profit-motive question: single phase adapter for 3-phase motor drives

Since the difference in price between a 3HP and 7.5HP VFD is only about $200, I have a hard time believing this would be economical.

Add in a 2000-3000VA transformer and that's worth another couple of hundred dollars - that's the isolation function. geoff

Usually, the derating factor is 0.66.

You cannot feed a three phase rectifier with DC and utilize all three inputs. So you have to feed the bus (as you say), which is easy only if DC terminals are available.

Very expensive. But the need for it rare.

I would suspect that people needing isolation, would have three phase available.

Interesting.

Not according to several manufacturers' web sites. They say 50%. Work out the peak currents in the rectifiers and the capacitors.

Depends - for transformer, read filter reactors. Again the highly non-sinusoidal input waveform causes problems when you're drawing 20A because the harmonic currents (like 180Hz and 300Hz) are pretty high and the rest of your appliances may not like them.

Perhaps. Isolation is a good thing - otherwise you can get very substantial ground currents from the transformer frame even on a lowly 1 or 2 HP motor. Doesn't do the bearings any good, for instance. This is true with a 3-phase feed as well. It -will- work without isolation. It works better and longer with it.

geoff

gwes sez: "If I sold a relatively inexpensive fully-isolated power-factor-corrected,

Get real ! It seems you are describing a VFD. Or did you mean a plain, ordinary rotary phase converter?

Are you also selling plans for a steam powered pickup truck?

Bob (seen it all before) Swinney

power-factor-corrected,

ordinary rotary phase

Nope. Just a front end which lets you use a smaller VFD, smaller input wiring because of lower harmonic currents, isolates it so you don't draw sparks in the bearings or your hand or have big ground currents, and keeps the RFI down.

If it's not what people want or need I'm certainly not going to bother making any, that's for sure. geoff

You need to consider that many hobby-garage shop types buy used VFDs on eBay for a fraction of new cost. The cost to them of derating the drive is quite small. I've been running a 1 Hp Bridgeport from a 1 Hp-rated, 3-phase-only Magnetek drive for years, because I didn't know any better when I bought it. It has not had any problems at all, and runs cool. When all this stuff about derating the drive came out, I looked up the power module in it, and it has rectifiers rated at 30 A continuous. As a 1 Hp drive, it shouldn't draw more than about 4 A RMS. Given the worse power factor for a single-phase input, I'm guessing the peaks may be around 10 A at full load. I had to put a line filter in the input to keep it from trashing some equipment in the shop, that probably helps limit the peak I. So, the rectifiers look like they are more than adequate for the job. If the capacitors ever croak, I can replace them, although the whole bank could get expensive.

You have to remember that these units are designed to run in enclosed cabinets at 50C ambient for 10 years continuously, sometimes with a motor start every minute. That is so much more severe than what most hobby-garage shop users put it through.

Jon

Good point. And the low-HP drives keep getting cheaper. More than one net discount drive supplier has 1 and 2 HP 1-to-3 drives for under $200. The box I was thinking of making would allow running 220V units from 110 or 440 from 220, but that's very much a niche.

There are these really cute power control and PFC ICs and planar transformers these days so it would be a lot of fun to find a good application to tinker with.... geoff

minarik already offers a drive that runs 220V units of 110, I have one sitting in my "test drawer" - I use it for testing 3 phase pumps