CNC and phase converters

[pipelathe]

I'm moving workshop soon and may not have 3 phase at the new palce. Does anyone know of any problems running a 5HP CNC lathe on phase converter?
Thanks for you input!

[Al_The_Man]

Is it a rotary or static converter? or do you still have to aquire it. I know most have no problem with rotary type, They are quite easy to build. I have not always heard good reports on the static type.
If it is just the Spindle motor that is 3phase you may be able to feed the control side with 1ph and use a 7hp VFD on the spindle, unless its DC.
Al

[pipelathe]

Thanks Al, no I don't have a converter yet. So my next question is which one? I also had heard that rotary is best, but I don't know much about that stuff, it's a lot simpler to just hook it up to the 3 phase!! I believe only the spindle motor is 3phase, but I'll check that out.

[eman5oh]

Just as a note some cnc machines will not run on a static phase converter. If it will run on a staic converter you will only get 2/3 rated HP out of the machine. I have a static converter for my mill that is one Hp and it works but starts the spindel slowly. The trade off is the static converts are cheap. If you have an extra 3 phase motor that you can hook up to the static converter it is almost the same thing as a rotory converter.

[gus]

Do NOT get a static.

You can make a suitable rotary from a large[say 7hp in your case] 3 phase motor and a small single phase motor momentary start switch to spin the small motor and ge tth ebig one turning, then power to the big one, voila , 3 phase. You will not have as much starting stopping torque, so you may have to adjust some timing if your machine is really fast. You may also have to swap the legs around because the control won't want to run on the 'fake' leg.

I ran 2 cnc mills on a pair of homemade converters, so it can be done.

[Al_The_Man]

This is a 7HP convertor I made up for running a couple of 3phase mills, it can also be fed into a machine with VFD.
I based my design on the articles by Fitch Williams in the Metalworking.com drop box under the 1998 retired files they are JPG files FRW-1 to 7. And with communication with him over tuning etc.
I elected to go with a totally 240vac control circuit to avoid transformers etc.
A 4pole motor is recommended over a 2 pole, although either will work, the cap sizes will be different. Also it is important that caps Cs,Cp & Pf are AC motor start rated, Cstart can be intermitently rated. I used various 10µfd & 20µfd in parallel during the tuning process.
The start button is pushed only long enough for the motor to come up to a good speed, usually barely a second.
If any one is interested in persuing this I can dig up the original communication tuning notes.
Al

[murphy625]

Static converters should have never been invented. (unless you want to consider a Variable Freq Drive a static converter)

Build yourself a rotary phase. It will run anything.
As the absolute most basic rotary converter is nothing more than a 3 phase motor that is running on only 2 phases. You can start it with a pull rope and dont even need capacitors, switches and all that other stuff. (it works but is a pain to work with).
The capacitors and other stuff are only there so you can install a button switch for control and they make life easy.

If you call up a phase converter manufacturer, they will tell you that you need a "balanced" converter or your CNC stuff wont work. This is not entirely true so dont take it to seriously. I was doing research on my servo drive (Digitax brushless servo) and found out that the input to my servo drive was rectified into DC anyhow!! I had a short conversation with the Digitax tech guys and got the feeling that all servo drives get rectified into DC internally.

Hope this helps.
Murphy

[Ito-Brazil]

Hello guys:
Well, I have same problems like you guys. I bought a 1CV - 3 phase -motor (i know, it´s like a toy for you guys, but i´m starting my shop) but in my house i don´t have 3phase power.
Ideas are welcome. Please pos some pictures and schematics. I downloaded that PDF already.
Thanks guys!

Ito-Brazil.

[lerman]

I bought a commercial rotary three phase converter. It was a quick way to get up and running.

If I had to do it again (knowing what I now know), I'd probably buy a VFD for my mill and lathe. (Well, actually, I did, but haven't installed it yet.) You will find that most of the small VFDs (up to about 2 HP), are available in models that will run off single phase power. I bought one on ebay for under $100. (But the guy who sold it didn't realize that it would run on single phase power -- and when people asked, he said it required three phase intput.)

I believe you can get a brand new 2HP VFD for under $300. So, if you don't feel like making a phase converter, just buy a VFD. -- Small, quiet, efficient -- And you get the advantage of variable speed.

Ken

[bdrmachine]

I quick rule of thumb is to double the VFD size when connecting to single phase. So a 10hp VFD will work fine for your 5hp application. The first stage of a VFD is Rectification (AC-->DC conversion). After that it chops the DC up into the selected frequency. Most manufacturers will still warranty a VFD wired this way but, that varies from company to company. All in all VFD are my first choice.

Good luck
Brian

[Ito-Brazil]

Thanks guys! And what about increasing speed? Like speed up mey 1800 rpm motor up to 3600 or more?
Have ya some ideas?

Ito-Brazil.

[tool_man]

O.K....I used to own an electric motor shop.I worked on many commercial rotarys and found most to be nothing more than a 3 phase motor with a starting capacitor and a timing relay to start the converter.There are many ways to make a converter start.I have my own design that has proven itself to be completely hassle free and bullet proof.I will try to post it later.In the mean time allow me to give a little advice.Just starting the converter is only the first step.To supply true 3 phase to a motor,you should,"balance the load".After the starting circuit opens,you have one leg that is open.Try this....add a running capacitor from each of the powered legs to the open leg.This will make your output from the converter,true 3 phase.Keep adding capacitors and checking the amps on all 3 legs until they are close to equal.As for starting....here are the approximate values for the starting capacitor needed to start your converter.

1hp=150mfd
2hp=250mfd
3hp=450mfd
5hp=650mfd
Hope this helps some of you.
I would highly recommend building your own converter,rather than buying one.It is not rocket science.Used 3 phase motors are dirt cheap.I built one converter from a 7.5 hp 2 pole motor that was destined for the dumpster.The only thing wrong with it was a front bearing that was locked up.It has been running for 5 years without a single glitch.