The goal of employing a Variable Frequency Drive <vfd> is generally to provide a variable speed range, often replacing
the change of pulleys on older equipment. Since VFD's are usually targeted at 3phase motors -- even this isn't cast in
stone, as single phase VFD's are appearing on the market for frac horsepower applications -- one gets the "side effect"
of converting to single phase line use
if its a single phase ->3P VFD. Many are 3P->3P.
The usable range is -- very generally -- only about 50% of the speed range, as torque falls dramatically below some point
on the duty cycle. Keep in mind that that insulation and magnetic structure of older motors was not built with
freq changing in mind, and will most certainly not be ideal.
On a lathe, this "bandwidth" falloff isn't too big a deal, as depth of cut generally falls with torque, and surface finish
can actually improve. Time in cut can become excessive. For hobby uses, many don't care, and "make it work".
On a mill, this
no big deal isn't necessarily the case, although some appropriate values can be determined by changing the number
of teeth in the cut, restoring adequate "chip loads" to get the cutter to work effectively -- or at all.
With that brief explanation, I think having a VFD that supported both drive motor and X axis would be a huge
Pain In the Arse. Not to mention that one cannot hang two motors on a single SCR bank and have it
work, or be durable, or obtain a warranty.
If someone has actually tried it and not fried the semi-conductors, please weigh in with specifics
about brand, model, power factors, back-EMF, speed stability and all the other concerns that make this "
a bad idea"
from the outset. Its a conversation I'd find interesting. Of course, a "two channel" VFD probably exists, but probably
costs more per delivered HP than two cheap ones, as it has specialized application concerns and a market willing
to pay a premium for an
oddball feature set that works.
Last; almost every VFD I've seen has a warranty exclusion for switching the load side. In other words, "don't do that".
One can only imagine what severing the load at speed might do to the electronics, as the load spike travels
back up the firing path! So, no; you won't be "just wiring a VFD to the reversing switch", much less
through it.
--frankb
So I'm going to try and get my VN 12 running very soon. I originally thought I could just buy a VFD and feed the now 3 phase...
</vfd>
