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30HP RPC questions


Feb 18, 2014
Southern Illinois University
For the last few years I've been limping my shop along on a patched up 100A service I'm the last one on the line, and only one on the transformer that is new (I blew up old one). Service was 6awg aluminum overhead about 150'. Started out with a 15hp cont 20 for 30min VMC, on a home made 15hp rpc with a 5 and 10hp idler, and had to back the spindle acceleration down to keep it all happy, but I could run it pretty hard on most of the stuff I was doing.

Since then I've added a few other machines small, and am working on a turning center. I just completed a major service upgrade a 320/400A service with one of those dual 200A breaker meter bases, and I think 500kcMill aluminum underground. One side is for the smaller single phase stuff, lights, outlets, compressor, house. The other side is strictly for my 3 phase setup. I just picked up a 3 phase panel and installed it with the service feeding the outer lugs, and my rpc setup powered and back-feeding through a 50A breaker. (Yes I know i could single phase my equipment, No employees, and I want it this way) So now that I have some real power I've been searching for a deal on a big motor for a bigger RPC. Thursday I picked up a Baldor Super E 30HP, 1760 RPM TEFC 78FLA $100. I have a size 3 contactor for it and a 100A breaker for my panel. My hope is that I could start it with my 15hpRPC and avoid pony motor and other hassle. Curiosity got the best of me, and I tried - it does, but I have questions.

So first off my current RPC. This was a salvaged commercially made 5hp rpc. Baldor 5hp shaftless generator at 1760RPM. I had to put it back together, and I changed the input contactor a bit. Moved it to a different box next to a size 2 for my 10Hp. I set it up so you can't start the 10 till the 5 is running. The 10hp is a baldor 1760rpm 10HP motor, and has the run caps on the load side of the contactor. It starts easy, dims the light just a bit draws 150A peak. I think I'm right at 15A idle current on the whole setup. Idle voltages are like 250 from utility and 248/245 from utility to generated. It works good and the only trouble is occasionally the vmc spindle drive will error on low voltage if I kick the coolant on when the spindle and air compressor is running. It's got like 6hp of coolant pump though. I've never seen much current on the generated leg at the VMC. On spindle ramp I've seen peaks of 150A, and like 30A from the rpc.

On to the 30hp deal. I just have a couple outlet on my panel and connect machines with cord/plug for now. So I wired a plug on the 30, put on a face shield and my welding gear, fired up the rpc and flipped the 70A breaker for the outlet. It all grunted and took right off. Maybe 2sec to get up to speed. A few more tries and I was able to measure some current and voltages.

Utility legs was right around 400A peak, generated leg was 78A peak. Line voltage unloaded is like 250, lowest I saw was 210. Measuring the utility lines it idles 50A 240V with the RPC on i think the generated leg had 13A at the 30HP. If I shut the RPC off the current is 53A on the utility legs at the 30HP. I think the 30HP by itself voltages were 240 225 218.

Curious of how it would act with a pony, I shut the rpc off, shut the 30HP breaker off, let it slow down a bit and flipped it back on. Right back up to speed with about 200A peak.

At this point I figured no reason to mess with pony start, not much peak current improvement.

Anyway last night my brother came by and I wanted to show him it starting. RPC on flipped the breaker and nothing, it sits still and grunts, pops breaker. I roll the shaft with my foot, try again and it takes right off. Several more restarts and it was fine.

Today I wanted to do some more checking, and it no started again today, but started right up on the second try.

When I first got my 10hp I used to start it with a drill/belt and throw a knife switch. Wanted to try that with the 30. No dice, just grunts and slows down. I'm guessing that I didn't get it close enough to synchronous speed, maybe 1000RPM at best.

