Looking for VAX6000 items

Guy Sotomayor ggs at shiresoft.com
Wed Jul 14 14:53:27 CDT 2021

On 7/14/21 9:50 AM, Paul Koning wrote:
>> 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?
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 

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

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