17 Jul
2025
17 Jul
'25
12:01 p.m.
On Jul 17, 2025, at 10:41 AM, Jon Elson via cctalk
<cctalk(a)classiccmp.org> wrote:
On 7/17/25 08:45, Paul Koning via cctalk wrote:
VFDs (variable frequency drives) produce ~340 V PWM "square" waves, Given
enough inductance in the motor windings, this causes roughly sinusoidal currents. But,
feed this into a transformer, and you will get high frequency spikes. Now, MAYBE, due to
the way a "Sola" transformer works, it might smooth out the square waves, but it
is real hard to predict what will happen. Also, VFDs are usually designed for balanced
3-phase loads.
Yes, the question is whether the transformer could be made sufficiently happy. As for
3-phase: the CDC mainframes (6000 series) in fact use 3 phase 400 Hz power feeding their
power supplies -- you're dealing with 3-phase transformers and 6-diode rectifiers
behind them.
paul
On Jul 16, 2025, at 3:42 PM, Van Snyder via
cctalk <cctalk(a)classiccmp.org> wrote:
I wonder about "they were certain it would not work". That
should be a question of fact, not belief.
As for the motors, that's an obvious issue (if they are induction motors rather than
universal motors). The modern solution is a VFC -- variable frequency motor controller.
Those are pretty cheap and work great with motors. I've heard that they are not so
good with power supply transformers, not sure if that has been experimentally confirmed.
For power supply transformers, 50 vs. 60 Hz is unlikely to matter. People with CDC
mainframes that want 400 Hz power do need a solution, with motor-generators as the
traditional answer. I wonder if a VFC would work for that, perhaps with post-VFC
filtering to turn the waveform into something closer to a sine wave.
...
The Computer History Museum in
Sunnyvale, CA has a working IBM 1401
computer from Germany. It has ferroresonant power supplies. They bought
a converter to supply 50 Hz power because they were certain it wouldn't
work at 60 Hz. And it has motors in the card reader, card punch,
printer, and tape drives, that would all run at the wrong speed using
60 Hz power.