Cray J932SE (was Re: Straight 8 up on Ebay just now)

[Chuck Guzis]

On 07/20/2016 05:59 AM, Paul Koning wrote:

> Three phase power shows up in a bunch of places.  Some high current 
> power supplies (pre switching era) use three phase input to increase 
> the ripple frequency and reduce its amplitude, which significantly 
> reduces the size of the required filter capacitors.  I remember that 
> in the KL-10.  CDC 6000 mainframes go further, not only using 3
> phase but also 400 Hz power for that reason (that also shrinks the 
> transformers).

Indeed, I was going to mention this.  A full-wave 3-phase rectifier
configuration produces ripple at six times the distribution frequency
and the output is largely DC.   Even more interesting is the use of a
transformer with both wye- and delta-configured secondaries.  This
introduces a bonus phase shift of 30 degrees, with the result that the
ripple frequency is twelve times the mains frequency (e.g. 720Hz on 60Hz
mains).

I remember working summers as a projectionist at a drive-in movie
theater that used carbon-arc lamps.  Many such installations simply used
a motor-DC generator for the arc supply, but one theater used a
transformer-rectifier setup on 3-phase power.  Even above the noise of
the projector and the exhaust fans, you could hear the 360 Hz "whine" of
the arc lamps.

3 phase induction motors are simple in the extreme--no starting
capacitors or coils.  I think (but am not sure) that they also deliver a
lot more starting torque than the typical single-phase induction
motor--at least that's been my experience with machine tools.

FWIW,
--Chuck