Ideas for powering a 7.5/3.75 hp lathe

Yes, I know about RPCs and VFDs, this is not the typical question; at least not as far as I know...

I started running my Bridgeport and Rockwell 11x37 off a 2hp GS2 VFD. Wound up building a 3 hp RPC that I generally like better, VFD got "repurposed". I've been very happy with my setup for over a year.

Now I bought a 17x60 lathe with 2 speed 7.5/3.75 hp motor. It has contactor reverse on the head and apron plus e-stop and brake triggered stop contactor. The complexity of rewiring that through the VFD, combined with the expense and size of a VFD suitable for that motor, plus the problems of 2 speed motor on VFD and not needing the variable speed aspects; not too hot on VFD power...

So the obvious solution is build/buy a 10-15hp RPC. But that's way over-kill for the rest of my shop.

And I've noticed "staged" commercial RPCs where you turn on the first stage idler, then if you need it, you start the second idler. This sounds like it would be THE setup for me. I've also got a really nice 5hp Baldor Inverter Duty motor just taking up space. And I'm told that a 7.5hp motor will run one of these if you start it on the low speed winding and hot-switch to 7.5. And I'll likely NEVER need to push 7.5hp on a cut anyway.

So, assuming this is not a pipe dream, how would one go about wiring a staged RPC? I would love to be able to keep my existing 3hp RPC for running my shop, and just use it to feed my 3ph breakout box and circuit for most stuff (mill, grinders, big drill press). Then throw the switch to bring the 5hp on-line when I need to fire up the lathe. Maybe even find a 7.5-10 hp for the second stage when "a deal comes along".

Seems the existing 3hp idler could easily be used as a "starter" to get the bigger second stage started (without separate cap/pony start), but I don't see how to get a secondary mains feed for the second stage. Any thoughts other than "just build another RPC that is big enough"?

a Manuel motor starter



and hook your number 2 motor onto the prc output side is the simple answer
Sky's the limit for sophistication from there

BadDog;

Just search this forum for "multiple idler RPC" or something similar. There are lots of posts about these and some good sketches by Peterh5322. The short answer is just connect two RPCs in parallel and the results will add.

If you have a 3 phase breaker panel then there is an easy solution.

Connect your big single phase circuit (5 hp +3 hp =8 hp total or about 50 amps single phase is needed), to two of the buss bars in the breaker panel (I use the outside ones and the middle bar will be the generated leg)
Next, use a 30 amp breaker to feed the 5 hp RPC and a 20 (or 30) to feed the 3 hp RPC and be sure to connect the generated leg to the middle pole of each breaker.
Only one RPC needs a starting circuit. Start this RPC first then the other one can be started from the 3 phase producec by the first RPC.
With this setup, you have both RPC with short circuit protection, they are in parallel and you can cut off the power by throwing the breaker.
Both RPCs need overload protection --overload relays and heater elements.

Be sure to wire your shop with the middle leg as the generated leg and do not connect any control circuits to the generated leg.

Go to the Phase Converter plans at the beginning of this forum and look for the RPC by B.Nelson. I think I remember it using this scheme.

Thanks for the info, and sorry for asking what (it seems) is a common question. I did do some reading and searching, and even saw some discussion on series RPCs and related problems, but didn't see anything about what I wanted to do. I guess I'll spend some more time searching, starting with the terms you provided...

BadDog;

here is a sketch I posted for comments some time ago. it is an idea for a multiple idler RPC but does not show the detailed wiring.

Bruce Norton