So whats the deal with the no start? Not enough RPC? Dead spot in motor? Needs high speed pony start? If I did a pony any reason to keep the 5 and 10hp stuff? I wanted to keep it for small loads. Current equipment 15/20HP VMC, 5/7.5HP VMC, 2hp lathe, Small OD grinder, 5HP gear hob, 2hp cold saw. Probably a small turning center sooner than later.
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Do you have start caps on the new 30HP idler motor? They probably need to be significantly bigger than for the 5/10HP versions. Or are you trying to start it off the already-running 10HP motor? That could be pushing it.
I agree, need to bring in start caps on that big motor. Put them on an "on delay' timer for maybe two seconds. should start 100% now.
I'm starting the 30hp as a load motor with an good working existing 15hp RPC. So I need to add a start circuit to the 30 to do that? I thought it wasn't good to use a starting circuit after 3phase had been established?

I guess what I'm asking, is this the behavior I'd see with not enough RPC to start the load? The shaft sits perfectly still, all of it growls and it pops the breaker. If i let it cool for a few minuets, turn the shaft a 1/4 turn and try again it takes right off.

If it's too small RPC I'd expect the shaft to turn slowly or something then kick out - or is that wrong?
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If you have a dead spot in the rotor, you should be able to feel it if you turn the shaft by hand, in certain positions you will feel more torque than others. There will be a minor variation, like say 26 to 34 divots equally spaced round the rotor.

You can connect the motor for 480v to perform this test, it will draw half the amps or less, giving you a lot more time before the breaker trips.

Your 30hp motor delivering 240,225,218 volts is good, better than my 5hp motor which is 93% efficient, 91% power factor, delivered 240,216,210 volts. Basically motors get more efficient the bigger they get, its not real intuitive.

Your 30hp rpc may work fine as an rpc all on its own, if you have enough start capacitance to start it. It may work so much better you dont notice much difference if the 10hp is in parallel with it.

As for adding more starting capacitors to an already running RPC, it is no problem. The capacitors and contactor will experience a harsher inrush current, because you are adding them in parallel, to a randomly charged capacitor, but they should be able to handle it.

Start capacitors actually reduce the amps drawn from the utility, up to a point, add as much as is needed to start the motor.

You do need to make sure they are on the same leg as the start caps are in the other rpc, otherwise they will be trying to start the motor in the wrong direction.
Dead spot in a motor may indicate an issue with the rotor.

A three phase motor with no load on it should not have any dead spots where three phase input will not start it. Even an RPC.
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I tinkered with it a bit more today - and I feel better about it now. I guess I had been sort of concerned with the 2 no starts it had, the no pony starts, and the high idle current draw. I'm not even 100% on what happened on the 2 no starts as I was so caught off guard by them I didn't pay enough attention. I went looking for a no start when powered by my RPC. I tried like 8 times, with the rotor turned 1/8 turn each time. Then I started it a half dozen other times measuring current and voltages. 100% start rate today.

I did find my 5hp by itself will not start it, and the rotor didn't really move before the breaker tripped. The 10hp will almost start it before the 70A breaker trips. I think it would start on a 100A breaker with the 10HP idler.

I did get it it to pony start - I just was not spinning it fast enough.

I also spent a little time watching the meter. Looks like my total RPC setup draws about 1.4KW when idling.

So now I need to decide how to integrate all this in. I want it all outside, but my 5 and 10 are open drip proof motors. Is there any reason to keep the 5 and 10hp idlers? Is it the worst thing to fire up the 30hp to run the 2hp southbend lathe for 5min or the cold saw for a curt, or should I keep the 5 for that use.

I'm thinking the 10hp is going for sure. it's 85.5% and 76 power factor. The 5 is 87.5% and 78. The 30 is 93.1 and 80.
Do you know what size your utility transformer is? And hoe many feet is your service cable to your meters and shop?
I do not know the size of my transformer. I'm guessing it's a smaller one as they did not replace it with the service upgrade and we had 100A or less. The cable is approximately 150' of aluminum 350kcmil from transformer to meter, then 3' of 3/0 copper to the panel inside.