of the building, far from the computer room.  Yes, it was noisy, but no one spent any time in that location.  The computer room was -- by mainframe standards certainly -- rather quiet.  Of course it helps to have liquid cooling, so there weren't many noisy fans to deal with.


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