Rotary phase converter - simple version

[precisionworks]

There are no power factor correction capacitors,
no balancing capacitors, and no starting capacitors.

This is a quote from my friend Jim Rozen on the PM site. RPC's don't get any easier

The 1 hp pony was a take off from my Burr King, while the 10 hp idler was purchased for $50 from Joe's Junk Jungle. The pony sits on a pair of strap hinges that allow tilting it back to tension the belt to the idler. Even though I have a number of VFD's in the shop, this RPC was built to power a gear head lathe without a clutch. The lathe motor is 3 hp, so the 10 hp idler is probably larger than necessary, but I won't have to build it twice

The pony drives an adjustable pitch sheave, so I was able to fine tune the idler rpm. The "ideal" (no load) idler rpm is 1793, and I ended up at 1812 rpm (according to my strobe tach). The frame is 1" square tube and 1.5" angle, butt welded.

[Ironman]

It sure is simple enough.
Do you think the pony motor needs to be 1hp? or would 1/2 work?

[Cavalry]

Mine might be even simpler. 5HP motor with a small engine pullstart cord wrapped around the shaft. I have been meaning to make something permanent for 4 years now

[Cavalry]

Originally Posted by Ironman

It sure is simple enough.
Do you think the pony motor needs to be 1hp? or would 1/2 work?

All it has to do is turn the shaft to get it going. I have seen old washing machine motors used

is there ant downside to having a much larger motor than current needs?

oops fat fingered the "t" also would this work on a plasma cutter?

[Lu47Dan]

Originally Posted by redfarm

is there ant downside to having a much larger motor than current needs?

Not that I have seen so far with mine, it has a 10-1 hp ratio(30hp Idler - 3hp lathe). I will never want for capacity with mine, I can run a bunch more equipment without having to build a larger RPC The only major drawback is that your wiring has to be able to handle the idlers power requirements. Since I have had mine set up and running I have not had any problems.
Dan.

[cutter]

Originally Posted by precisionworks

. RPC's don't get any easier

Barry, the problem with the pony motor converter is that you take all the suspense out of the build.

[Charlie C]

As soon as the pony motor starts rolling the RPC you can go and close the switch to the RPC. It really just needs a nudge to get started.

[precisionworks]

you take all the suspense out of the build.

The cap start RPC, like the one you built, is a nicer piece. This design eliminates the timer (or potential relay), and one of those two parts will fail eventually - which takes out a starting cap.

would 1/2 work?

I believe 1/2 hp would have been fine. I had the 1 hp sitting on the shelf, plus it already had an attached switch, so it was an easy choice.

All it has to do is turn the shaft to get it going... It really just needs a nudge to get started.

While that's correct, the ideal speed for the idler is about 103% of nameplate rpm. A motor runs at nameplate rpm while under full load, and runs about 3% faster under no load. If the idler is brought up to no load speed before power is applied, there is no large current surge - the current surge is needed only to spin the big rotor up to speed. Since the pony already has the rotor spinning, the lights never dim when the disconnect is thrown.

any downside to having a much larger motor than current needs?

Just like Dan said, the larger idler requires larger supply wiring. It also draws more current, with my 10 hp drawing 7 amps at idle. Surprisingly, a huge motor like Dan's 30 hp needs only 12 amps at idle, and even a 50 hp monster (Jen?) pulls just 15 idle amps.

A big benefit to over sizing the RPC is that it can start a heavily loaded motor without a problem, so a gear head lathe (without a clutch) comes right up to speed. I've read dozens of threads about under sized RPC's, or about the person who added a Bridgeport or other 3 ph machine, and didn't want to have to do this again