View Full Version : What size RPC for CNC mill and 5 hp compressor?

02-26-2006, 06:51 PM
I'm at a home residence attempting to get a Bridgeport Torq-Cut 22 CNC 3 axis mill going. I have a couple of options. The compressor I will be running is a Powerex 5 hp "Rotary Scroll" air compresser with a 3 phase baldor motor. What size convertor do you guys suggest if I were to run both the compressor and mill, or should I just run the mill off 3 phase power and change the motor on the compressor to a single phase? The mill has a 7.5 hp spindle motor. Acutally here's what the manual says about the spindle motor, "The Torq-Cut 22 is equipped with a 5 horsepower AC induction motor. This motor is driven by a 7.5 horesepower vector drive. The spindle motor speed is controlled by the drive system thru an encoder that is mounted in the motor. This combination enables the motor to produce 7.5 horsepower for 30 minutes." OK whatever that means??? Anyway I guess we'll call it a 7.5 hp spindle. Now the back of the machine states: 240 Volts, 3 Phase, 60 HZ, Full load current: 50 amps, Rating of largest motor: 20 amps. Several people I have talked to say that the full load current is grossly overstated on CNC's and that you will never see that amp draw, they say to simply double the size of the spindle motor. Now, when I add the compressor to the equation where am I at as far as RPC size goes? I'm guessing being a "rotary scroll" compressor that it is easier to start up than a piston compressor??? Anyway what do you think my options are as far as a rotary phase convertor? Does 15 hp sound right for just the mill and maybe 20 or 25 for both. Are there any drawbacks to running more than one component off of a single RPC? Like wasting electricity running a big RPC when I'm only using the compressor at times? Also what size breakers would you suggest for these size RPC's. I can't believe the different size RPC's that are being recommended to me and even the vastly different size breakers being recommended for the same size RPC's. Any and all info on what you guys think I might need to get up and running and also what you guys think would be my best option as far as running the compressor off of 3 phase or converting it to 1 phase and only running the mill off the RPC??? Thanks, Mike.

02-26-2006, 08:24 PM
Normally when you run a large motor off of the RPC, the motor being fed acts to aid the rotary converter and also produces current for the third leg, But the fact that the mill has a VFD isolates this feature, so there is no derating factor coming from the Mill side, the compressor however should be able to offer this.
Therefore if you want to use the mill up to its maximum rating of 7.5hp, allow 10hp RPC for this, This should also allow the starting and running of the compressor, if you want to add any margin, I would think 15hp is the highest HP RPC you are going to need.
In a RPC setup with only 3 phase motor loads, the RPC can be as low as half the total HP of the motors being fed, as long as it is as large as the largest load motor.
Make sure that any single phase transformers wired in the mill control circuits etc are connected to the main 240v legs and not the artificial phase.