Flywheel on a rotary phase convertor

Cuts down on a lot of confusion, it does.

Jim

Dan sez: "Suppose I take a three phase motor and use some method to get it

Yes, Dan (I think) you'd have 3-phase current going to the rectifier bridge. This is true by virtue of the fact a 3-phase motor running on single-phase delivers 3 phase currents to a resistive load. Notice, I did not say "generates" 3-phase current because the original single-phase line terminals are connected through to the resistive load and the other "phase", if you will, is derived via transformer action within the 3-phase motor. The 3rd. leg voltage, and current, into the rectifier will (obviously) be less than the single-phase line voltage suplied directly to the rectifier.

Now, stop at this point and forget the rectifier. The description above would apply equally well if the load on the idler was composed of 3 light bulbs, also a resistive load. In that case, the light connected directly across the single-phase line would burn at full brilliance while the other 2 bulbs connected from the 3rd leg to L1 and L2 respectively would burn at less than full brilliance.

IMO, your other point re. a RPC needing a load motor to work is moot. This because an idler motor alone is not a RPC.

Bob Swinney

Again, taking some point another poster has made, as if it was his own, Iggy bleats:

"You are right, it does not need a load motor. You could run a resistive load (a 3 ph heater), or, like I do, a 3 phase welder that starts with a transformer and rectifier."

Bob (does not suffer trolls) Swinney

Sorry for taking some time to reply to this thread.

As Jim says, this does seem to be a semantic minefield. But I suspect that were people face-to-face they could hammer out their differences fairly quickly. It is often the way on Usenet.

My take on the contentious issues is as follows.

The idler motor will generate a voltage at the third terminal when not connected to a load, but it cannot supply any generated power because no current can be drawn. The difference here is between generated voltage and generated power.

I would say that the RPC idler and load motors are in parallel in a topological sense. However, the current flow within the windings of the motors will be very different from what would be seen if the two motors were connected in parallel across a three phase supply. I suspect that this is the root of the disagreement here.

All are fair points.

Chris

Out of interest, what are the horsepower ratings of your RPC idler and lathe and mill main motors?

Chris

Thanks for that thought, Wayne. It actually entered my head a month or two ago, but for some reason I'd forgotten about it again. I have a couple of two-groove cast iron pulleys which fit this motor. Each weighs about 10 lb and is about 8" in diameter. I'm not sure if I can fit two on the shaft, but it'll certainly take one.

It seems like there is no data regarding flywheels on RPC idlers. A few people have suggested that it may help with plug reversing (which is what I was thinking) but it seems unclear what the effect will be while the convertor is running in a steady state. My motor already has a pretty heavy rotor (about 8" diameter), but the energy stored will be reduced by the fact that it spins fairly slowly (940 rpm). My aim is to get the best performance out of a convertor with a limited idler size.

If I get chance to experiment with a flywheel and acquire some data, I will. Thanks for all the input.

Chris

Christopher Tidy wrote (in part)...

I don't understand how it's possible by the laws of physics for both of these statements to be true.

Jim

Chris sez:

"> I would say that the RPC idler and load motors are in parallel in a

Well, how's that for getting at the crux of the biscuit, eh? Semantics are us! You display a depth of understanding that goes way past most of the arguments set forth in this thread. Your description, above, very aptly conveys a depth of appreciation for the manner in which currents must flow in a RPC. Perhaps in trying to describe those currents, my use of the term "convoluted" was an unwise choice. I suppose it was not "technically precise" enough for some of the intelligencia on RCM for they sought to jump on words and skip over any element of mutual comprehension. Apparently there was no "mutual comprehension" save for maybe Jim Rozen and yourself.

Parallel was another bad choice of words - although it apparently had more technical appeal to those that demand precise engineering definition. Many of the respondents chose to pick on the absolute definition of "parallel" while totally overlooking the context in which it was used.

Bob Swinney

I think he refers to a case where the third leg of the idler has no external connection; that terminal is just hangin' out in the breeze. It could still exhibit a voltage, but no current flows because there is no circuit in which it can flow. If no current flows thru that terminal , then no power is transferred into or out of it.

