RPC vs Motor Generator

I currently have the following equipment running off 3 phase 220v:

Cincinnati 24" shaper (7 1/2 hp) Excello 602 vertical mill (1 1/2 hp) B&S #2 surface grinder (1 hp) Cincinnati #2 horizontal mill (5 hp, I think)

I have serveral other pieces of equipment not currently under power that it would be very convenient to run on 3 phase. (It is either that, or buy a bunch of single phase motors, which seems silly).

What I'm using for an RPC is a 25 hp motor, fired up by a pony. No balancing caps, no nothing. Been meaning to balance it and do a proper install for ages, so far it hasn't got to the top of the project list.

So: I'm asking myself if I should balance the RPC (and if so, to which load?). Or if I should just get one 10 hp single phase motor, drive the 25 hp through a lovejoy, and make true 3 phase. Which would probably make any future 3 phase equipment happier, (ie CNC stuff, welders or plasma, anything that is fussy about its power).

Comments?

Thanks,

Adam Smith Midland, ON

Reply to
Adam Smith
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Try measuring voltages between all 3 legs under the loads that interest you. They should be pretty close. That should give you a good answer as to whether your supply is adequate. I did just that on my RPC.

i
Reply to
Ignoramus25888

You didn't say what "3-phase" voltages you are getting from the 25HP idler, nor how much of the total 15 HP load is running at the same time. I would be surprised if there is much of a voltage imbalance, even at full 15 HP load. I wouldn't consider adding any "balancing" capacitance unless there is a significant amount of imbalance, say, > 10% from phase to phase.

You said, "So: I'm asking myself if I should balance the RAP (and if so, to which

This seems like you are considering driving the 25HP idler motor with a 10 HP single-phase motor through a flexible coupling to make a sort of "motor-generator set". That is not a good idea. Gen sets are not made that way - doing so would simply leave a starting (pony) motor permanently connected to the idler motor. Perhaps you are thinking about making an induction generator. In order for an ordinary motor to be used as an induction generator it must be driven faster than its normal operating speed; there are other problems concerning excitation and regulation. Check into a good Electrical Engineering handbook and I think you will be discouraged re. an induction generator.

Check your phase voltages. Unless they are seriously unbalanced now, I would leave them alone. Voltage balancing must be done for a single load value - be that total load or the load of any single machine. Individual balancing of each machine (separately) would not be effective unless and until all machines were on line. Essentially, you can only balance for 1 load.

Bob Swinney

Reply to
Robert Swinney

NO! You should not drive the 25 hp motor through a Lovejoy. You need to drive the 25 hp using a belt drive that is adjustable. The reason is that if you drive it with a Lovejoy, you may or may not be actually driving it. Say the RPC with no motor driving it turns at 1740 rpm and your 10 hp motor runs at 1725. Then the RPC will be driving the 10 hp motor.

If you do connect the RPC and a 10 hp motor with an adjustable drive, you can measure the current going into the 10 hp motor and adjust the drive so that the current is at or somewhat below the rated nameplate current. It will then be driving the RPC slightly faster than the RPC would turn at on its own. And life will be good 8-).

Dan

Adam Smith wrote:

Reply to
dcaster

I don't understand how the motor could be driven by the generator, I'm afraid. Wouldn't there need to be some kind of loop for that to happen? I'm contemplating a situation were the generator would not have any electric feed lines, would simply be mechanically driven. I would have guessed the effect of driving the 1740 generator at 1725 would be to produce 59.48 hz power, but I may (probably) be missing something.

I'm not at all determined on this, my current situation is not very broken. If you and Bob come back and say I'm crazy to think of this, I'll just balance to the vertical mill, and let the horizontal and shaper be under-balanced.

Thanks,

Adam

Reply to
Adam Smith

Adam, your concerns are with respect to the afore mentioned induction generator. Speeds such as 1740 and 1725 are not absolute. Induction motor speeds vary slightly with loading and are stated on a motor's nameplate with respect to the rated HP. Check into induction generator theory before making any such speed / voltage calculations.

Bob Swinney

Reply to
Robert Swinney

Sorry, I misunderstood what you were thinking. If I were doing this, I would not simply drive the generator with no electric feed lines. Induction generators do not work well when not connected to the power grid. But they do work well if they are connected to the power grid. So if you leave your RPC connected as is and change the pony motor to a ten hp motor driving the RPC through a belt drive, it will work very well. By leaving the RPC connected to electric power, you will not have to generate that phase. You could probably get by with less than a 10 hp motor.

About 25 years ago I made someone a three phase source using a 7.5 hp three phase motor and a two hp single phase motor. It powered a centerless sander that had a 5 hp three phase motor as I remember. It worked well for some twenty years until the business was sold and the sander was moved to a location with three phase power.

