View Full Version : Newbie RPC and air compressor questions

01-21-2010, 09:27 AM
Hi guys

I just joined today but have been reading about RPC's on here for awhile.

Let me tell you what I have

Duplex air compressor 1-3hp (9amp) pump and 1-5hp (13.6amp) pump
10hp (idler motor) RPC

My questions go out to guys that run compressors off a RPC or maybe any other high load machine

I am trying to balance the run caps. I have been able to get voltages closer then 5% with no load.

problem I am having is under load.

Both motors will draw over there rated amps when pumping around 100 psi of air.
the 3-hp has a standard pulley the 5-hp has been reduced by 25%

both motors are kicking out on the heaters 3hp is set at 10 amps (high as it will go) 5-hp is set at 16 amps (high as it will go)

I think the 5 hp is very close as it will run for a period before it kicks out. But seems odd that amps are over name plate with a reduced pulley.

anyways here is what I have under a load (3hp maintaining 100 psi)
L1-L3= 105MF
L2-L3= 80MF

L1-L3= 213V
L2-L3= 232V
L1-L2= 242V

L1 Amps = 10
L2 Amps =12
L3 Amps = 2 or 3

Im sure my problem is the low generated leg, my question how do I fix it (keep adding run caps?) or did you find that you had to oversize your heaters?

Thank you for any input you may have


01-22-2010, 10:48 AM
I found this article here in the Phase converters and VFD Forums here on CNCzone.com


I will trying this soon and post how it worked for me

04-14-2010, 11:23 PM
Ok I got this finished.

Here is how I did it. I balanced the RPC with just the Idler running and no load.
Idler is 10 hp

L1-L3 = 70 MFD Volts = 257
L2-L3 = 90 MFD Volts = 244
L1-L2 Volts = 258

L1 = 14.3A
L2 = 13.3A
L3 = 13.7A

When the compressor starts then the following run caps are added
the following reading were taken @ 130 to 140 PSI

L1-L3 50 MFD (total on L1-L3 120MFD) Volts = 248
L2-L3 40 MFD (total on L2-L3 130 MFD) Volts = 229
L1-L2 Volts = 253

L1 = 17.2A
L2 = 13.5A
L3 = 9.5A

I would have liked to have gotten the amp reading (with compressor running) a little better but that the best I got. It seems to be working so far, but it does not have a lot of run time on it. This compressor also has another pump (duplex Compressor)powered by a 3hp. When I get some time I am going to do the same set up. once I find what run caps work best. My reason for doing this was to reduce the voltage when the compressor was not running. this way if the RPC is running for a hour and no air is being used I don't have high voltage like I would if all the run caps were wired together.