Rotary Phase Converter

[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.

[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.

[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]

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 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]

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.

[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.

[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

[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.

------------------
-Christian D. Sokolowski

[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.

[jdunmyer]

Thanks for all the info, it's kinda as expected with regard to RPCs. Anymore, I think that VFDs are the way to go, they're not much more expensive than a commercial phase converter. In fact, I use one on my B'Port instead of using my factory 3-phase power.

Like others, I've made phase converters by using an old 3-phase motor. Mine have always been started mechanically (rope or another motor) and they work OK, but no substitute for the real thing.

[J Tiers]

<font face="Verdana, Arial" size="2">Originally posted by sauer38h:
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.
.</font>

Actually, it works.

The key is, as usual, buildup.

In the case of a DC generator, residual magnetism is needed to induce some field current, which then creates a larger field. It is a positive feedback system that ends up at full voltage.

In the induction AC generator, a small residual field induces rotor currents, which then create stator current, and it will build up if the load is right.
The usual system apparently is to load it with capacitors to draw current for buildup, otherwise unloaded, and then connect the load.

I actually tried it, and got output from the induction motor. I had to "zap" it with a small battery to get some residual in the stator iron before starting it up.

[Dawai]

Voltages are all over the place on the generated leg.

Best deal I did? hook my 10hp 3ph motor into a sub panel add in a 3rd phase insulated bus. The more motors you run, the smoother they get. Use a single phase panel and 2 pole breakers are cheap.

DO not expect to operate a transformer from the generated leg. Unless major voltages are generated to DC and a 3phase timing is throwed out via power transistors.

Yes, I put a VFD on my mill, single phase in, 3 phase out. It charges a dc bus then makes 3phase out of the dc.

The 5hp hydraulic pump runs quieter on the RPC than the inverter. I found that kinda humorous. I am sure it has to do with the current feedback compensation inside the inverter. Easier to hit it with a hammer than finess.

[KDRedd]

I set up a 20 hp RPC through a 220 to 440 3ph transformer for a water jet. The water jet was set-up at another shop with a 25hp RPC and This trasformer. The water jet is working on the transformer.

Kent

[wierdscience]

<font face="Verdana, Arial" size="2">Originally posted by 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</font>

Thanks for that,this explains the difference between the cnc converters and the plain vanilla models like Ronk,Arco and others.
The commercial units I have been into just have the caps feeding the third leg and that's it,no potential relay nothing,you flip the power on they start and that's it,well one did have a contactor as a main switch but it was the exception.

The isolation characteristic I mentioned in my mind can be only one thing,the the generated leg,since it has no lead in power and it is generated by residual magnetism in the rotor it is esentially isolated.That's the only thing I can attribute the behavior to.