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