On 7/14/21 9:50 AM, Paul Koning wrote:
On Jul 14, 2021, at 12:33 PM, Guy Sotomayor via cctalk <[email protected]>
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?
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*.
* 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.
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