Dan

Reply to
dcaster

Asdan said, "> Induction generators do not work well when not connected to the power

And therein lies part of the problem. If you've got a mechanically driven induction generator connected to the "grid" for excitation purposes and if you mechanically drive it faster than slip speed you will, in fact, generate AC. If this were done, you'd see your watt/hour meter running in reverse compared to its usu. direction of rotation. This would indicate power was being supplied to the grid rather than consumed from the grid.

In the more general case of a "line excited" induction generator there would have to be some way of supplying excitation to the generator rather than it being directly connected to the grid as is done in the motor's case. Don't confuse RPC's with induction generators - they are quite different animals.

Bob Swinney

Reply to
Robert Swinney

You learn something every day.

Thanks once again, Bob and Dan.

I'll stick to running the motor as an RPC for the foreseeable future. If I want to get into experimentation for its own sake, I might do that sometime, but I won't plan on running the shop from the experiment.

Regards,

Adam Smith Midland ON

Reply to
Adam Smith

However if the mechanical drive to an induction generator is an electric motor, then the power meter will not run backwards as any power generated by the induction generator comes from the electric motor.

As far as I know most induction generators are directly connected to the power grid. And while you think that induction generators and RPC's are quiet different animals, I think of the usual RPC as being an induction generator with the drive being a three phase motor run on single phase. It may not work for you, but it seems to explain exactly how a RPC works for me. To me a three phase motor connected to single phase power can supply mechanical power or can be supplied mechanical power and the in between point with no power being supplied or being supplied is exactly what most RPC's are.

Dan

Robert Sw> As dan said, "> Induction generators do not work well when not connected to > the power

Reply to
dcaster

That's how I visualize them as well.

Reply to
Don Foreman

Reply to
Robert Swinney

I think an induction motor identical to the load motor might make a nearly perfect RPC under the following condx:

mains excited on two phases, externally driven to negative slip equal to the positive slip of the load; e.g., if load is 1725 RPM, RPC is driven at 1875 RPM.

Now the phase currents in the RPC are all in the "generating" direction. One phase voltage will be EMF - Iphase*Zphase by definition. With the third leg of the RPC is connected to the third leg of the load, third leg voltage will also be EMF - IphaseZphase

-- which is exactly what the third leg voltage should be.

The imperfection here is that if the load motor slows due to load then the slipspeeds won't be equal anymore so the third leg voltage will be a little off -- and phase might drift a bit due to slightly different Iphase Zphase contribution -- but it might be pretty close over a fairly wide range of loads; probably considerably closer than a self-driven RPC running at positive slip.

This setup would not run the electric meter backwards because the drive motor draws at least as much power as the driven motor generates.

In practical terms, it's probably easier just to use the biggest idler you can find. Bigger is better because Zphase is less so there is correspondingly less IphaseZphase drop in the driven windings, correspondingly higher EMF, and correspondingly less IphaseZphase drop in the third leg.

Reply to
Don Foreman

That was my own answer. I got the biggest idler I could humanly handle with my hands and circuits. If I need higher capacity (as may soon be the case for a certain application), I would just add extra idlers.

i
Reply to
Ignoramus9118

Don,

Point well taken. While we are bouncing around in the vagaries of induction generators, I believe what you propose is a "perfect" induction generator (for the load that is). But for an externally driven induction generator it would seem the scenario is: The 3rd leg is exactly restored by generation and slips are totally complimentary; then the watt/hour meter would still be going in the normal direction because of small excitation losses in the induction machine. Now, if external drive is increased by a small fraction of rated slip speed, the excitation losses will be overcome and the induction generator will be driving the grid.

And you wrote: "> In practical terms, it's probably easier just to use the biggest idler

Yep! This the rationale of those that forget about all that pesky "Fitch balancing", and just use the biggest honking idler that they can figure out how to start. Sometimes known as the the "Rozen system"; inelegant but workable.

Bob Swinney

Reply to
Robert Swinney
"

I like elegance, but I sure understand "field expedient" . I continue to find the subject of esoteric interest, but my self-starting RPC was whipped together in an afternoon over a decade ago, works fine, I'll not be fixing what ain't broke. I didn't do any fussy balancing. I threw a couple of caps at it in balance formation and called it good since it worked. I don't need an ammeter to tell me if my lathe is working.

I hit a switch mounted on the lathe, RPC starts. I can then use lathe or mill at will. Done making chips, kill the RPC. The RPC is a bit noisy, but not so noisy I can't hear "dinner's ready".

Reply to
Don Foreman

Hmm. I'm not entirely sure that this noteriety is a *good* thing...

And it's not a case of actually forgetting about balancing the thing. It's just been a busy summer, that's all.

Starting even large idler motors is easy however. My 1/8 hp pony motor spins up a 5hp idler in about one second.

Jim

Reply to
jim rozen

Hey Jim,

The best possible notoriety you could get came >

Reply to
Robert Swinney

He didn't actually *agree*, he just didn't disagree strongly!

I know that converter's a hack. But I'm too busy making chips with it to upgrade.

Jim - the guy who's becoming known for doing things that just basically kinda sorta work OK. Mostly.

Reply to
jim rozen

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