View Full Version : New(?) idea on VFD *combined* with RPC?

12-21-2006, 12:47 AM
I just had an idea (AKA Brain Flatulence).

What if one were to start with a VFD for 1ph to 3ph. Then use that to power an RPC idler (technically not an RPC in this scenario, but follow along...). This then would be hooked to mill, lathe, whatever.


1) This would eliminate any need for start caps or run cap phase balancing.

2) Would this allow you to use the normal drum switches and contactors to control the machines? In other words, would the idler motor shield the RPC from the damaging effects? What about instant reverse? Hmmm, I think not...

3) You could still control speed using the VFD, though obviously it would affect all motors. But in a home shop, where you’re only using one at a time (typically), who cares.

Not economically ideal, but if #2 above works out, this could be a rather nice combination and perhaps an easy/cheap improvement for VFD owners. But, alas, I'm sure someone will confirm that the idler will not be sufficient to buffer the VFD against the eddy cause by switching down stream, much less instant reversing.

12-21-2006, 08:47 PM
Posting just to bring it to the top again. Maybe all the great folks who know lots about this stuff and helped during my recent RPC problems missed it till it fell off the front page...

At the very least is seems this would eliminate the RPC phase balance problem, though the idler seems to add nothing to the mix unless it can eliminate the problems with switching on the VFD output circuit. Unfortunately, I don’t know enough to be sure how it would affect things, and can’t afford to gamble my VFD on a test! :D

J. Randall
12-21-2006, 09:36 PM
Baddog, I am no expert by far, but my take on it is that the VFD puts out pretty well balanced legs to start with. I would think if you hooked a socalled idler motor in between it and the machine it would see it as an extra load and not as an RPC. I think I have read that if you have the right input terminals on the VFD you can wire your original controls to it with no problems. James

On edit, just reread your post and saw that you were thinking of running multiple machines off of one VFD, seems like that has been hashed over in the past on on the PM phase converter forum. I think I remember some talk of wiring in some chokes or reactors to take care of some of the surges or spikes. I think Forrest Addy may have been one of the guys with a theory on it. You might do a search over there.

12-21-2006, 09:46 PM
That's absolutely true. If I have a VFD running my mill, I can wire the original drum switch to the VFD external control inputs for FWD/OFF/REV (among other things) and it will work fine as long as the VFD output itself is not switched. Problem comes in when you want to have all your machines run from the same VFD. Not only do you have potential problems with the VFD being tuned properly to each motor, but you also have potential problems with multiple switches depending on what those switches are and how they work. On some VFDs, like my GS2, you can also remote mount the main control panel itself by getting the correct wire.

I guess it's probably a pipe dream, but it would be nice to have the benefits of the VFD and RPC without their particular limitations and requirements. I really don't think this is likely to work out, just thought I would throw it out and see what those who know more might say. :D

J. Randall
12-21-2006, 09:49 PM
Baddog, we just cross posted, see my edit on my original post. James

12-21-2006, 10:18 PM
Yep, I've talked to several folks (don't recall names) about doing just that some time back. I've still got a diagram somewhere that uses various heaters, breakers, and such to deal with tuning/safety required to drive multiple machines. But before I got to a point where I was actually using it for more than my Mill, I was given a nice 3hp motor and decided to build a RPC and make my life simpler. The VFD has now been put on a shelf awaiting use on my pending 2hp grinder project. And then I had that random brain flatulence that caused me to wonder if the 2 could be combined to good effect. :D

12-22-2006, 02:03 PM
Your VFD has to be set up to run the motor you want to use, change the motor & you have to change your parameters. Switch in another motor & your parameters are wrong, unless it's a really small motor in comparison to the main motor,

12-22-2006, 02:20 PM
as J. Randall said, a VFD puts out the closest to real 3-ph that you'll get (short of one of those fancy electronic phase-matching gadgets that has been posted about recently). adding an idler to the mix won't help. in fact, the VFD is only rated for a certain HP, so if you add an idler you will be using a portion of the rating to basically spin an extra motor to generate heat. :)

andy b.

12-22-2006, 09:51 PM
1)This would eliminate any need for start caps or run cap phase balancing.

Sounds like an expensive way to get rid of some cheapo capacitors. I've never needed balancing caps on my rotary, and the start cap was about $5 retail.

12-23-2006, 02:00 AM
There was considerable discussion about RPCs on a thread here recently. Several knowledgeable participants indicated that if you don't use the run caps, you effectively derate the driven motor to 2/3 stated. Then the problem became about simple run caps being tuned to a narrow load band where it would be balanced before falling out of balance. The VFD provides truly balanced power at all loads up to it's stated max, so that's what led me to wonder what would happen if you fed an idler with a VFD. My guess is that, as others have said, the idler is just extra complexity and load that the VFD sees without providing a buffer to protect the VFD from output side switching.