Faraday's homopolar motor revisited

[PhysicsProf]

It is a classical Faraday motor whose Harvey gave the schematics in the left view (no effect is to be expected for the right view):


Due to the electric field from the battery between the center of the disk magnet and the rim, there is a radial current in the neo. The electrons are crossing the magnetic field so the Lorentz force is tangential. The electrons transfer the Lorentz force applying on them to the lattice of the metal and make rotate the magnet which would accelerate indefinitely if there was no friction.
Under the effect of braking due to losses or if we try to extract a mechanical work or during the phase of angular acceleration, there is a reaction force from the disk lattice on the electrons. Therefore the electrons move not only radially under the effect of the voltage, but also partly tangentially due to the mechanical force from the rotating metal lattice in which they are embedded. It follows that the Lorentz force onto the electrons acquires a radial component that opposes the electric force provided by the battery, and so more current is needed to maintain the rotation. Action/reaction works perfectly.

Juxtposing Harvey's clever figure (right) with the one I presented in my initial post #1, we see that there IS AN EFFECT, a force on the wire, for the one on the right (Harvey's Fig) , due to the Lorentz force on the current-carrying wire, arising from I x B.

[PhysicsProf]

   Now let us consider this experiment (getting back to my questions regarding angular momentum conservation). 

   Let's support Harvey's RIGHT fig., say on a free-to-rotate disc -- again, with the wire connected (soldered, say) to the CENTER of the neo, and the other end of the wire connected to the CENTER of the battery.  Then -- there is NO torque (r X F) on the battery+neo.  BUT, there IS A TORQUE on the wire as I showed in my previous post.

    Experimentally, I would like to see a time-switch in the wire, to close after a certain time so there is NO TOUCHING of the system when the switch closes; also I would add an LED so one can see when the switch closes and the current flows.

    NOW -- will the entire system thus configured begin to rotate, or not?   There is a torque on the wire, so it seems there should be rotation.
HOWEVER -- in that case, it also seems that angular momentum would NOT BE CONSERVED, for there is nothing counter-rotating.  Hence, an important question arises about whether or not Angular Momentum is Conserved -- which question I have raised before.

[GibbsHelmholtz]

   Now let us consider this experiment (getting back to my questions regarding angular momentum conservation). 

   Let's support Harvey's RIGHT fig., say on a free-to-rotate disc -- again, with the wire connected (soldered, say) to the CENTER of the neo, and the other end of the wire connected to the CENTER of the battery.  Then -- there is NO torque (r X F) on the battery+neo.  BUT, there IS A TORQUE on the wire as I showed in my previous post.

    Experimentally, I would like to see a time-switch in the wire, to close after a certain time so there is NO TOUCHING of the system when the switch closes; also I would add an LED so one can see when the switch closes and the current flows.

    NOW -- will the entire system thus configured begin to rotate, or not?   There is a torque on the wire, so it seems there should be rotation.
HOWEVER -- in that case, it also seems that angular momentum would NOT BE CONSERVED, for there is nothing counter-rotating.  Hence, an important question arises about whether or not Angular Momentum is Conserved -- which question I have raised before.

Professor,

For Harvey's left picture:
         1. Soldered       - no rotation
         2.  not soldered - rotation
For Harvey's right picture:
         1. Soldered      - no rotation
         2. not soldered - no rotation

There is a torque on right picture, but if you follow the wire, the bottom portion contain the opposite torque.  Since negative and positive torque on the same wire.  It can't move.  However, I find it difficult to hang the whole thing without wobbling. 

[PhysicsProf]

Professor,

For Harvey's left picture:
         1. Soldered       - no rotation
         2.  not soldered - rotation
For Harvey's right picture:
         1. Soldered      - no rotation
         2. not soldered - no rotation

There is a torque on right picture, but if you follow the wire, the bottom portion contain the opposite torque.  Since negative and positive torque on the same wire.  It can't move.  However, I find it difficult to hang the whole thing without wobbling. 

