Re: Three Phase Power

Hi Jon,

Amps in equals amps out, no ?

I like to oversize drives anyway, as my indoctrination some 30 years
ago instilled that VSD's were inherently failure prone.

It would seem that the rectifier (front end) of the drive will create
the DC needed for the chopper end to create the 3 phase and the RMS
value of the 3 phase would higher than the same rated motor on single
phase, but the total current would be the same.

If we determine that the drive is a 9amp and each incomming leg is
rated for 3 amps load, then a 9 amp drive would handle a 6 amp motor
(using 2 legs) at it's full rating on the inputs, but it would only
use 66% of it's full rating on the chopper.

Running a 3 phase motor from a single phase supply is the norm, so
the only reason I see to do a phase converter would be if you started
with a gas/oil/steam/diesel engine.

With the cost of drives so cheap, and so little price difference
between a 3 and 5 hp, the additional cost of a phase converter is
much better spent on the bigger drive.

And in Jon's case, good planning expands the capability so an even
larger unit with more features can be used to run more than one
machine. that offers greater savings overall.


Dave

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> jeffalanp wrote:
>
> >Hi All,
> > Has anyone ever used a single phase to three phase rotary phase
> >converter to power a variable frequency drive VFD(which itself
runs a
> >three phase motor)? Is it a good/bad idea?
> >
> >
> Far better is to use a slightly larger VFD, and run it directly
from the
> single phase!
> Most manufacturers will now admit that you can run a 5 Hp motor at
full
> power
> indefinitely from a 10 Hp VFD. For home shop use, you generally
can
> even run
> a 5 Hp motor from a 7.5 Hp VFD without much trouble. The VFDs are
rated
> for some pretty harsh conditions, such as only 208 V 3-phase, and
convection
> cooling only in a 45 C ambient temperature inside a cabinet. In 30
C
> room air,
> and with 240 V single phase power, the rectifiers are under less
stress,
> and everything
> else is about the same.
>
> What you propose is workable, but a huge waste of space and energy,
not
> to mention
> making a lot of noise in your shop. It is simply unnecessary. I
am
> using a 1 Hp
> Magnetek VFD to run a 1 Hp Bridgeport mill on single phase, and it
has
> no problem
> at all, although the manufacturer would have a cow. I also use the
same
> VFD to
> run a high speed spindle on 80 V @ 400 Hz. This draws more current
from the
> VFD, and I put a fan on it when I do this.
>
> Jon