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old blue
01-30-2010, 01:46 PM
Ok here is the deal. I have a 5hp 240v lathe and I have a 5hp rpc that I run
my mill on. The lathe will start up and run on the rpc but starts slow in
the higher gears. I have a 2nd 5hp motor I can use for a 2nd idler, will
this help with the slow start up if I add run caps to the 3rd leg just like
the current rpc? If I have my mill running when I start the lathe it does
not seem to make a difference, but it does not have the extra run caps.

This is the info I used to build the 5hp rpc.
http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

lakeside53
01-30-2010, 02:44 PM
I have the same problem with my lathe - 4hp on a 5hp RPC.. it's just not big enough.. and the higher gear start is slow enough to pop the lathe motor overloads. Not good. And.. it sure doesn't like "instant reverse :)


Just add the second 5hp idler. Run caps may or may not make much difference. If your third leg voltage is too high (or low), add caps, but if not I wouldn't bother.

Put a delay between the orginal RPC and the second idler start up, and make sure your breaker can handle the extra current from the second.

Carld
01-30-2010, 02:54 PM
Yes, having two RPC's on line will help. It would also help to use balance caps between the two legs to the generated leg to balance the load.

My lathe will not start in the higher rpm's but if I start the mill it will help starting the lathe at two gear changes higher. I still can't get the highest rpm and maybe balance caps will make mine work.

Mike Burdick
01-30-2010, 02:54 PM
Old blue,

You might try this for a test...

If the lathe and mill are connected to the same idler then turn on your mill and run with no load. THEN, while your mill is on, try running the lathe in high gear. This would accomplish the same thing as adding another idler.

Edit: oops, you already tried this...sorry!

BadDog
01-30-2010, 03:20 PM
Likewise, I had a similar problem. Most of my shop had been running fine on a 2hp RPC. Then I bought the 7.5 hp lathe! :eek: I didn't really want an RPC big enough to run the lathe running all the time, nor did I want to fool with a start circuit big enough to cold start a 10+ hp idler. My existing start circuit could handle a 5hp start with only the addition of some bigger caps. And once the 5hp is running, I use it to start a larger 10hp idler. I used some relays and A-B momentary switches in a NEMA (?) cabinet to allow me to start the 5, then (only when the 5 is running) start the 10. Once it's running, I can drop and pick up either idler at will. Then there is one big red "e-stop" that shuts down the whole thing. Complete build with schematics pictures and discussion is over on PM in the RPC forum.

Anyway, my RPC running in 15hp mode will start my lathe easily/quickly even in the higher gears.

old blue
01-30-2010, 07:33 PM
Likewise, I had a similar problem. Most of my shop had been running fine on a 2hp RPC. Then I bought the 7.5 hp lathe! :eek: I didn't really want an RPC big enough to run the lathe running all the time, nor did I want to fool with a start circuit big enough to cold start a 10+ hp idler. My existing start circuit could handle a 5hp start with only the addition of some bigger caps. And once the 5hp is running, I use it to start a larger 10hp idler. I used some relays and A-B momentary switches in a NEMA (?) cabinet to allow me to start the 5, then (only when the 5 is running) start the 10. Once it's running, I can drop and pick up either idler at will. Then there is one big red "e-stop" that shuts down the whole thing. Complete build with schematics pictures and discussion is over on PM in the RPC forum.

Anyway, my RPC running in 15hp mode will start my lathe easily/quickly even in the higher gears.


I would guess you have run caps on both idlers.

BadDog
01-30-2010, 08:16 PM
I would guess you have run caps on both idlers.
That I do...

Weston Bye
01-30-2010, 08:51 PM
Adding a flywheel to the RPC may help. The RPC is depending on stored rotating energy and magnetic field to carry over to the extra phase. The flywheel won't help store any magnetic field, but will keep the armature rolling at speed for a little longer.

Chris Alton
01-30-2010, 11:22 PM
I also have a 5hp R.F.C. and a 5hp on the lathe. I start the converter and then the mill with the belt removed. I will be getting a 10hp R.F.C. to add on and run primarily with the 5hp to come on when running the big welding machine.
5hp R.F.C. will start and run a 5hp motor but not a 5hp with a belt to a gearbox even with the lathe in neutral. My lathe just has too much resistance.
Chris.