View Full Version : RPC Caps question

12-07-2006, 01:24 PM
I'm running a 2.5/5hp lathe and am picking up a 1hp Tree knee mill in a few days. I don't plan on running both at the same time, although I understand that it can increase the capacity of the RCP circut.

My wiring guy is helping with service upgrade and while we were re-wiring the garage, we set up a VERY rudimentary RPC.
Nothing more than a 7.5hp 3ph motor, pull start at this point, what a PIA!!
But it DOES work and I used the lathe for a quick job right away.

Anyway, he really dosen't seem to know much about RPCs (he measured voltage for each leg, not between the legs) and claims that no balancing is necessary. From what I've read in these forums, this isn't the way to go, and I'll prob just do the RPC thing myself.

The idler motor I'm using is 7.5hp 60hz, 230V/22.2A 1740rpm.

As i don't have much spare cash to waste on trying different capacitor ratings, can anyone give me a close starting point for:
a) Starter cap
b) balance caps

I don't want to go the route of slave motor to start it, and pulling just isn't gonna cut it!

Can anyone get me in the ballpark?

Thanx, Dan

12-07-2006, 05:12 PM
Mine is the same size motor as yours I will look up the values I have, and I used a contactor with a 230vac coil so no control transformer was needed, with PB start.
I have a previous link to mine and it references the metalworking drop box. see the FRW* files.
Your start capacitor can be start type but the tuning need to be motor-run style.

12-07-2006, 10:44 PM
From the PracticalMachinist web site, Peterh5322 posted:

I use a ROT of 1, 2, 3 and 30 for each 1/4 HP that a motor has, which is somewhat crude, but is very easy to remember. I call this my "1-2-3-30 Rule". 1 mF/quarter HP for the A-C (PowerFactorCorrection) cap, 2 mF/quarter HP for the C-ManufacturedLeg cap, 3 mF/quarter HP for the A-ManufacturedLeg cap and 30 mF/quarter HP for the start cap. The 30 mF/quarter HP start cap is placed across the A-B cap, for reasons which were previously described. (The higher the idler HP the less capacitance you're probably going to actually require). For 7.5HP motor, that’s 30uF PFC, 60uF C-MfgLeg, 90uF A-MfgLeg, 900uF start cap.

This formula works for me.

12-12-2006, 05:22 PM
thanks for the info, I'll post some pics when it's finished.
Just got ther mill home and need to throw chips!
As it is right now, I'm more concerned with getting this service upgrade done before real bad weather hits.
Also need switches and relays but that was a given...
Whatever the case it will be a clean instalation.

12-12-2006, 06:04 PM
:o Whats an RPC and an ROT ? :o

12-12-2006, 06:20 PM
RPC Rotary Phase Convertor. ROT Rule Of Thumb.

12-12-2006, 06:24 PM