Rotation by Lorentz or by Faraday? 3

[Intermesher]

The following assumes the application of a constant DC current to rotors that have no rotational inertia to start with.

Some illustrations show the flux from the pole of a permanent magnet causing rotation by 'pulling' the laminate core of a coil wound electromagnet into alignment with the permanent magnet; before stopping.

http://www.unicopter.com/1904_7.gif

However, other illustrations show the flux from the pole of a permanent magnet causing a portion of a coil of wires to transition across the full width of the flux; before stopping.

http://www.unicopter.com/1904_8.gif

Are both of the above statements correct ~or~ where is my limited intelligence coming off the tracks.


Thanks,
Dave

 

[Intermesher]

The following is a questioning of the most rudimentary part of the large motor question.


Lorentz force shows that if a current carrying conductor is located in a flux path the conductor will move.

This sketch is the Lorentz force diagram in a slightly different format.

http://www.unicopter.com/1904_15.gif

It shows a pair of ring shaped permanent magnets, which are fixed in space (stator). A single wire conductor is located between the two magnets and this conductor is free to rotate about the axis that is common to the ring magnets. In some un-described way a current is applied to this conductor.

The questions then become;
A/ Will the conductor continuously rotate about the axis if the direct current is a constant one?
B/ Will the conductor continuously rotate about the axis if the direct current is a pulsed one?
C/ Will the conductor oscillate if the current is an alternating one?


Dave ????

 

[Intermesher]

Bill,

Thanks for the lead to homopolar motors.

It's too late to look deeply into tonight.(2:00 AM) However, it shows that it is possible. Perhaps today's super magnets will make it more attractive, particularly for a counterrotating application. Putting the coils in the stator also eliminates the slip rings.

It's worth looking a little further into. Thanks

Dave

 

[electricpete]

The questions then become;
A/ Will the conductor continuously rotate about the axis if the direct current is a constant one?..

As I see your most recent post is a simplification of the motor for purposes of clarifying the underlying principle. It looks at the same problem in a different inertial reference frame. As you no doubt know, the force is the same in any inertial reference frame. If it works on dc, the principle is the same as your earlier motor and the motor I proposed.

Namely, the principles which suggest these motors will produce torque:
1 – We know there is force F=qVxB = Length I x B on a current carrying conductor in a magnetic field
2 – We think that that F=qVxB = Length I x B force still occurs when the conductor is moving relative to the magnets that produced the field (as long as current and field don't change).
3 – We think that at least part of the equal/opposite reaction force associated with that force on conductor occurs on the magnet which is located on the other side of the airgap. This amounts to torque transfer accross the airgap.

Items 2 and 3 are subject to further discussion (are they correct?) but 2 and 3 must be true for these devices to work. Even if 2 and 3 are correct, we still need another step of analysis to understand torque vs speed characteristics.

fwiw, I have done an F.E. analysis of a simplified/reduced version of my proposed motor in the condition (attached). It shows that in the static condition of this simplified/reduced device there is a force on the conductors roughly predicted by F=qVxB, and that the equal/opposite reaction force occurs primarily on the permanent magnets. What it means is for this particular geometry in the static condition torque is produced.

Also we know that any dc motor of the type we discuss must include generator action. If you rotate it backwards it should produce dc voltage. If you rotate it forward under load, it should produce a voltage opposite in polarity to the applied voltage which permits conservation of energy (I*E_induced = Pmechanical including mechanical losses). I am still trying to figure out how that would occur. As a start, in my motor diagram, the flux from a single magnet linked to a single partial arc coil changes over time which could possibly provide the mechanism for induced voltage. But one would think the device should work even if the rotor was uniformly loaded with magnets, in which case it is very difficult to see any mechanism for change in linked stator flux over time. Without that piece of the puzzle, it can't work.

The questions then become;
.....
B/ Will the conductor continuously rotate about the axis if the direct current is a pulsed one?
C/ Will the conductor oscillate if the current is an alternating one?

As I see it, if these devices as built do not work on dc, then they will not work on ac or pulsed dc. To take advantage of any of these, you would need more than one phase in your stator winding.

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[electricpete]

If you rotate it backwards it should produce dc voltage...

Correction - I should have said polarity of induced voltage will reverse depenpding on direction of rotation and motor/generator action depends on comparison of terminal voltage to induced voltage.

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[waross]

Here's another typo, Pete.