  Yes, I also find there is an opposing force on the bottom portion as the B-field reverses direction over that portion.  However, it is not clear whether the resulting torque EXACTLY CANCELS  the force on the rest of the wire loop, so that the torques add to ZERO, OR NOT.
  Also, one can place a magnet at the top also...   
  Travel time!  Hope to be back here tomorrow morning.

[GibbsHelmholtz]

 Yes, I also find there is an opposing force on the bottom portion as the B-field reverses direction over that portion.  However, it is not clear whether the resulting torque EXACTLY CANCELS  the force on the rest of the wire loop, so that the torques add to ZERO, OR NOT.
 Also, one can place a magnet at the top also...  
 Travel time!  Hope to be back here tomorrow morning.

That is a good point professor.  It could be zero, it could be not.  I think the charges exert a force on the magnet, so if the magnet does not move in some other direction, momentum would be violated.  

I've tried the Faraday disk experiment.  I could not get a stable reliable voltage.  I think it makes sense to say that polarity reverse because it is the same as the Hall effect.  However, I think that even if the external magnetic field is not apply, as the disk rotates, the charges create its own magnetic field.  Because this magnetic field due to mechanical rotation move with less than sound speed comepare to electron with light speed in magnets, the self generate B field is very small.  I think this is the forces that keep electron orbits and not going into the nucleus .  We cross the B field generate by electron and its speed and see the force pointing outward.  If electron move in the other direction, its self generate B field also change direction, the cross product of B and V again point outward.

[GibbsHelmholtz]

I've come up with a model for electromagnetic induction.  This concept is based on Lorentz forces and charge movement.

The set up is we have a primary loop and a secondary loop.  Let's walk through step 1-6:

1.  Current is injects into primary loop.  Test charge move as indicated by the arrow.

2.  As the charge move circular around the loop, a B field is set up denotes by green dots going out of the page.

3.  Lorentz force pushes the charge outside.  We can see by V x B or using the right hand rule.

4.  Now the charge resides more on the outside of the wire than the inside.  This set up an electric field going outward denotes by the gray arrows. 

5.  The secondary loop charge experiences this E field and moves outward.

6.  As it moves outward, it subjects Lorentz force cause by the B field due to primary current.  Using the right hand rule, it pushes the charges in the opposite direction to the primary loop.

[Harvey]

Interesting Homopolar Motor:

Superconducting DC Homopolar Motor

[GibbsHelmholtz]

Interesting Homopolar Motor:

Superconducting DC Homopolar Motor

Notice that they emphasize on the field coils must be superconducting.  Why couldn't they use a strong PM?  It also occurs to me that Gotoluc experiment saying that power output isn't about input current but it's all about how strong the magnetic field we have.   

[Paul-R]

Notice that they emphasize on the field coils must be superconducting.  Why couldn't they use a strong PM?

I suspect because their numbers are so huge. 36 Megawatt.

[PhysicsProf]

Notice that they emphasize on the field coils must be superconducting.  Why couldn't they use a strong PM?  It also occurs to me that Gotoluc experiment saying that power output isn't about input current but it's all about how strong the magnetic field we have.   

1.  Gibbs, can you point to (URL) "that Gotoluc experiment", pls?  I have respect for his work.

2.  Harvey -- very interesting.  I understand this to be an example of magnetohydrodynamics, and I'm thinking the ocean-water is used to provide the moving conductor, to provide thrust to the (Navy) ship.  Am I close?

[GibbsHelmholtz]

Professor,

This is one of his latest experiment vid.

http://www.youtube.com/watch?v=OxuotFUWVGQ&list=UUwXI4FD09cVYyofvzF2sUrA&index=2&feature=plcp

He try to increase output by stacking up magnetic field which is easily seen with equations and related to magnetohydrodynamics drive.

F = iL x B or F = qV x B

Luc can see the term i or q as input and F as output.  The amplification factor would then be magnetic field B . 

[BEP]

I have a great deal of respect for the efforts of Luc, as well.

The first question I would ask myself is, what determines the amount of lift or stroke length under load?

I will offer that the stronger static field provides for quicker coil movement to the attracting polarity. With a weaker static field, velocity is slow enough that all the stored energy is dissipated before the coil reaches the attracting polarity.

[Peterae]

[BEP]

Wow!

