Another Rotary Phase Converter

I believe those are capacitor banks for voltage correction, not receptacles and he does show 3 phase output at the right side of the schematic.

Ted

Well Jim, I guess we were typing at the same time !

I gotcha now -- what i thought were receptacles are caps -- i see now.. thanks

My fault for the confusion, I should have use the correct symbol for a cap. I started drawing a pictorial rather than a schematic and didn't correct the drawing. :confused 3:
 
np ..every trade uses their own. I think the thing that also threw me were the switches on each cap - do you plan to switch the caps in and out?
 
np ..every trade uses their own. I think the thing that also threw me were the switches on each cap - do you plan to switch the caps in and out?

I'm thinking about switching them in and out. That might be overkill, since it's just for setup. Sometimes I over design things. :rolleyes:

I do have a question about setup. For initial testing I'm thinking about building a load bank out of stove top elements. I'm wondering if a resistive load will work to get kinda close to a balanced output. I would rather fry an $8.00 heating element than a rather expensive servo drive.
 
The load bank sounds like a great idea Jim.
Also having a switch per cap (use cheap house wall switches) will help tune it in fast.
Mark S.
ps no timer in the circuit?
 
The load bank sounds like a great idea Jim.
Also having a switch per cap (use cheap house wall switches) will help tune it in fast.
Mark S.
ps no timer in the circuit?

In this one I don't need a timer or start caps. The pony motor is manually switched in, then when up to speed, I will manually switch to the idler motor. I thought about putting in a potential relay, but I can flip a switch :)
 
You can use whatever rotgut loads you have available as long as the voltage is compatible. For a jiffy test, I have gathered up whatever available, including old space heaters, and have even mixed 3-phase devices/appliances with single phase devices - wired accordingly , and roughly eyeballed for balance. If balancing is your concern, borrow a 3-phase motor close to the actual load. All those switches are probably overkill in that you are asking for impractical precision. I would just estimate cap values you need based on your actual loading, and if you want to play with it and fine tune, you can manually connect a candidate cap while monitoring current on that line. If anything, you probably want to fuse the caps, and throw in a bleeder resistor across the caps to drain them. I found this attached design online from the practical machinist forum -- it appears robust to me and well thought out and includes the calculations used - however, it is for a 10 hp motor.
 

Attachments

  • FitchWConverter.pdf
    2.1 MB · Views: 39
Thank you for posting the PDF, very informative.
 
One thing to bear in mind is that you over engineer, but often everything else is over engineered as well. My 4HP Hurco mill actually has a 5HP motor on it. It came that way. I figured I needed a minimum 5HP (after derating VFD) but it had a 3.7Kw 3ph derated to about 2.4Kw (3HP~ish) since I am feeding it single phase. I didn't have the cash for the VFD I wanted at the time so I just ran what I had. Its been that way for many years now, and I use that mill all the time for aluminum, 4140, 4140HT, 1018, mystery metal, brass or whatever else needs cutting. I don't have to put a 5HP load on it just because it has a 5HP motor. In all my calculations I put 1.5 down as my max spec, and it removes metal at incredible rates. Sure I could cut at 3HP loads still, but its hard on that old 1981 Hurco mill.

Anyway. Yes a 15HP RPC might be a little light if he was running all his motors at peak load, but in reality that just isn't going to happen. The worst current will be inrush, but we aren't talking about a universal motor. We are talking about an induction motor. Its sure not going to be 10hp 7.5kw to start. Then how would I know. Maybe he plans to do high feed high DOC hard steel all day long and throw chips big enough to take out small animals in their path. LOL.

If you want another example. Go look at your home service entrance. Its very likely if you add up all the breakers they exceed the current of the main breaker. How often do you trip your main breaker?

Now we are talking about shop equipment. If you are running all your equipment at the same time, and it all starts a hard cut, hard start up, or crash at the same time... Then you will be lucky if your main trips right away. LOL.

Anyway, I would agree in general to go bigger, but as long as he isn't running actual 10HP loads I wouldn't worry to much about it for occasional use.
 
I can't say that I have ever seen Power Factor correction capacitors used in a phase converter. That's a new one. Your values for your run caps are not shown. All I can say is watch overdoing it with the run capacitors on the RPC. Too much capacitance and you will get a wild leg in the 300 to 500 volt range with no load.
I have tuned most every one I have touched down to within 5% A to C and B to C voltage wise and they are typically very stable under load.

Now as far as the motor. Is it compressor rated or not?
If compressor rated, what type of compressor?
It's important to understand that rating. If it's rated for a screw compressor then your fine. It's 100% duty cycle.
COmpressor motors are typically NOT rated to be run for hours on end. Think about how your compressor works. You use air, it comes on and pressure builds up and it shuts off. It's off for a while then cycles again.
RPC's run and run as long are you are using them. You also give up some efficency for starting torque with a compressor motor. Meaning it will draw a bit more power, and generate a bit more heat than a pump or fan motor.
 
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