If you rotate it forward under load, it should produce a voltage opposite in polarity to the applied voltage

Induced voltage or back EMF is in the same direction as the applied voltage. As the motor is overdriven by an overhauling load or other factor, the back EMF or generated voltage increases. At the null point of changeover from motoring to generating, the applied voltage and the back EMF or generated voltages are equal and no current flows. As the machine speed is further increased, the generated voltage exceeds the applied voltage. At this point a reverse current may flow from the higher voltage source (the machine, ie; motor cum generator) to the lower voltage source IF the external circuit allows reverse currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I guess "opposite polarity" or "same polarity" depends on the particular circuit your are picturing. Here is the circuit I am picturing: draw a loop including the source voltage and the induced voltage and traverse that loop in the direction of current flow during motor operation.... then the induced voltage has a polarity opposite the applied voltage (if you disagree I can post a picture). On a more practical level, the applied voltage is in a direction to increase current flow and the induced voltage is in a direction to decrease current flow (during motor operation). But I personally don't care what you call it.

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[electricpete]

The polarity question is not important and I know Bill understands it better than anyone else here.

I just wanted try out to see if I could actually draw it here in the forum using Takoma 10-point font. Looks ok in my preview window so here goes:

DC Motor Drawings:

1st drawing highlights the series aspect of voltages.... same polarity with respect to the current loop:

?==== -Vsource+ ======== R ====== +Vinduced- ============?
? ?
?== =============================================?

2nd drawing highlights the parallel aspect of voltage sources: opposite polarity with respect to common node at bottom:

?==================== R =========================?
? ?
+ +
Vsource Vinduced
- -
? ?
?== =============================================?


The interesting thing: you might be inclined to change your terminology based on which drawing you look at, but they're of course the same circuit.

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[electricpete]

Whoops the terms opposite/same were of course reversed in my labels. Should've been:

1st drawing highlights the series aspect of voltages.... opposite polarity with respect to the current loop:

2nd drawing highlights the parallel aspect of voltage sources: same polarity with respect to common node at bottom:

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[waross]

Just put a voltmeter on the machine terminals and an ammeter on one of the lines. As the motor is over driven from full load to full generation, the voltage will increase slightly and the current will fall to zero at the transition point and then rise to maximum in the other direction.
The back EMF will rise more than the terminal voltage but it is awkward to measure directly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Hi Pete;
Try this to get your end characters to line up.
[ignore][tt]MonoSpaced Text[/tt][/ignore]

The current changes direction depending on generating or motoring mode.

Try a 12 volt battery in parallel with a 10 volt battery.
The 12 volt battery is the source and the 10 Volt battery is the back EMF of the motor. Be sure to add some conductor resistance and an equivalent resistance for the commutating poles and armature resistance.
Now change the 10 volt battery for a 12 volt battery. The back EMF has risen 2 volts to 12 volts. equal voltages and no current.
Now change the motor equivalent battery for a 14 volt battery. You will have full current in the opposite direction. This agrees with both your sketches.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

As I overheard on a jobsite (actually about someone younger than me);
"He's old enough to remember DC."
We don't see the big DC machines on new construction that we used to.
I used to tell my students, sometimes the only difference between a motor and a generator is a couple of volts or a couple of RPM.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Bill - thanks for all the continued tutorials. However there's nothing new in there for me I'm afraid. You'll recall this whole discussion started when you said I had a typo regarding polarity. I think I have addressed that and I have clarified my comments applied to motoring mode.

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[electricpete]

I was reviewing Electromagnetic Field Theory" (1979) by Zahn and found the following statement on page 431:

It is impossible to design a commutatorless dc machine. Although the speed voltage alone can have a dc average, it will be canceled by the transformer electromotive force due to the time rate of change of magnetic flux through the loop. The total terminal voltage will always have a zero time average.

I have to admit I don't totally understand that (like a lot of stuff in that book), but Zahn ought to know.


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[waross]

Check out homopolar motors, Pete. They do have sliding contacts of some type such as conductive bearings, brushes or a conductive liquid dip, but they do not have a commutator. Unfortunately they have little practical application due to their very low efficiency.
Now there's a challenge for someone; figure out how to improve the efficiency of a homopolar motor. I suspect that figuring out how to add some iron to the magnetic circuit may help.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Thanks Bill. But once again it is nothing new to me. I mentioned that homopolar motors use brushes 24 Mar 10 23:23. Zahn covers Homopolar motors on page 420. He also makes the statement about impossibility of cummutatorless motors on pages 430-432. From the context it is clear he is not considering homopolar motor in this category

http://ocw.mit.edu/NR/rdonlyres/resources/RES-6-002Spring-2008/Textbookcontents/chp06_text.pdf

If you don't like the terminology, feel free to take it up with him.

The reason I quoted this is as it relates to the original post and the OP's followup questions related to his device (nothing to do with homopolar motors).

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[electricpete]

To me, the Zahn quote is important because it makes me think it is less likely that "my device" and the OP's device would work. But the exact reasons are strill tricky and I still can't understand completely why.