That is a whole lot of hoopla in one video 

[PhysicsProf]

  Thanks, Gibbs. 
And here is the thread for the GotoLuc work:  http://www.overunity.com/8429/mostly-permanent-magnet-motor-with-minimal-input-power/

[PhysicsProf]

  I put together various versions of Faraday's homopolar motor, as my wife and I visited family from Maryland to Missouri to Utah in the last several weeks.  Fun and educational.  Perhaps you will find something that YOU would like to build.  It's quite easy (see videos) and fascinating to study.     The simplest version, shown in the first segment, requires just one 1-inch diameter disk magnet with conducting coating (about 3 mm thick), a 9" piece of 14-gauge wire, and a battery (C-cell or AA-cell is best).   You can buy the needed neodymium/rare earth magnet(s) from supermagnetman.com and other sources.  I went to a hardware store to get the copper wire -- pulled one wire out of ordinary electrical wiring for a house. Note the ground wire has no insulation and is easiest to use.

The next-to-last version shows the screw and entire system suspended by a thread.  It is interesting to see the motion of the wire and the battery+magnet as the wire intermittently (on its own) touches the metal-casing on the neo.

http://www.youtube.com/watch?v=YfKzleMWE60

[GibbsHelmholtz]

I've been studying the characteristic of motors and graphs of torque, rpms, efficiency etc...


It seems that the elimination of BEMF in motors are desire to achieve high efficiency, possibly greater than 1.  I do not see how BEMF can be totally eliminated except to suppress it enough to not effect the torque.  When looking at a motor in general, torque decreases as RPM increase.  The cause of course is BEMF.  The efficiency is:

effiency = Torque x RPMs / U*I

U*I is energy from a battery.  Let's say we operate at region where speed starting to give BEMF that decrease input current by half.  We have saved half of the potential input energy, RPM increased proportionally, but torque is now decrease by 4 times.  The overall efficiency doesn't change much.  If current input and torque stays constant with RPM, then efficiency only increase and possibly exceeds 1. 

Homopolar motors can put out constant torque for a large range of RPMs.  The reason being BEMF is relatively small for homopolars.  This can also be achieved with pulse motor, however, the drive coil has to be small in inductance as possible.  This allows us to operate at high speed and constant torque.  Inductance reduction can be achieved in various ways such as low turn coil, bias magnet etc...

[PhysicsProf]

  Hurray! I succeeded with my first homopolar GENERATOR!  Photos below will help a brief explanation.

First, I designed a simple motor using a 1" diam neo magnet with metal casing, held magnetically to a short/fat bolt.  (Extra mass helps keep it spinning during the generator phase.)  This in turn holds to a screw which I punched through a box to hold the system from above.  The screw-tip provides a good "bearing".

1.5 V is applied from a DC power supply shown, and the 1.77 A is typical for revving up the motor.  +1.5V to the screw, ground via contact wire to the rim of the magnet-disk.

Then, with the magnet+bolt spinning magnet spinning and power supply disconnected, it now has a potential due to {E = velocity X B effects} -- that is, it serves now as a generator.  Yes, even with the magnet co-spinning with the bolt. 

To measure the effect, I have connected one probe of an ammeter to the screw and the other via a separate contact wire, touching the rim of the spinning magnet.  One hand holds the contact wire, the other hand holds the camera -- and you see 0.9 mA measured on the meter produced by the simple homopolar ammeter, demonstrating output power from the homopolar generator.

This experiment was repeated several times.  About 1mA current was typical.  The voltage will be small with such a small diameter rotating magnet and has not been measured, but current generation has been demonstrated and measured.

[PhysicsProf]

Note:  I am seeking (long-range goal) to extract power WITHOUT any external (non-moving) brushes or contacts...!

   I would be interested to hear if anyone thinks this is even POSSIBLE.  Thomas Valone experiments, etc., may suggest it is not possible.  I wonder what he thinks about this today...

[GibbsHelmholtz]

I say we just connect the homopolar to rotating magnets and coils to extract power.  No brushes for generation there.

