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The Homopolar Generator: Unanswered Questions

  1. Sep 11, 2006 #1
    I thought about the different configurations of the homopolar generator. Asking the right questions helps to lead me in a particular design direction. I will present the information I've found online, my questions, and my current (if partial) answers which may or may not be correct. I would appreciate more complete answers if anyone has an idea so that I can fully understand alternative constructions.

    Coil design. At first, I wanted to use one of Tesla's http://www.tfcbooks.com/patents/coil.htm" [Broken] (bifilar coils) to amplify the magnetic field of a given current because he stated that the "capacity" of the coil was increased. However, the actual goal of the coil design seems to be to eliminate its self-inductance at higher AC frequencies. Therefore, I don't suppose it would be an effective electromagnet.

    [Q1.a] Is the coil design supposed to increase or eliminate the magnetic field of the coil? Parallel bifilar should increase it, and antiparallel bifilar should eliminate it.

    [Q1.b] If it is increased, by how much is it increased (250,000 times as stated in the patent)? Does this refer to magnetic field strength amplification?

    Electromagnets vs. Permanent magnets, field strength and power required.

    [Q2.a] What is the limitation in field strength of a permanent magnet? Neodynium seems to be at about 1.2T. Requires no power to maintain.

    [Q2.b] What is the limitation in field strength of an electromagnet with an air core, such as Tesla's design above? Normally with an air core you'll never drive enough current or coil it enough to get an appreciable field strength. If Tesla's coil were to magnify the effect by 250,000 times somehow, then it would make an air core viable because you can push the generator current through the electromagnets as part of the circuit and possibly exceed the field created through other methods. Requires no extra power if it can be done this way, otherwise not a vialbe option.

    [Q2.c] What is the limitation in field strength of electromagnets with a core? Some materials can reach 1.8T with an applied field of only 0.05 gauss, such as http://www.metglas.com/PRODUCTS/page5_1_2_1.htm" [Broken]. This means that 0.1 amps of current and a reasonable number of turns of coil can fully saturate it. Requires an insignificant amount of power to reach field strengths above most permanent magnets.

    [Q2.d] Electromagnets have the advantage that their fields can be adjusted. It's easier to put them into position initially when they aren't turned on. The high current in the generator can be run through the coils perhaps to increase intensity.

    Geometry. Attached is a picture of a program I wrote to test the relative field strength and vectors given different coil geometies. I found that two solenoids or flat coils do not create uniform magnetic fields along the surface of a disk between them, but it's fairly close at long as the disk is smaller than the coils. It appears that two hemisphere-shaped coils (as shown in the picture approximately) can create a uniform field across the disk surface. Another example is shown http://web.mit.edu/6.013_book/www/chapter8/ch8-853.gif". This is a relatively complex thing to accomplish, and doesn't suit well to using a core. If [Q2.b] is true, then it might be a good option.

    [Q3.a] Are some of the properties of the generator derived from the non-uniformity of the magnetic field on the disk? http://www.stardrivedevice.com/Tesla_notes.html" [Broken] on the device kind of states that when only a part of the disk is covered by magnetic fields, it can be made self-exciting. However, it will function a bit differently when the whole surface is covered, and perhaps especially if the field is more uniform.

    [Q3.b] Consider Tesla's notes (link above). Examine figure 5. Tesla states that the current can be drawn from the disk, through what is essentially a coil, and out into the external circuit. In other words, you can force the current to create a field in alignment with the original magnetic field. Taken one step further, you can power the electromagnets this way.

    Configuration. According to one http://www.stardrivedevice.com/over-unity.html" [Broken], its author believes that from Tesla's design notes: "eddy currents will be confined to those radial sectors of the disk which do not lie directly between the shaft and an outer pickup brush". Reducing or eliminating the back-torque/counterforce from the system is really the entire premise for over-unity operation; the other is that the power derived from the induction of the magnetic field in the disk increases more rapidly than the power consumption of the motor used to drive the shaft and rotate the disk. The power used to generate the magnetic field is little or none depending on design, and so isn't the limiting factor probably.