He highlights the subtleties of calculating induced voltage in similar situations. A very good concise summary of approaches for caluclating induced voltages (more concise than Zahn imo) is on figure 9 the last page here:

http://www.physics.princeton.edu/~mcdonald/examples/EM/cohn_ee_68_441_49.pdf

I focus first on my device which I think is a little simpler. It is clear there is flux cutting = motional induction on only half of each loop (the half inside the core). There is also transformer action, but to figure it out we need to know the return path for the flux. Flux could either turn in direction of rotation or opposite direction of rotation once it enters on the core. On the surface, it seems like we could control the polarity of that transformational component by our positioning of flux return paths relative to coils which would dictate whether the transformed component adds or subtracts. But that is just a thought... could be way off base.

Now looking at the OP's device. One version of it is very similar to the homopolar motor, except that conductors are stranded / series instead of a solid conductor disk with terminals around OD and ID. Zahn analysies induced voltage in the homopolar motor page 423. He uses equaiton (11) which is the Galilean transformation E = E' - v x B. It applies when the contour moves compared to the conductor. It is a fair analysis when the conductor is solid and contour remains inside the conductor. I'm not sure whether we can apply it when we have multiple conductors insulated from each other.

Let's set aside the question of induced voltage and just look at the force. To me it seems pretty clear all of these devices produce force on conductor F = qVxB in static condition (anyone disagree?). The question is then: if we hold the current constant, what is it that would happen when the rotor begins to move that would invalidate or cancel that force. It's tough for me to visualize what's going on that would prevent these devices from working.

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[electricpete]

I focus first on my device which I think is a little simpler. It is clear there is flux cutting = motional induction on only half of each loop (the half inside the core). There is also transformer action, but to figure it out we need to know the return path for the flux. Flux could either turn in direction of rotation or opposite direction of rotation once it enters on the core. On the surface, it seems like we could control the polarity of that transformational component by our positioning of flux return paths relative to coils which would dictate whether the transformed component adds or subtracts. But that is just a thought... could be way off base.

Actually, I could certainly make a case that tranformer-action emf is zero and motional emf while rotating is constant non-zero. Considering that the rotor can be modeled relatively uniform with respect to angle (looks no different when we rotate it) the radial flux that it generates is uniform crossing the airgap. Then even if the stator is not unfirom with respect to angle (and it cannot be due to the requirement for flux return path that doens't cross the coils), and if stator current is held constant, I think the flux linking stator coil should be uniform over time resulting zero transformer action voltage. If true (it seems true at the moment), that supplies a plausible answer to the question I have been wondeirng since the other thread: what is source of induced voltage. (The answer would be the motional emf). It makes the pendulum nugdge back a hair the other way towards believing it is possible this thing might work.

Assuming we control this thing as constant current source, it would be a constant torque device (torque ~ Radius*F where F = I L x B where I and B are constant). For a constant torque device, power increases linearly with speed. That is convenient because induced voltage also increases linearly with speed, so energy would be conserved. If it were supplied by constant voltage, it would act more like shunt dc motor. That's how it might work anyway.... (or maybe not).

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[Intermesher]

Peter, after rereading the description of your radial flux motor concept, I agree that there is much commonality between your concept and my axial flux concept. A primary similarity is that the conducting wires are on the stator and the permanent magnets are on the rotor(s). This obviously has the advantage of eliminating the slip-ring.

Pursuing Bill's mentioning of the Homopolar motor, this web page came up 'Faraday Paradox'

http://en.wikipedia.org/wiki/Faraday_paradox.

"The experiment proceeds in three steps:
1/ The magnet is held to prevent it from rotating, while the disc is spun on its axis. The result is that the galvanometer registers a direct current. The apparatus therefore acts as a generator, ....
2/ The disc is held stationary while the magnet is spun on its axis. The result is that the galvanometer registers no current.
3/ The disc and magnet are spun together. The galvanometer registers a current, as it did in step 1."

Further on it says; "There is no paradox or difficulty if one invokes the special theory of relativity."
Now it is really becoming confusing.


WHAT IF:

The previous sketch 1904_15 was changed to

http://www.unicopter.com/1904_16.gif

In the sketch, the commutators (plural) are the stator and the magnets become the rotors. These two magnets are now composed of many thin sectors, which all have their polarity in the same direction.

Might the 'laminated' PMs cause experiment 2/ to now operate as experiment 1/?


Dave

 

[electricpete]

I'll have to study that when I get a chance.

I am still interested in the commutatorless (and brushless) variety. It should be pretty easy to set up a simple check for dc generator action spinning the rotor and seeing if I can generate a few millivolts change in meter reading. I will do that and let you know results (this weekend or latest next weekend).

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[electricpete]

Looking at that wikipedia article, there is never voltage created unless the conductors spin... in which case there is a measured voltage in the non-spinning ref frame (given by E = v X B times number of turns). That doesn't seem to bode well for my experiment.

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