> On May 5, 2021, at 12:22 PM, Jon Elson <[email protected]> wrote:
>
> On 05/05/2021 10:37 AM, Paul Koning via cctalk wrote:
>> Incidentally, a way to get three phase power at a frequency of your choice
>> is to use a "variable frequency drive". That's basically a high power solid
>> state inverter intended to drive three-phase motors with a chosen frequency
>> resulting in the RPM of your choice. Depending on the model, those can go up
>> to 120 Hz or so, or all the way to somewhere around 400 Hz. I have a very
>> cheap one at home that runs on single phase 220 volt power, producing up to
>> 3 kW or so at anywhere up to 120 Hz. (Made by a company named Teco,
>> amusingly.) Most of these and especially the larger ones want three phase
>> mains input, though I'm told that even for those you can typically just
>> connect them to single phase power (between two of the three inputs, leaving
>> the third unconnected) at reduced power ratings. These devices are
>> surprisingly cheap, in particular they tend to be cheaper than "rotary phase
>> converters" which is how machine shops traditionally produce three phase
>> power when their mains is just single phase. paul
> Note that VFDs are designed to run motors exclusively. They approximate a
> sine wave with pulse width modulated 400 V pulses. DON'T EVER try to run
> electronic gear with a VFD, at least without a massive smoothing filter to
> convert to a true sine wave. With a motor, the winding inductance solves the
> problem, and the coils see nice sine-wave currents.
>
> Jon
I've heard that. But why? It's not like the electronics we're talking about
actually runs on AC. Instead, it goes right into a transformer (an inductive
load not much different from a motor) and after that into a ripple filter.
That filter IS the "massive smoothing filter" you're talking about.
It's certainly worth a test especially before connecting something rare and
ancient, but I see no theoretical reason for it to cause trouble.
The voltage issue is a different one. I've never seen a VFD that offered
anything other than frequency change -- indeed, it produces the same RMS output
voltage as what you feed it, and it isn't insulated. Again, for the
application we're talking about that is likely to be fine. For example, the
CDC 6600 wants 208 volt three-phase (in the US model) both at 60 Hz for the
motors and at 400 Hz for the main power.
Chuck mentioned an "idler motor", that's what is sold commercially as a "rotary
converter". Those are nice and can be had for quite high power, but often a
VFD does the job more cheaply (and with the added benefit of speed control) for
much less. I think my Teco converter cost about $100, way less than a
commercial rotary converter rated at 3 hp as that one was, and for a lot less
effort than home-building a rotary converter would be.
paul
