Re: [CAD_CAM_EDM_DRO] Re: UPS to VFD hardware hack

roundrocktom wrote:

> Doug,
>
> You're on the right track for a UPS to VFD, but as already pointed
> out your can find surplus VFD's for less than you can build one!
>
> However, I am a long time "learn by doing" so by all means do mess
> around with a UPS to see what you can get it to do. Yes,
> the "reference frequency" is most likely generated by a small
> microcontroller.

Cheap UPSs may even use an R-C oscillator, either a Royer converter
or a 555 timer and a flip-flop to drive 2 power transistors feeding a
transformer in push-pull. The fancier ones have a micro to monitor
battery charge left and signal the computer to shutdown before the
battery gives up the ghost. I don't know if they use the micro for
frequency control or not. Some brag about tight frequency control
that sounds like a quartz crystal time base, although most computers,
copiers, time clocks, etc. could care less about the line frequency.

Most lower-cost UPSs (up to $1000, at least) generate a 'modified sine
wave', which means essentially a square wave with some off time between
the voltage pulses. Larger motors don't particularly like this waveform, and
will run quite hot on it.

Jon