    [Q4.a] The same author stated that input power for rotation increases as radius^2 while power generated increases as radius^4. Why would this be true? If it is true, what strength of field do we need to reach before o-ratio*r^4 > i-ratio*r^2 and we benefit? Can this field strength be created under normal conditions? If not, would it be so much energy as to eliminate any gains in the system?

    [Q4.b] If back-torque is eliminated through the configuration of the current draw, then we can use more than one approach. 1) Tesla suggested splitting the disk into radial sections. 2) Tesla also suggested forcing a few turns of wire in the right direction would do the same thing. 3) http://www.stardrivedevice.com/over-unity.html" [Broken]) rather than a copper disk. The proper relationship between magnetic field, direction of coil turns, and direction of coil rotation should yield a self-adding effect to the coil and prevent the eddy current losses at the same time. The coil forces the current to move "radially" as in Figure 4 or 5 of Tesla's notes as it is induced.

    [Q4.c] The coil design can be two electromagnets with cores and a flat coil between them that rotates. It could also be two permanent magnets or magnet arrays with the same coil in the middle. Tests would include spinning the coils all together, or just the center coil to see what the result is. Multiple inner coils might be tried with different exit points to see if "symmetrical current draw" can help. Perhaps even small wires ("spokes") arranged around the axis would work.

    [Q4.d] You want a design where applied current to the rotor produces no movement. Then you've got a better chance of having the correct generator action.

    [Q4.e] Why do magnets connected to a disk create a situation where there seems to be no counterforce?

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    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 11, 2006 #2


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    Staff: Mentor

    Welcome to PF. I did a quick forum search on homopolar and got lots of hits. Here's the hit list for your reading enjoyment. I have no idea what a homopolar generator is, but it looks like it's been discussed a lot:

    https://www.physicsforums.com/search.php?searchid=461743 [Broken]
    Last edited by a moderator: May 2, 2017
  4. Sep 11, 2006 #3
    Yes, I read over 100 posts on the long thread about it. I figured with the length of my initial analysis that pushing the old thread wasn't a good idea. That thread is from 2004. I'm also trying to get a fresh start on it without some of the older arguements.
    Last edited: Sep 11, 2006
  5. Jul 28, 2010 #4
    OK here's my two cents worth;

    I've used bifilar coils in switch mode power supplied to reduce the skin effect on the conductors. I've never seen coils amplify current or at least what I would consider amplification. Voltage out is the same as in but current is greater. Never seen that.

    The issue here is power. Current alone does not equate to power. You need to consider what the voltage is doing.

    Air gaps in transformers are used to keep the core out of saturation.

    I've never seen a rotating device that generates electricity without back-torque. Would love to see one.

    Your "this picture" is a pancake coil. I've only seen them used as primaries on Tesla coils and radio antennas. If somebody knows of other uses please tell us. Spinning a pancake coil in a magnetic field will not produce a homopolar generator.

    Over-unity devices don't exist. If they did, they would use a little power to release or tap into a greater source of power. Consider this; A water pipe coming from a dam through a valve into a turbine generator set. The valve is spring loaded so it always shuts off. If I open the valve by hand, the generator would produce more power than I used to open the valve. If I then connected a solenoid to the valve and took power from the generator to keep this valve open, I would have an over-unity device. I put in a few watts of power to open the valve and from that point on I have free energy. I need to be oblivious of the existance of the dam of course.

    Nothing is free in this life.
  6. Jul 28, 2010 #5


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    What I have found concerning ANYTHING to do with Tesla is that one needs be very careful about what you believe. People seem to worship Tesla and I think over the years alot of his work has been distorted into claims of free energy and garbage like this. Although I wouldn't doubt Tesla himself was a slight bit off kilter concerning free energy.
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