Flywheel on a rotary phase convertor

Hi all,
I'm trying to figure out if there is any benefit in adding a flywheel to a rotary phase convertor. I've heard varying opinions on the subject.
Having thought about it myself, I've reached the following conclusions:
(i) The sag in voltage on the third line is caused by the fact that it is not connected directly to the supply. The flywheel doesn't change this. Nor will it change the steady speed at which the rotor turns, so unless it has some averaging effect on a cycle-by-cycle basis which I haven't considered, it won't affect the quality of the three phase output when the convertor is running in a steady state.
(ii) It might be an advantage when trying to plug reverse the load motor. As far as I can see (on the most simplistic level), the motor with the most kinetic energy will win.
I can't seem to find any used flywheels to fit my motor, but I can get a brand new flywheel for 40. I'm not sure if it is worth it in order to satisfy my scientific curiousity. If I get a different motor, I can get a flywheel for next to nothing, but that will involve lots of effort, bartering and deals in order to get a motor which isn't quite so cool.
Any opinions and arguments? Thoughts would be appreciated...
Best wishes,
Chris
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On Tue, 3 Jan 2006 21:07:42 +0000 (UTC), Christopher Tidy

Correct. It can only provide limited power.

Correct.
I am not sure why you think so. Would you clarify why you think that plug reversing a load motor would somehow slow down the idler motor. The idler, after all, spins with the frequency of the AC mains.

Consider also the limits of your circuit and increased startup current and duration of startup, if you add a flywheel to the idler.
As I learned about my own 10 HP idler, its startup current was whopping 120 amps. It starts up quickly (1 sec), but that would not be the case if I added a heavy flywheel.
i
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On Tue, 03 Jan 2006 21:14:34 GMT, Ignoramus29795

No it doesn't. It rotates at less than synchronous speed. If it ran at synch speed there would be no slip hence no induction in the rotor.
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    [ ... ]

    I have read reports here of under-rated rotary converters actually reversing, instead of the load motor, when plug reversing. I think that the flywheel might indeed solve this problem.
    Enjoy,         DoN.
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There is no experimental data on this subject as far as I can see.
I have seen coherent, cogent arguments from respected folks that support both views - one that it will help, the other that it will hinder.
Those who suggest a flywheel is bad say that rotary converters deliver transient power to the generated phase by allowing the rotor to slip, and a flywheel prevents this.
Those who suggest a flywheel is good say that that flywheels store rotational energy and will this is made available to transient loads.
Those two preceeding statements are pure paraphrase on my part, and I of course apologize if I have mis-represented anyones comments. But there's no empirical data out there as far as I can tell.
It wouldn't be that hard to instrument and measure.
Jim
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jim rozen wrote:

Then maybe one needs a "dual-mass" flywheel like they are putting on the diesel pickups now.
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[...]

That sounds like an interesting thingy. Got any details on it?
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wrote:

Some kinetic energy is necessary for the thing to work, but my bet is that the rotor has more than enough and more would not help.
Kinetic energy is necessary for the idler to produce power in the third leg during parts of the cycle when less or none is being drawn from the mains. Energy is also stored in the magnetic field, but its ebb and flow is in quadrature with third leg power. This is a cycle-by-cycle event: it accelerates (accumulates energy) during part of each cycle and decelerates (gives up energy) during other parts of each cycle. The result is speed ripple, which would be greater for rotors with small moments of inertia.
The power levels drawn and delivered are a function of slip speed which governs both stator current and induced emf -- back emf in the case of the driven windings and generated emf in the case of the third leg. As the third leg produces more countertorque from higher current flow thru it, the rotor will slow until slipspeed has increased to the point where enough power is drawn from the mains to regain equilibrium.
Observers (Jerry and Fitch) have said they didn't note much change in idler slipspeed with varying loads. However, resolution of 1% or better would be necessary to see speed variations because the slip speed range from no load to full load in most induction motors is only a few percent at most.
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wrote:

Dont forget a nice heavy rotor IS a flywheel.
Gunner
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Regarding plug reversing, I recently rewired an older BP head. I was surprised to see considerable evidence of arcing near the contacts in the drum switch. I figured that plug reversing it was the reason - a LOT of current flows, and motors with all their inductance do NOT like current changes. So regardless of what you do with your phase convertor, I strongly suggest that you not plug reverse anything using a drum switch unless that switch is extremely heavily built.
I know of no value in adding rotary mass. The armature of an idler motor is already quite a bit of rotary mass.
GWE
Christopher Tidy wrote:

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Christopher Tidy wrote...

