20 Jul
2016
20 Jul
'16
11:14 a.m.
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