View Full Version : RPC Sizing

11-16-2006, 12:11 PM
OK I've read thru the posts here and just wanted to get my ducks in a row before I spent some money. I scored a older CNC mill on EBay and it has a 7.5 HP spindle and as far as I can see from this foto :


it appears that only the spindle is 3 phase. I'm putting this in my garage and I will be using an RPC to convert from 1 phase to 3 phase. I have nothing else in my shop that is 3 phase so I only need the RPC for this machine.

$64000 question : What size RPC do you knowledgeable gentlemen :) recommend ? There is a 12 HP RPC for sale near me - would this be a good fit for this mill ?

11-16-2006, 12:16 PM
A 12hp should be be good for it, I would assume this has a spindle controller on it so the spindle motor is not right across the incomming supply, but if you run any other 3ph motor off the RPC, it can add itself to the RPC capacity.

11-16-2006, 01:58 PM
Hey thanks, Al for your reponse in this thread and the other one I started.

One last thing, if I can't get this RPC, what would be the lowest HP rating for this mill (it's the same as the one on EBay from the other thread - Acromax MX-16). Would 10 HP work or what would be the lower limit I should look for ?


11-16-2006, 02:36 PM
I would think a 10hp would be ample.
Make sure that if there is any 1 phase circuitry fed from any of the 3phases, connect the 1ph to the two main feeds, IOW not the artificial leg.
This way this circuit will not be affected so much by spindle current etc.

11-16-2006, 09:33 PM
Your machine should state the maximum current draw. Use this amperage draw, multiply it by the voltage you will be using and you will get the wattage needed. Utilize that number and call Temco (510) 490-1507 or visit them at www.phaseconverter.com and they will recommend the proper size RPC

11-20-2006, 01:58 AM
OK, now I'm a bit confused - I searched the forums, but didn't see an answer to this ???

Considering that I have a 7.5 HP spindle on my small VMC and I want to put it in my garage (which has 220V 1 phase) would I be better off going with a VFD or and RPC ?

If someday my (old) machine dies and I need to clear out all the old electronics and replace them (say with Geckos or Rutex) would the VFD be best since it could provide me with a variable speed spindle (considering all the electronics would be dead).

Advantages/disadvantages of one or the other ? (If there is a thread that discusses this, please point me there)


11-20-2006, 10:17 AM
I did not suggest a VFD before as I assumed this mill had a variable speed spindle, if you have no other 3 phase equipment to power the VFD is the way to go for the variable speed feature, your controller may have a ±10v analogue spindle output that you could program spindle speeds in part program.
The problem may be in finding a VFD that will allow you to run on 1ph at 7.5hp, many makes have a recommended cut off around 5hp.
Try some of the manufacturers like SEM and WEG as well as Shneider and Mitsubishi.

11-20-2006, 11:03 AM
Sorry, Al, didn't mean to cot turned onnfuse - the mill does have a variable speed spindle, I'm just trying to think ahead - plan for the worst case. So the only difference between the VFD and the RPC is the variable frequency part ?

If I do run other 3 phase equipment, pretty much it would be when the mill was not turned on - if I was using the VFD to provide 3 phase to some other piece of equipment - like a grinder for example, could I just set the frequency on the VFD to 60 hz (and never change it) and then it would act just like an RPC ? Or is this a dumb question ? :)

11-20-2006, 11:39 AM
I would be inclined to go with the RPC, as a rule, when running other 3ph motors across the artificial 3ph, they can aid the capacity of the 3ph supply as they also are now acting in tandem to the RPC motor.
I personally would be leary of using a VFD for over 5hp on 1ph, unless is is specifically rated so.

11-21-2006, 03:01 PM
OK - getting ready to actually spend some hard earned cash. The RPC I mentioned above (which is listed as 12 HP) says on the ID Plate that the maximum HP for single motor is 5 HP - my spindle is 7.5 HP.

Should I pass on this or is it ok ?

Also I might be wrong about the controllers - they might need 3 phase too, if so, do I need one of those special RPC's that keep the voltage within 5 to 10 percent ?

Thanks again - don't want to make a mistake.

11-21-2006, 04:01 PM
The RPC I mentioned above (which is listed as 12 HP) says on the ID Plate that the maximum HP for single motor is 5 HP - my spindle is 7.5 HP.

If this is a commercial unit, I would run it by them as to what they recommend, if you have a variable speed spindle, then mention your motor is not directly across the RPC, check to see if the servo's have 3ph input. If they are not SCR, but PWM then they are not as critical to slight variation.
It sounds to me like they have it sized fairly conservatively