Flywheel on a rotary phase convertor

No load, no generation, Don. An idler running with no load motor does not constitute a RPC. The network and supported current flow through that network makes a RPC. Remember the idler is running as a single-phase machine and the 3rd leg is open, that is, until it is connected into a RPC.

OK. So they aren't series resonant circuits when there are no run caps - granted. But the interconnection of idler and load and their associated current paths are the same, even without run caps.

Don Young sez:

"Since the running idler and load motors are directly connected in parallel . . ."

You are right about there being "many ways to understand and describe how things work" but the concept of an idler and load motor's respective windings being in parallel is not one of them.

Bob Swinney

[...]

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

Dont forget a nice heavy rotor IS a flywheel.

Gunner

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Theodore Dalrymple,

No current flow, yes. But the third leg does come up in voltage, even when open circuited. While it won't do any work, folks would be tempted to say that the third leg is indeed "generated" even when it's open circuited. Another one of those semantic mine fields....

Jim

Don, I think you have a great insight here. The two motors are clearly in parallel; swap a lead on either motor and the "bigger" one wins. I still am not sure if a flywheel has any significant effect on which one is bigger (though it seems like it would).

Steve

D>Since the running idler and load motors are directly connected in parallel,

In my opinion you need to realize that a RPC is an induction generator.

As far as flywheels are concerned, a flywheel will keep the slip angle from changing as quickly. So a RPC without a flywheel will draw power from the mains more quickly when the load is increased. Score points for that side. On the other hand, a RPC with a flywheel will draw power from the flywheel when the load is increased as well as from the mains. So score points for the other side.

In the real world, it does not make much difference as the change in speed of the RPC should be slight, and therefore only a small amount of power can be drawn from the flywheel. Having a flywheel would help with an undersized RPC when the load motor is plugged.

Dan

Don sez: ">> Sure it does. With the idler spinning, a voltage is generated in

I'm not sure what you mean, Don. You said "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".

Firstly, I don't understand why the issue must be complicated by bringing the rotor field into the picture. It is well known the stator field and rotor field are more or less locked into rotation at the same speed, but it is incongruous to speculate the rotor field is solely responsible for the stator field's third leg voltage. Remember we are essentially talking about a single phase motor here with an open coil connected to the center point of the line-fed main winding. I respectfully submit the third leg voltage is not in quatrature with line voltage. The only way for that to be a true statement would be in the special case of a precise amount of capacitance connected from one line side to the end of the 3rd leg coil; an amount of capacitance (start cap if you will) necessary to achieve an exact 90 degree phase shift between line voltage and the 3rd. leg.

Bob Swinney

Hey, Bob, what about delta-wound motors? Sure looks parallel to me!

Interesting! The relative impedances are also important here. The larger motor with lower impedance (and probably higher inertia) will govern. Look at the terminal voltage where the two third legs are connected. If the motors were perfectly matched, their effects would cancel and this terminal voltage would be zero. If they are not matched, the voltage (phase) of that terminal will be determined by the motor with the lower impedance, and the phase of this voltage determines (or indicates) the direction in which both motors turn.

Right, up to here.

Only if the rotor has some significant permanent magnetism -- not usually the case.

Hey, Don, it sounds like you are beginning to go off half cocked, sort of "Iggy style".

Do this: Visualize 2 deltas connected in "parallel" if you will.. Obviously the current paths through the branches, where the lines are connected, are in parallel. Now look at the common point where the other 2 legs of both deltas connect together. Those points are no more in parallel than they would be if they were between two wyes.

It may be helpful to look at the configuration in its wye equivalent. Same thing. All this speaks to the very complex current flow in an idler and load connected as a RPC. Two 3-phase induction motors running on the same

3-phase line do not constitute a RPC. A RPC is two 3-phase induction motors running on single-phase current. Capacitor augmentation assists in tuning the network such that it appears to be operating from a 3-phase line.

Bob Swinney

Don,

See my previous post, where I tried to show 2 induction motors operating from single phase current in a RPC configuration cannot be in parallel.

