14 Jul
2021
14 Jul
'21
12:53 p.m.
On 7/14/21 9:50 AM, Paul Koning wrote:
Spec sheets for various rotary converters that I
looked at.? I'd have to
go back and find them again but they typically drew full load power all
the time...and they were *loud*.
The one I looked at produced full sine wave output for all 3 phases.? I
don't recall the THD but it was sub 1%.
21KVA I think works out to 15 or 20HP.? The input for what I was looking
at was 75A @ 220v single phase.? So it's quite a bit more than 2KW and
the MOSFETs they use are *huge*.
Yes, the "small" VCFs are relatively inexpensive if they are just PWM
outputs.
I'm was concerned because there are ferro-resonant? transformers in some
of this gear and the IBM specs for these devices was pretty tight on
frequency and THD.? Given the nature of this gear, I'd rather not have
to go and start replacing unobtainium parts due to poor quality power.
--
TTFN - Guy
On Jul 14, 2021, at 12:33 PM, Guy Sotomayor via
cctalk <cctalk at classiccmp.org> wrote:
I've found 2 issues w.r.t. "rotary converters".
* They *always* consume lots of power regardless of the actual load
Really? That
seems odd. A rotary converter is merely a three phase motor with run capacitors. Just
like any other motor, its power demand depends on the applied load. A normal motor
spinning without anything connected to it consumes power to overcome electrical,
magnetical, and friction losses, but none of these are particularly large.
Can you cite a source for this? * They typically don't have great frequency
regulation as they are
really designed for machine tools (which are pretty tolerant) so if
the load varies, the frequency will vary until the "mass" catches up
They have no frequency regulation at all; what comes out of the third wire is a phase
shifted version of the line input.
You may be thinking about motor-generators, where the output frequency is defined by the
construction of the generator section and how fast it spins. Yes, under high load those
will slow down some, reducing the output frequency.
I did a fair amount of investigation of this in
order to power the peripherals for my IBM 4331. The peripherals in total require on the
order of 21KVA of 3-phase power and with them (printer, card reader/punch and tape drives)
the load will vary *a lot) which would screw up the DASD (string of 3340 drives and some
3350 clones).
Yes, I would expect that. Power supplies would not care much.
Another example is the CDC 6000 series, which uses 400 Hz M/G sets feeding power supplies.
The disk drives run off mains power, so any M/G speed variations is not a factor.
I ended up looking at a solid state phase
converter (takes in 220v single phase and produces 208v 3-phase). It has a good (< 1%
frequency regulation) and only consumed 100W at idle. Plus it's relatively small and
quiet. The downside is cost (~$5000).
$5000 ??? I have a VFC on my lathe (3 hp
rating, so about 2 kW electric). It cost only $150 or so as I recall -- TECO Westinghouse
brand. I think they are still around. That particular model was rated for single phase
input. Larger ones are not, though I'm told that they still work if connected that
way (220 to two of the input terminals and the third left open) at reduced rating.
Here is a current example, 3 hp single phase input:
https://www.wolfautomation.com/vfd-3hp-230v-single-phase-ip20/
The concern with VFCs is the pulse width modulated output waveform, which I am told will
bother some types of loads (some electronics) but not others. Motors will certainly be
fine with them, so if you're looking at feeding disk drive motor loads, this is the
perfect answer.