Re: VFD for a 15HP CNC lathe

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <info@t...> wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "fuddham" <fuddham@a...>
> wrote:
> > I have a CNC lathe that has a 15 HP spindle motor. What size VFD
> > would I need if I only fed the VFD single phase power? Would it be
> > better to get a 15 hp VFD and use a rotary phase converter to feed
> > the VFD? What size VFD and Rotary converter?
> >
> > Thanks,
> >
> > Scott Hamilton
>
> 15HP! Now THAT's a CNC lathe!!
>
> Scott, 15HP VFD's get pretty pricey. Most of the solid state ones
> are rated for 1 to 5 HP. On a lot of the older VFD models the HP
> rating is with 3 phase in, and they have to be derated 50% for single
> phase in. That motor probably draws 35A @ 230VAC and 10 times that
> amount on startup for a few seconds.

One of the nice things about a VFD is that it can slowly bring the
motor up to speed so that the rotor inertia (and load) don't result in
such high magnetizing current (the current on startup that it takes to
build up the flux). By accelerating the motor over a period of, say,
15 seconds, the current drawn from the mains likely won't even
approach the full load current of the motor.

Most VFDs I've encountered (Baldor, Square D, TB Woods, ABB,
Magnetek/Yaskawa, Toshiba) *can* run on single phase power but must be
derated by 40%. The reason they must be derated is because the
capacitors inside are only big enough to smooth out the ripple from a
3 phase input (A VFD converts the incoming AC supply to DC, then chops
that DC up in a particular manner that induces sine wave currents to
flow in the motor) - with only one phase the DC voltage is not nearly
as smooth so you start to get Undervoltage faults above 60% of the
horsepower rating. A partial, but very simple, solution to this is to
connect an oil-filled capacitor from, say, L1 to L3. This will
phase-shift the sine wave going to L3 in an amount proportional to the
load current drawn (which is good). A uF value similar to what would
be used for a static or rotary converter of the same hp rating should
do the trick (yes, I've tried this and it does work - not as good as
rotary phase converter, but it's a helluva lot cheaper!).

Finally, I think the best place to get an excellent deal on VFDs is
eBay - I know because I've sold a few there myself and I almost cry at
the miniscule prices I get for each one - typically 10-20% of the list
price?! My user id over there is allpraisebob (shameless plug) but,
unfortunately, the only 230V drives I have are a 30hp Baldor and a
10hP Square D Sensorless Flux Vector (this control algorithm is not
needed nor really appropriate for a lathe). If you guys have more
questions on VFD usage I'll try to help out - as long as I get some
help in return on converting my Harbor Freight Micro Mill to CNC use ;)

(Yeah, I know about the group dedidcated specifically to it, and I've
read every post over there!)

Regards,
Jeff