[rescue] phase converters, was Re: Rescued: SGI Altix 450

Dave McGuire mcguire at neurotica.com
Thu Jul 28 19:43:17 CDT 2016

On 07/28/2016 07:01 PM, Bob Darlington wrote:
> These work great for running the spindle motors in my shop.  I don't see
> any reason why they would't also work to power 3ph computers.  If you're
> running them on single phase power you have to derate them (also for
> altitude), typically a 5HP unit to run a 1 or 2 HP motor.  I buy mine from
> AutomationDirect.com, GS2 units if I remember right.  You can select the
> output voltage, typically 208 volt, 3 phase for my stuff with 240 volt
> single phase residential power in.
> They work by taking in AC power, rectifying it, then inverting it back to
> AC at a frequency of your choice.  This is a pretty clean way to go.
> Do NOT use rotary phase converters unless by chance they had a major design
> change over the years and spit out clean power.  Great for motors but they
> cause early death of the computer running the milling machines.

  Do you have any idea of the failure mode here?  It was my
understanding that rotary phase converters should work fine for this
sort of stuff, while (not to sound contradictory) the solid-state phase
converters mentioned above would not work for non-inductive, non-motor

  The issue here is that with a whole lot of big transistors operating
in their active region, which would be needed for a 60Hz sine wave (or
three of them), which means a much bigger heat sink than those little
DIN-rail-mounted boxes could contain.  So they use a few banks of
smaller transistors restricted to either cutoff or saturation, not
active region but just switching, and create a stepped rough
approximation of a sine wave.  This will tear a switching power supply
apart, but with a big fat motor they're fine because the winding
inductance of the motor smooths out those stepped waveforms.


Dave McGuire, AK4HZ
New Kensington, PA

More information about the rescue mailing list