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Posted: Sep. 3 2007, 15:27 ET

I need three phase powe to power my milling machine. It has a 1/2 hp motor. I am considering a vfd and  i was wondering if amyone knew if you loose any of your hp with a vfd. I just cant seem to find astraight answer on the web. i don't want a static due to power loss.

Posted: Sep. 3 2007, 16:28 ET

From what I understand some VFD's have to be de-rated when using single phase input, but some don't. http://www.ctiautomation.net/AC-Tech-Drives-FAQs.htm  
On this page they say some of their models have to be de-rated by 40 %, so if you got a VFD that had to be de-rated, you'd have to ensure it was over sized by the appropriate amount.

If variable speed control isn't necessary, you could build a rotary phase converter ...... lots of info in these pages ...




Just looking on flea-bay ....  VFD's of the of the right size for your application seem to be fairly cheap ...... probably easiest to get a VFD double you horsepower requirements and feed it single phase.

Posted: Sep. 4 2007, 00:15 ET

     So if I just order a vfd for 2 time the motor hp then i should be fine?

Posted: Sep. 4 2007, 02:19 ET

Yep, sounds about right, oh, you DO NOT loose any power when you run thru a VFD. You actually gain a speed control for the motor, bonus, change speeds with the flick of the wrist, marvelous invention.

Posted: Sep. 4 2007, 10:33 ET

My adventures in VFD retrofit may help a bit.  I am a huge VFD fan and I think they are great, although a bit expensive compared to some other options.  Some of the cool things a VFD can do that cannot be done with other methods is a user-controlled acceleration and deceleration.  Because the VFD will bring the spindle to a stop in X seconds (whatever you program), you don't have to mess with the spindle brake anymore.

Good luck, it's easy to do.  I bought my VFD's new from Drives Warehouse in Dallas.  They have a big presence on eBay.

Posted: Sep. 6 2007, 00:05 ET


Posted: Sep. 6 2007, 08:33 ET

check out ac tech drives they are fairly cheap and we use a lot of them at work. the have good customer service and are really easy to program.ac tech 1hp on evil-bay

Posted: Sep. 6 2007, 09:57 ET

Quote (Lowpull @ Sep. 06 2007,00:05)
Your motor is rated for x horsepower at x freq. So you will see differences when you run that motor under or over that x freq. The further away you are from that freq the more the difference is apparent. But at that freq (60 Hz) it will be the same as not using a vfd.
example running a motor at 60 hz and gearing it down to turn the chuck at 10 rpm will be stronger than using a vfd to  just make a direct driven chuck turn 10 rpm.

I think Ive muddied the waters enough.

I don't disagree at all, and I'm not a EE guy, but I was very pleasantly surprised at the torque my Hitachi delivers at very low speeds.  The final setup I went for on my mill was switching from a 1.5 HP 3P 2 speed Chinese motor, to a 2HP, 3400 RPM Leeson.  I figured the extra 1/2 HP would help, and it does.  The pulley setup I have right now is basically 1:1, so the spindle peaks at around 4,000 (I was able to jack the top frequency up a bit) down to maybe 240 RPM?  Somewhere around there.

There's some sort of logic/feedback mechanism in the VFD, maybe something that boosts the juice when a load is applied.  The torque at low RPM was much better than I had hoped.  Of course, if you leave the original step pullies in place, you can do the bulk of your work at some middle pulley selection, and if you need it, you can still swap the belt(s) around for more torque or more speed.

Posted: Sep. 6 2007, 10:15 ET

I am not a big fan of VFDs but they are the best choice in some applications.. I bought VFD for the spindle drive of my CNC mill from AutomationDirect..I had a bit of trouble setting it up and AD's tech support sorted my problems immediately.......Ill need another VFD for another cnc project and I will get it from AutomationDirect.