[jevs]

My dad built a rotary 3 phase converter to use on an old lathe. I plan to use this to run a CNC mill. What is important to check on the output of this? Do I just need to make sure the voltages are the same or can the phase angles be screwed up as well? I need to tune this thing so that it doesn't have any problems with the CNC machine. My machine specs say +/- 10% on the input. I do not currently have an oscilloscope so I am not sure how I could check the phase angle. Is it possible for this to be off or does it matter much if it is?

[NBrigode]

My dad built me a rotary converter also. First I have a couple of questions. How many horsepower is the converter? (How many horspower is the motor on the conerter?) How much horsepower does the CNC mill require? What type of CNC mill did you get?(my own curiosity) We have a 60 horse converter that we use to run our 7.5 horse manual lathe and our 3 horse prototrak mill. It dims the lights when we fire it up, but other than that it runs great. You need to make sure that the two main legs (voltages) are as close to equal as possible. The 3rd will be a little wild, but + or -10% is acceptable. We actually put an ociliscope up to all three legs and tested them with just the converter running then adding turning machines on and watching the scope. We are pretty balanced. If your not balanced correctly you run the risk of burning up electronics. My dad is a saftey freak when comes to electronics so we tested and retested just to make sure nothing would burn up.

In all this I'm just trying to say make sure you have enough horsepower converted to run the machine as balanced as possible.

Nick

[Al_The_Man]

See some of my previous post on building converter, it gives links to sites for tuning etc. The two main legs are going to be balanced as they are not produced by the converter, they are the 230v provided by the supply company, you just need to tune for good power factoron these two, the artificial leg should be slightly higher than the main two when unloaded, the nice feature of the rotary converter is that any other motor type load fed from the converter, becomes part of the system and in turn assists in the production of the third phase, that is why you can run mutilple motor loads on a converter that is higher than the converter itself.
Al

[jevs]

I believe the motor on this phase converter is 7.5HP. The spindle motor on my machine is 5HP I believe. The mill is a pratt & Whitney Tapemate c. Is it possible for the phase angles to be out from 120 degrees with the plans on the net? I can check the voltages easily, but I can't check the phase angles because I don't have a scope. Also, should I be "tuning" the phase converter unloaded or with the CNC spindle running? I am also very picky about having things as perfect as they can be. I am also an Electronics Technician so tuning it is not a problem as far as figuring out the caps etc. I just never bought a scope because I never wanted to do much with electronics at home after work If you enjoy something I think it is best not to make it your full time job or you get tired of it LOL.

[NBrigode]

Al described what I was trying to get at in my first post. I shouldn't reply before I get some sleep. (working nights). The key advantage of a rotory sytem is that you can run more load than the converter itself only when you are running more than one machine. I do not believe that it is possible to be out of phase angle too much. I can see it being a problem if your windings on the motor is incorrect. Please keep the post updated on how well the converter works, sounds like it should. Like Al said above your third legs should be a little higher than the other two with no load. That leg is only used to start the spindle motor once the spindles turning, the motor itself acts as a secondary rotary converter until turned back off.

Nick

[Al_The_Man]

I can check the voltages easily, but I can't check the phase angles because I don't have a scope. Also, should I be "tuning" the phase converter unloaded or with the CNC spindle running?

If you perform the tuning as outlined in the articles I posted, an example of the Fitch Williams postings on the Machineshop drop box shows how accurate the phase angle can be due to tuning, although, don't forget that the 3ph synchronous motor is basically a transformer, with the secondary being the rotor conductor bars, in a converter it is acting as an auto-transformer, producing the third leg from the sum of the other two legs.
The tuning caps are used mainly to raise the voltage on the 3rd leg.
In fact it will work without the caps, or even the starting cap, if you have a 'pony' motor to start it turning, some people have even used rope starts to get it moving before you throw the switch, it then continues to run under its own power.
You tune with the converter unloaded.
Al