I just thought of some way of loading.  I am analyzing which method of loading is the best for energy and torque drag on the generator.  There seems to be better ways than others for energy we get.  Anyone has an opinion on this? If we increase the number of coils, we get more energy for stacking resistance and less drag for reducing turns.

[PhysicsProf]

I say we just connect the homopolar to rotating magnets and coils to extract power.  No brushes for generation there.

Thanks, but how would you do the connecting?  and extract power?

My idea lately was to have two conducting disks on the same shaft, but separated by say 10 cm or so, and one with NtoSouth to the right and the other, NtoSouth to the left.  That way, when the shaft is spun, the rims acquire OPPOSITE charges (also the area near the shaft)... and one should be able to extract power by allowing current flow between the rims  -- also, between the disk-central areas, which would need to be insulated from the shaft.

[GibbsHelmholtz]

Thanks, but how would you do the connecting?  and extract power?

My idea lately was to have two conducting disks on the same shaft, but separated by say 10 cm or so, and one with NtoSouth to the right and the other, NtoSouth to the left.  That way, when the shaft is spun, the rims acquire OPPOSITE charges (also the area near the shaft)... and one should be able to extract power by allowing current flow between the rims  -- also, between the disk-central areas, which would need to be insulated from the shaft.

My thinking was to connect the homopolar motor to a generator via gear coupling or belt. 

I think your mode of power extraction allow for high current to flow, but the voltage is too little even at high speed.  Higher current would also create a torque to stop the disk. 

I think I've made a mistake assuming that torque is proportional to current squared.  I'm based on force between magnetized core.  If one is electromagnet and one is permanent magnet, the pole of permanent magnet doesn't change strength, leaving torque is a linear function of current.  The efficiency formula then:

efficiency = Torque x RPM / U*I

If input current go down by 1/2 due to BEMF, then Torque also go down by 1/2 as RPM increase.  Efficiency would actually go up proportional to RPM.  Hm.... So to have high efficiency, we actually needs to operate at the region where BEMF dominate.  The torque would be so low, power so low, but we could exceed COP 1. 

[exnihiloest]

Note:  I am seeking (long-range goal) to extract power WITHOUT any external (non-moving) brushes or contacts...!

   I would be interested to hear if anyone thinks this is even POSSIBLE.  Thomas Valone experiments, etc., may suggest it is not possible.  I wonder what he thinks about this today...

I have often though about this question for the Faraday disk generator and I didn't find any solution.
For instance, if you connect directly the rotating disk (axis and rim) with two flexible wires able to wrap when the disk rotates, there is no DC because there is a reverse effect of the Lorentz force onto the electrons in the wires that are now crossing the magnetic field with a not nul speed.
I have also tried with a capacitor rotating with the disk and connected between the rim and the axis: it doesn't charge for the same reason.
It's a question of relativity. We must have a referential at rest, and one at a speed v, and let pass the electrons from one to the other. There is a potential difference neither in the referential of the disk nor in that of the circuit at rest. It comes only from the speed gap.

[exnihiloest]

...
I'll do the experiment today to see if polarity change.

I've tried to analyzed the forces on Harvey's drawing.  The red line indicates Lorentz force pushing the loop out of the page.  However, notice that the blue force on the magnet section is pushing the magnet into the page.  If we add the blue to the red, the forces exactly cancel out to zero!
...
Therefore, the conclusion is... the net force is zero! no movement. 

It's logical. There is no movement but only if there is no sliding contacts.

The "red" Lorentz force applies to the wire and the blue one, to the magnet. So in case of sliding contacts, if you maintain at rest one of the two objects, the other one will rotate under the effect of the force.

(Sorry for the delay of my reply).

[GibbsHelmholtz]

It's logical. There is no movement but only if there is no sliding contacts.

The "red" Lorentz force applies to the wire and the blue one, to the magnet. So in case of sliding contacts, if you maintain at rest one of the two objects, the other one will rotate under the effect of the force.

(Sorry for the delay of my reply).

Welcome back,

Yah, relative motion is require for sure.

Glad you're back, hope everything doing well over there.  I'm just struggling right now to find out is the torque of motor linear or squared to current of the coil, then I would go on to which flavor of OU I want. lol