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

Bob Darlington rdarlington at gmail.com
Fri Jul 29 11:08:07 CDT 2016

On Thu, Jul 28, 2016 at 6:43 PM, Dave McGuire <mcguire at neurotica.com> wrote:

> 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
> loads.

Shooting from the hip here.  The rotary phase converters I played with put
out what looked damn near like a square wave.   Sure, you'll have a 60 Hz
component in there, but also every other odd harmonic at decreasing levels
out to infinity.  I haven't tried my hand at building a 60Hz LPF with a
180Hz notch filter for that much power!   10MHz at maybe a watt, but not 60
Hz.  Ordinary laptop power supplies start to sing in conditions like this.
Audibly making high freqency noise from what is assumed (on my part) to be
SMPS components (inductors in particular) operating at something other than
the design frequency.  Aside from acoustic noise, you will also get heating.


>   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
> --
> Dave McGuire, AK4HZ
> New Kensington, PA
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