Rotary Phase Converter

[sauer38h]

<font face="Verdana, Arial" size="2">Originally posted by rsr911:
would it be possible for those machines to use a 7-8HP single phase motor to drive a 6-7HP 3 phase motor as a generator?</font>

If it's a garden-variety squirrelcage induction motor, I don't see how. You have to get a magnetic field from somewhere in order to generate any current, and an induction motor doesn't have any magnetic field by itself. It needs a current in the field windings to generate a magnetic field, which in turn induces both currents and a magnetic field in the squirrel-cage rotor. Without current in the field windings, the rotor is just a hunk of iron and won't do anything for you no matter how fast you spin it.

Try these pages for some simple material on AC generators -

http://www.tpub.com/content/doe/h101...1011v3_110.htm
http://www.tpub.com/content/doe/h101...1011v3_111.htm

Note that both stationary armature and rotating armature generators are described. That "armature" stuff came up here a few days ago. As used - correctly - in those pages, the "armature" of a generator is where the output current is actually generated.

[rsr911]

I currently have my machines at work but I'm building a new house and shop and plan to move my personal machines there and an ide popped into my head. My largest machine is a 20HP lathe. The other machines are less than 5hp, would it be possible for those machines to use a 7-8HP single phase motor to drive a 6-7HP 3 phase motor as a generator? Reason I ask is that I happen to have an 8hp single phase 220 motor, came on a mixer we bought. We pulled it an put on a 3 phase since the shop is wired for it and they are more economical (or so I've been told). I'm thinking I could run the big lathe with an RPC and everything else with the generator I'm thinking of.

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-Christian D. Sokolowski

[J. R. Williams]

Wierd..
I have an ARCO CNC-3 rotary converter and it has capacitors across all three legs. There is a starting section controlled with a back EMF relay. The output voltage on the system can be adjusted / balanced by changing the number of capacitors used.

JRW

[JCD]

You are probably, without question, correct!
I have been using my home made generator for about 3 years, seems to work O.K., using a 5 hp motor for the converter, a capacitor for starting, and a few between the legs to optimize the load and minimize the temperature rise.
I don’t understand it either.

[wierdscience]

I've always wanted to see what the"CNC" converters looked like on the inside as compared to a regular rotary.I wonder if there really is anything special about them or if it's just a paint job.

[J Tiers]

I don't quite see how you can ever get to correct phases with a regular RPC. Better than a static converter, naturally.

I set one up once, and checked it out with voltage probes. I found that the phase was indeed NOT at 120 deg between the various phases.

I did find that with the exact loading I had, the capacitors COULD adjust phase a bit. Not much, and not enough, but somewhat. The third phase was however stuck at around 90 from the others.

But, then, I tried running a 3 phase on single phase. Once started, there was indeed a capacitor value to the third wire that would result in least current draw overall. It also produced least humming from the motor.

So, for any given loading using the motor with no RPC , there should be a best capacitor. But I believe it will change with load..... not much help there if it changes much. I didn't check that, don't have a good mechanical load.

The slickest setup I have seen was a motor repair guy with a basement shop. He had a big 10HP 220V motor running on single phase, and a 3phase generator driven by it. No arguments about phasing and balance that way.......

I don't know what he really did, I was there for a sale, he had passed away. Nobody knew what he did, but there were lots of motors there, some with customer tags. I hope they got their stuff back, the executor was going to toss them.....

The only machines there were a 10" Logan and a mill-drill. clearly he didn't do John Stevenson type work..... the motors were bigger than teh Logan.

[wierdscience]

Okay maybe not,but why is it you can have a dead short between conductors on a driven machine and not trip a single breaker?I'm all ears.

The big pile of run caps you see on most factory converters only start the converter,they do little to change the performance other than causing more heating when the unit is idling and not in use.I've tried it both ways,with and without caps and have seen no difference so I leave them out and only use a simple start circuit to start the RPC.

[J. R. Williams]

Wierd..
Isolation X-former??? No way--- as both of the supply lines are at the output end. If the voltages are unbalanced, you need to add a few capacitors.

JRW

[wierdscience]

I have heard figures from 75-90% nameplate hp and I believe it.I have used both commercial units and my own homemade converters all work equally well.

On my homemade converters I have checked the volts to ground and found two legs reading 120 and the third leg reading 150-160 with the unit just idling,during startup that third leg drops to 60-70 volts.

The other thing I have noticed is the RPC's ability to act as an isolation x-former.

[jdunmyer]

A friend has a 2Hp-rated rotary P.C. that he got with his Bridgeport. Got it connected, all works fine, but a question arose:

Does this thing make power that's anywhere close to "factory"? It seems like the RPC is going to have a generated 3rd leg that's 90 degrees off the others, not 3 phases that are 120 degrees apart.

Although I have an education in electronics (nearly 40 years ago, now), 3-phase wasn't discussed a lot, and I'm really not well-versed in 3-phase power stuff.