Because it's there, producing phase shifts and emf's that cannot be produced by a network of similar topology having only R's and L's.

The rotor field is always in space quadrature from the stator field. This is well-established in about any textbook on the subject. That being the case, the emf it induces in the third leg is necessarily in quadrature with the emf impressed by the line (and countered by the stator field) in the other two windings.

Now consider a Y-connected motor. Rotate the Y 30 degrees clockwise so the right hand leg is horizontal, with line connected across the lefthand legs. Note that the vertical components of the excited coil windings add while the horizontal components cancel. Therefore, there is no emf induced in the horizontal third winding by direct transformer action. Emf induced in the third winding is therefore solely due to the rotor field -- and since that field is in quadrature with the stator field, the emf in the third winding must be in quadrature with the excitation voltage. QED.

The terminal voltage on a loaded third winding will vary from exact quadrature due to I Z drops, which have opposite sense in the excited windings from those in a loaded third leg -- look at the directions of current flow. . But if you connect a scope from third terminal to neutral (center of the Y) in an unloaded idler, it would show an emf in quadrature with line voltage.

Credit to Jerry Martes for showing me this aspect of induction idlers.

I thought it was the rotor *current* that was in quadrature.

Basically the rotor currents cause a rotating B field to exist inside the stator. How it does this doesn't matter much, but suffice it to say that the phase of the rotating B field agrees with the incoming excitation (which of course supplies all the power to the gizmo) which means it will cause the correct phase voltage to exist on the third lead.

Jim

Look at

Colored lines are windings, white lines are connections. 'Splain to me how the windings of same colors are not in parallel...

Don "Half-cocked" Foreman

I saw it. I just don't agree with it. See recent post showing things in 3D.

Transform to Y using the usual Y-delta transforms if you like. See any textbook on the subject.

In the Y case they don't look in parallel if there is no neutral connection. However, since a delta depiction clearly shows that they

*ARE* in parallel, they are in freakin' parallel, BOB! Must I glue up some popsicle sticks for you?

Can you explain the discrepancy? :

Sorry, Don. We were discussing RPC's and I assumed (we know what that does) you were thinking of RPC's as well. Two 3-phase induction motors connected as a RPC are not, repeat are not in parallel. I'm afraid you have jumped to the conclusion that two 3-phase induction motors connected in RPC fashion are merely connected in parallel. That is not the case. See a later post in which I tried to explain the defference.

Bob (if it sounds like Iggy, it might be Iggy, No! it can't be) Swinney

Well, Don - you've missed the point again! What part of "2 induction motors operating from single phase current in a RPC configuration cannot be in parallel" did you fail to understand. Your well intentioned, and colorful, drawings were not of a RPC configuration. Draw out a RPC and I think you may understand. Oh! be sure to include some capacitors. They (in electronic terms) might be considered as steering capacitors, for it is their job to force the convoluted currents to flow in such a way as to

*emulate* true 3-phase. Note, I said *emulate* because current flow in a RPC is not the same as current flow in parallel connected 3-phase motors, no matter which transform is used.

Here's how my phase converter is wired when it runs a load motor:

idler Load

L3-------------------------L3 / \ / \ L1 --- L2 L1---L2 | | | | | +--------------------->---+

+-----------------------------+ Hot2 Hot1

L1 connected to L1, L2 connected to L2, L3 connected to L3. L1 and L2 are also connected to utility power Hot1 and Hot2.

How are these motors not running in parallel?

They are.

Leg numbering on the load motor is somewhat arbitrary, but however these motors are connected, the poles are connected in parallel.

Maybe you have a different definition of "connected in parallel". My definition is that potential between like points (L1 to L1, L2 to L2, L3 to L3) is zero. Current does flow from L3 of one motor to L3 of another.

i (now I have more bells and whistles on my new phase converter, like balancing and power factor correction caps, dual idlers, blah blah, but they do not change the basic fact that these motors are indeed parallel connected)