Phase Converter for a 20-HP CNC

      Machine owners share experiences with finding ways to supply three-phase power in sufficient quantity and quality. June 17, 2010

I have a 20 hp CNC, however I cannot get 3 phase to our shop so my options are phase converters or a 3 phase diesel generator. I have read a bit about phase converters and am leaning to "Phase Perfect" digital 3 phase converters with +- 1%. Is there anyone here that can help me do this correctly? This is a huge commitment and I don't want to screw up this machine.

Forum Responses
(CNC Forum)
From contributor B:
You need to make sure that you include all of your resource demands needed for 3 phase, at least for the machine. You may need one unit for the machine outside of the vacuum pump, one for the vacuum pump, and another for your air compressor, dust collection, etc. Your air compressor and dust system will be much more efficient with 3 phase. It often makes more sense to have separate converters depending on your electrical circuits and budget, small units are less expensive than large ones.

From contributor O:
You need to find the amp rating on whatever your running is and times it by 4.2 in order to get the needed start up amps, my 30 hp converter motor is 78 amps so I need 78x4.2 is 328 amps to get going, thatís one big generator. I am going to get a small pony motor to get it turning then switch it to my generator which is rated at 125 amps. My Komo is 480v 29 max amps and all of this is getting very expensive as the gen is 7000 plus the electrician.

From contributor L:
Like contributor B said, you will need two. I have a 30 hp rotary (CNC) on my CNC (15 hp). The converter should be two times the size of the largest motor started. I also have a 50 hp rotary (CNC) that runs everything else in the shop. The CNC should be on a dedicated converter. Power drop or surge caused by the starting of other motors on the same converter will cause problems on your CNC. I bought both converters CNC-ready for better balanced power to the rest of the shop also.

Converters : approx. cost $5,000.
Panel upgrade and larger service brought in by power co. and electricians : approx. cost $15,000.
Peace of mind that everything turns on when the switch gets thrown : priceless.

Definitely get it done right. If you have a 20 hp CNC you have a lot of money already invested.

From contributor O:
My apologies. My Innova is 480v 16 max amps. I bought a gen thatís rated for 125 amps and use a small 1 hp 3600 rpm motor to get the 30 hp turning, then hit the phase converter switch. Itís low tech and not for a real business type environment.

From contributor W:
I myself only have a small converter but with the help of a very smart electrician friend was able to run more than just the slider that I had it for originally. What was explained to me was Ohms law. If you have a 200amp supply to the primary panel of the three phase 240 volt phase converter when you plumb in/install a transformer to boost the 240 volt supply to 480 volts to the next panel you will have 100 amps to work with there. It has been far more cost efficient for my shop and allows me to run more machinery at a time.

From contributor P:
If you use a Phasemaster by Kay industries you do not need to double the size of the converter. I recently purchased an MA4 (20hp) on Ebay for $250. Excellent converter, much better than the 20hp generic rotary converter I also use (I wired them together and start the Phasemaster when running the 15hp vacuum blower for the sander or the two 10hp vacuum pumps for the router). There is presently an MA7 (40hp, 120 total hp) converter on Ebay for $500. Take a look at what they cost new. I've purchased three used Phasemasters on Ebay so far (two for a friend) and have had no problems.

From the original questioner:
The power company where we are is too ridiculous. I again sought their help and the veiled threat was their 240 with a "stinger leg" with 200 amp max, 20 hp max and no light dimming. For a 300k machine I will relocate and feed it good clean power.

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