A flywheel would reduce, not increase, the idler's ability to respond to load changes. When the electrical load on the idler increases, the idler's rate of rotation falls (I.e., the slip increases). This raises the current draw from the single phase source. The higher winding current increases the strength of the rotating magnetic field in the idler, which pushes the generated third leg voltage up. The upshot of all this is that the response rate of the third leg voltage to electrical load changes is inversely related to the inertia of the idler's armature.
That's my understanding. Perhaps one of the old regulars can explain it better. Is Fitch still around? I seem to remember his doing some tests on this very thing a few years back.
Jim
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Jim Wilson wrote:
Thanks for all the responses.

I'm not sure about this. Yes, it will take longer for the rotor's speed to fall, but surely the stored energy will be dissipated by driving extra current through the load?
Best wishes,
Chris
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On Tue, 3 Jan 2006 22:53:42 +0000 (UTC), Christopher Tidy

Yes, but power is the rate of energy flow. The amount of power it can produce for the third leg (energy delivered per cycle) is a function of slip speed, and field strength hence stator current which is also a function of slip speed.
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IMO, you need to lose the thinking of a RPC as being a form of generator. It isn't. Think more of the RPC as a network in which parts of it rotate in order to supply current throughout. Part of the RPC is the load motor. The idler generates nothing without the load as part of a network. IMO, a flywheel on the idler cannot act as anything more than additional dynamic load on the network. It would be aprox. the same to put the flywheel on the load motor instead. Forget flywheels and spend the money on enhancing the idler-load network with proper capacitance. Complex current flows in all parts of the RPC. In simplistic terms, the idler-load current paths can be viewed as series resonant circuits. Such circuits are "tuned" via capacitance placed in series.
Bob Swinney

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Granted this kind of tuning is the very *first* thing one would do before considering flywheels.
I specifically recall Gary Coffman claiming they would reduce transient response, and yet there's a considerable group of well-informed individuals on the practical machinist board who say they improve matters.
I have to say I find *both* sides to be persuasive, at least at the 'hand-waving' level.
My suspicion is that flywheels probably help up to a point, if one models the rotor as having zero mass to start with. And that the optimum flywheel size will wind up being about one rotor unit in size! This is what a former boss of mine calls 'the schwarz law of the initial maximum.'
Ie, if it works the first time you set it up, anything you do to it after that makes it work worse.
:^)
Jim
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wrote:

We just call that syndrome "fix it 'til it's broke".
Snarl
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Robert Swinney wrote...

Hrm. Is this in response to my post, or Christopher's? I don't think I view a RPC as a generator at all. Perhaps it's more like a rotating transformer.
<snip>

Most of what you said seemed reasonable (I snipped all the unobjectionable parts), but this statement can only be true under limited conditions. There would be a large difference in performance between the two systems for example when plug reversing is used.
Cheers,
Jim
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FWIW, you might view a plug reverse of the load motor as the worst case flywheel effect.
Bob Swinney
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On Tue, 3 Jan 2006 17:20:12 -0600, "Robert Swinney"

Sure it does. With the idler spinning, a voltage is generated in the third leg that is in quadrature to line voltage, even if there are no capacitors anywhere. Transformer action can not produce a quadrature voltage so it must be (and is) generated by the rotating rotor field -- which always is in quadrature with the stator field.

If there are capacitors. But idlers without any run caps still work. In fact, they work quite well if they're large enough.
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I agree, using large motors simplifies everything! You get the advantage of great kinetic energy with very understressed component parts. I favor a pony to "spin up" the first started (should be largest by 1.5) motor.
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