Re: [CAD_CAM_EDM_DRO] Re: VFD's / Skates, Tim & JohnS.

johnhe wrote:

> > You will probably find the 460 V version at a particular power level is
> > cheaper than the 230 V one. it will be much easier to convert single
> > phase 230 to 460 than the 3-phase output of the inverter.
>
> Problem, I am unable to find anywhere selling single phase 460volt input
> VFD's. All the 460volt ones I've found want three phase input.

Of course. Who has single phase 460 V? nobody. But, the inverters ALL
seem to run off single phase, the only question is how much to derate it.
(This applies to all VFDs made after 1990 or so.)

> Why is it
> difficult to step up the voltage after it's been VFD'ed?

The problem is leakage inductance. The motors have fairly high leakage
inductance, so the windings don't jump out of the stator slots when the
motor is started from standstill. If they didn't make them like this, they'd
need exotic starting controls to prevent drawing thousands of amps when
started.

Transformers don't have a 'dead start' problem, since there is nothing moving,

and have low leakage inductance. they will try to pass the 25 - 50 KHz
PWM waveform verbatim, and the iron losses will be huge, and copper
losses due to eddy currents will also be severe. the transformer will go
up in smoke in minutes, if the VFD survives that long. It took me a couple
of days to realize why a transformer on the output won't work (as the
manufacturers warn) but I believe that is it.

So, get a Yaskawa 3-phase 460 v input inverter, and I think you will be
very happy. If you go with a rotary converter, you will eventually want
to change it. if you go with a 'static' converter (really, a starting relay
and capacitor) you will want to replace it quite soon. Also note, besides
being more efficient and quieter, the VFD is so small you can mount it
on the wall next to the circuit breaker box (like I did) or on the side
of the machine, as many do.

Jon