VFD as 3Ph shop supply

I wasn't questioning the motor. I said, "the VFD isn't doing anything useful," and then, "The VFD isn't adding any functionality..."

Rephrasing again; for the situation in question -- powering multiple moderate sized motors in a small shop -- a VFD buffered by an idler doesn't seem to offer any advantage over a simple RPC.

Convenience for sure. No capacitor balancing, et al.

One can probably get away with a smaller VFD as well, although they are getting cheap enough that it matters less.

Joe Gwinn

That's great, but are any of these inverters 3 phase and capable of handling motor starting currents? A VFD is a 3 phase inverter that is capable of ramping up voltage and frequency to lessen the motor starting currents.

RogerN

It should work fine. I am running a 1-1/2 HP motor with a 10 HP drive. Seems to work fine although the motor protection settings did not go that low. No worries as the motor runs only when I am there. Greg

Better balance without having to add a ton of tuning capacitors. Here, the motor is serving only to absorb serges and switching transients -- not being asked to generate the third phase wiich is missing.

The ability to slow down whatever machines are actually being run (all at the same time, which may be a problem.

The ability to run your machines at 50 Hz (if they were UK made) instead of the local 60 Hz (or vice versa) -- so the speeds are what the manual says. (This is most important with grinders, as the extra speed running a 50 Hz machine on 60 Hz might be enough to grenade the wheel.

Enjoy, DoN.

I've never heard of anyone having to add "a ton of tuning capacitors" to a RPC in the size range mentioned in the original post, 15 - 20HP, to get acceptable results.

Even if you're willing to live with running one machine at a time, controlling the VFD frequency from multiple locations would be a pain. I suppose you could use a pendant, but if you're only going to run one machine and drag a pendant to that machine, you might as well start and stop the motor with the VFD as well.

Normal 7" and 8" wheels are rated 3600RPM, so it won't matter to the typical bench or surface grinder with a 2-pole induction motor and direct drive spindle. In fact, I have both an 8" Swedish bench grinder and a Japanese Mitsui surface grinder that are dual rated 50/60Hz.

The only situation I can imagine where a VFD + idler scheme seems practical is using a VFD to soft-start a very large idler that couldn't be started directly off the line. I suppose it also might make sense if you were in a situation where VFDs were cheaper than caps. But I don't think either scenario applies here.

If cap balancing is in fact more convenient than familiarizing yourself with the VFD's setup parameters and the subsequent debug. VFDs ain't simply plug and play.

I don't see how adding motors would reduce the size of the required VFD.

Ned, that Yaskawa article was great. Thanks a lot.

Sorry, that should be, "If cap balancing is in fact less convenient..."

To handle surges, so the VFD doesn't have to.

Joe Gwinn

De nada. Yaskawa is usually my first choice when choosing a VFD, if I have any say in the matter. Not because there are not plenty of other capable drives, but because they seem to have the best documentation, which can save a lot of time and headaches on a job.

What kind of surges? Current surges from starting motors or voltage spikes when disconnecting them?

Yes to both.

Joe Gwinn

Funny how people experiences are different. At work we have one piece of equipment with a Yaskawa drive. In a year it has been replaced once, and it is acting up again! We have many Allen Bradley drives, maybe 25 of them and never a hic-up with any of them. Greg

My choice as well, professionally.

And they seem far less likely to let out the magic smoke when mishandled

Gunner

"Not so old as to need virgins to excite him, nor old enough to have the patience to teach one."

OK, I'll bite.

I can see a motor permanently connected to the drive protecting it from inductive voltage spikes. (Though I've not changed my opinion that the best way to avoid the problem is to not use the VFD in the first place.)

But how do you figure that an idler motor is going to reduce the current loading on the drive?

Well I only design and build the stuff, someone else has to worry about maintaining it . Seriously, I've never had any complaints about the Yaskawas I've installed, and my customers' maintenance guys aren't shy about their opinions, and I wouldn't have it any other way. I learn more from them than from their bosses.

Like you, I've formed bad opinions of brands and products on limited information. (I'll show you my list if you show me yours.) Seems to me it's the most logical thing to do after a bad experience and when there are reasonable alternatives.

Because the spinning motor stores significant energy. This is how RPC are able to generate the missing phases.

Recall the long discussions of sizing RPC motors to handle the startup surges of various machine tools, and the staged startup scenarios (where one starts the smaller motors first, to have enough spinning capacity to later start the larger motors).

It makes no difference in this picture if the first source of 3-phase is a RPC, or a VFD.

Joe Gwinn

To my knowledge, for an induction motor to act as a generator, and that is in essence what you're proposing, it must be rotating over its synchronous speed. Clearly this is not the case if the motor is powered by a VFD, and it won't be the case if you switch in a second motor while holding the frequency constant. I guess you could drop the drive frequency at the same time the second motor is started and take advantage of the idler's kinetic energy, but that hardly seems practical.