[mgb1974]

Picked up a 3 axis cnc Bed mill, similiar to a prototrak that I am putting in my home garage. The machine came from Canada and is setup for 550volts but comes with a transformer so it can run with 220 3 phase. The machine is 3hp and the control is 110v.

Should I go with a Rotary or a VFD? I might have these mixed up but from what I have read on here you can use either one to convert 220 single phase to 220 3 phase.

Positives and Negatives??

I know a Rotary will work but not sure what the negatives are for a VFD.

Thanks

[bru102]

Hi mgb, I have at last count 2 mills, a lathe, and and an irrigation pump, all three phase. I have a 2hp or 3hp VFD on each, and they run great. The advantages are price, size, and functionality. Each of the machines can be programmed for acceleration and deceleration, and the speed is controlled by turning a knob. Note that the highest-end mills now come with variable frequency drive from the factory

I suggest a "sensorless vector" type unit such as the Hitachi SJ200 series...which means that the unit does a fair job of figuring out how fast the motor is turning without using an encoder (sensorless...just analyzes the load it sees) and adjusts the output accordingly. This gives you better (but not perfect) speed control in low speed high torque situations. I just got one from driveswarehouse.com at about $350 for a 3hp unit.

The downside is that these units don't generate a perfect sine wave, and do subject the motor to higher voltage spikes. That's why you'll see "inverter duty" motors advertised with better insulation. On the other hand, I haven't seen a problem yet, and would put in the new motor if the need arose. The other issue is cooling... if you run the motor slow for long periods and it is fan cooled, the fan is turning too slowly to have any effect. Some folks have replaced the fan with a computer-type unit that runs independent of motor speed...they are very inexpensive. I haven't seen the need yet.

(By the way, I mentioned a pump. I irrigate our property with a 2hp pump which drives large pasture sprinklers and small garden drip circuits. I got a 3-phase motor specifically so I could use a vfd. The vfd will also do closed-loop control, automatically adjusting the pump speed to keep a constant pressure as the flow rate varies from large to small watering circuits...the advantage here is energy savings, as most of the time the motor is running at less than half of capacity.)

Hope that helps. Ray

[bru102]

mgb... I just noticed the "transformer" in your original post. That does introduce another set of wires and insulation to worry about. Maybe you'll can find some specific help on that. But again, a new 3hp vector-duty 220V motor might be worth considering, the total package may be less than a rotary converter, and you get a mill with very rare belt or gear changes.

Suggest you also do a search on "vfd" both here and on the practical machinist site. Ray

[mgb1974]

thanks Ray!

Put another post up to find out about the transformer issue!

[Al_The_Man]

What you need to do is look at what is actually on 3 phase, if it is just the spindle and you do not have a spindle controller, and the motor is configurable for 220, the a VFD for the spindle would be the way to go, as the controller/power supplies etc are most likely on 1 phase.
If there is more than a couple of motors on three phase and or the spindle has a VFD type control, then you may want to look at setting the others to 220 if possible and use a RPC or converter.
You have to assess what basic voltage/supply the machine can actually run on after any transformation that exists.
Al.