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15HP RPC and Emco120 CNC lathe review (long winded)


Aug 15, 2004
So. Cal.

I finally hooked up the 15hp RPC I bought from American rotary.

This was supposedly a balanced, CNC compatible converter. It was...

I needed it to be as balanced as possible due to the lathe. The lathe is an Emco 120 CNC lathe. It is reported to be finicky with the supply voltage and the balance between the three legs.

I had to drop another 220vac circuit from my 100amp sub panel. I ran electrical metal conduit (EMT) from the sub across my work bench, of about eight feet, to where I mounted the control box for the RPC. The control box is a nice Hoffman enclosure that is large enough inside to run number six stranded wire without getting too crowded.

I ran three number sixes, two hot, one common, EMT grounded all the way.

I ran metal flex conduit from the RPC control to the 15hp Baldor idler motor.

I got some nice rubber isolator feet with the idler. She sits directly on the concrete floor. I had to remove some wooden subfloor to get to the concrete but I didnt want to run it on the wood due to possible noise.

From the RPC control box I ran the power line (rubber, looks like 10/4) for the transformer. The transformer is a step-up, multi tap to provide 380vac from a variety of inputs. There are also two additional outputs for plus or minus 5% the 380vac. Really a nice transformer (somewhat).

After getting her all wired up I hit the on button for the RPC control. Whoohoo, she started, and no crazy sparks. Oh yeah, I dont know anything about electrical stuff so dont follow my lead :D

Next I threw the disconnect. Oh, forgot to talk about that one. IMO you have to have a mechanical disconnect for machinery. Whether it be a removable plug from a socket or a mechanical switch. So I have a nice Square D, three pole switch there.

Cool, still no sparks or magic smoke.

The RPC idler was running smooth. No noticeable overhead lighting dimming or unsteady starting. Seemingly the soft start of this "special" RPC motor from Baldor is in fact soft start with a reduced current draw during light off.

With the transformer lit off I wanted to take some voltage measurements before committing the CNC control.

Basically I'll call the original house lines T-1 and T-2 and the generated line T-3.

I measured, with the RPC on:
T-1 to T-2 at 239vac.
T-1 to T-3 at 246vac.
T-2 to T-3 at 248vac.

All a little bit high?

But...If I figure it correctly (if not I'm sure to be corrected, I hope) the balance between the legs is 3.8%. The way I figured it was taking the highest voltage and subtracting the lowest voltage to get nine volts. Then nine divided by the lowest voltage to come up with my percentage. Again, I dont know a damm thing about math either so dont follow my path.

So, if I am somewhat close with my percentages 3.8% (3.7 actually) balance is well within the 5% needed.

I continued to measure and T-1 to ground was 119.5vac, T-2 to ground was 119.5vac and the generated leg was 217vac to ground.

This was all pre-transformer..

Post transformer gave me:
T-1 to T-2 at 388vac.
T-1 to T-3 at 408vac.
T-2 to T-3 at 392vac.

Ooh, the transformer is not as balanced as I had hoped, I think. Still not outta spec for the lathe. Unless my math is bunk, again... With the math I see 5.1% balance which is still alright.

Measurements again to the transformer common (close to ground).
T-1 to com was 229vac
T-2 to com was 222vac
T-3 to com was 231vac

That is 4% balanced.

Dunno...I finally switched the lathe power switch on and she fired right up. No smoke..

Two concerns were the sensitivity of the machine to line voltage and the regenerative braking.

I chucked up a piece of steel, basically the largest the lathe could handle. I ran the spindle speed from dead stop to max speed (4000rpm) many times trying to notice any problems with drive voltages/currents and braking currents. Evidentally the caps in the RPC are large enough to sink the re-gen currents and the RPC is overall large enough to keep the legs balanced even through power spikes. No error codes.

Overall I am real happy with the converter. I purposely bought one way oversize specifically to overcome the problems I thought would become an issue with a smaller unit.

I have a balanced leg, 5hp unit I built to run my Bridgeport mill. The legs are fine turned to run balanced throughout the load. I dont think it would have been enough for the lathe which IIRC, is only about 5.5kw.

I am glad it works and I would recommend the company. A little pricey but after you by the Baldor motor and electrics you my have a few bucks in it already..

So solly for the long winded post. I actually cut myself off from a few thoughts.

Thanks for the help, JRouche...


Aug 15, 2004
So. Cal.
Hey Rollo. Off the top of my flat head I believe the spindle is a 3.5hp DC drive. The overall power requirements are 5.5KW. The Emco 120 is a very small lathe. Not a lot of power required, just has to be relatively balanced. JRouche


Aug 15, 2004
So. Cal.
JoeFin... I did not measure the voltages during lathe operation. I was gonna let the lathe error out (which it will gladly do) if it didnt like the voltages.

I didnt break out the tek 2335, I used an old digital VOM (wavetek 235) and my handy-dandy 50kv rubber gloves. I dont trust my control these days and with the side of the xformer open I didnt want to use my hand or arm as a shorting bar... JRouche