Idler is 5 HP. Lathe is 3 or 5 HP, mill is 1 HP. My idler runs the lathe just fine when the lathe is set to the 3HP setting, but can't quite hack it on the 5 HP setting. If I needed the higher speeds (2x) of the 5 HP setting, I'd probably need a bit larger idler.

It might pull it if it were a belt-drive lathe or had a clutch, but it's a gearhead lathe. It tries to work on the 5 HP setting, but it blows the breaker in the lathe before it can get wound up to speed.

Adding some capacitors probably would help some, but I think a bigger idler would be the best solution. I'm content to use it as is. 1000 RPM is fast enough to do everything I want to do on that lathe.

The 1 HP mill has no problem operating at any speed. Smaller motor, less inertia.

Ah, hang on there. The field in the rotor has to be 180 degrees out from the stator fields, from Lenz's law, right?

Jim

Well, I had the benefit of reading all your posts before I wrote a response! Thanks for the compliment.

I'm cleaning and painting the idler at the moment. Should be looking at a first test run as soon as I've wound a new secondary to provide 240 V

-> 415 V conversion.

Best wishes,

Chris

That's exactly what I meant.

Chris

Thanks for those figures. All more data to add to the thought "melting pot".

Chris

Dan sez: "> In my opinion you need to realize that a RPC is an induction generator."

Dan, I know you have some experience with induction generators so I'll ask you to respectfully consider that:

An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it?

Bob Swinney

I was vague, sorry. I meant rotor current. Rotor field (the field the rotor produces) is in phase with rotor current by Ampere's law relating magnetic field to current.

That field is produced by rotor current resulting from EMF induced in the rotor by rate of change of stator field linking it -- Lenz's law, as you say. Rotor current is then this EMF / rotor_impedance.

The rate of change is at slip speed -- stator flux rotates at synch speed, rotor rotates at slip rpm below synch speed. This is from the perspective of an observer on the rotor, using the artifice of rotating fields of constant magnitude to represent sinusoidally varying fields in space quadrature. The equivalance is mathematically correct, and a convenient way to look at things. It isn't the only way to look at things by any means, but I find it easiest to visualize. Reference: "Electric Machinery", Fitzgerald & Kingsley, McGraw Hill.

Rotor field and stator field can be dealt with separately and independently, as when resolving vectors into components. They differ in phase because induced voltage in the rotor is the time derivative of stator flux apparently rotating at slip speed. They are usually very nearly in quadrature because the rotor impedance looks primarily resistive to rotor emf induced at slip frequency. Slip frequency is typically about 2.5 Hz at rated speed so rotor self-reactance is negligable.

Sorry, Dan - Make that overdrive via overspeed from the prime mover to make an induction generator. The induction generator (one made from a common induction motor) will generate when excited by the mains and when its rotor is driven by external means to a speed exceeding that of the motor's synchronous speed. Slip is said to be negative under these conditions.

A fella by the name of Swinney said elsewhere that idler and load motor function both as generators and loads. True statement.

Emf is produced in all three windings of the idler. In the driven windings, those connected to the mains, that emf is slightly less than applied voltage so the current in those windings is (Vline - Vemf)/Zwnding. A similar but phase-displaced emf is also produced in the third leg. This emf can then drive (supply power to) the third leg of the load motor which also is not connected to the mains.

So neither the idler nor the load are generators from the perspective of the mains, but the idler, regardless of what else you may call it, does supply current and power to the load motor's third leg.

I would therefore argue that the idler alone is a rotary phase converter (RPC) because it produces a voltage on its third leg that is of different phase from the mains voltage whether or not it has a load connected to it. It isn't a rotary power converter (also RPC) unless there is a load connected, because if there is no load connection then the idler's third leg has no current so there is no different-phase power.

In either case, this different phase is not exactly right in magnitude or phase to make the result balanced threephase, though if the idler is big enough (low impedance) it'll be pretty close. This is because the IZ drops in the driven windings of the idler are different in polarity wrt the emf than is the case in the third leg.

The discrepancy can be reduced with capacitors, at least for a particular load motor and particular mechanical or useful load.

OK, makes sense to me.

Jim