The Halbach Disc Motor Ebike Project

Discussion in 'Electric Bicycles' started by safe, Nov 18, 2009.

  1. safe

    safe Active Member

    The Halbach Disc Motor Ebike Project

    The theory was developed on the other thread and now I'm going to begin the actual "build" process of the Halbach Disc Motor idea. (after about two months of motor research)

    This will NOT be an axial motor.... the magnets will extend their magnetism radially away from the rear wheels hub center. Just want to be sure that people understand the concept because most of the Halbach disc-like motors that are standalone (not built into the rear wheel) make use of the axial design and two parallel discs surrounding the third which is the rotor that holds the copper magnet wires.

    In this design there is just ONE DISC and it has a Halbach array facing radially outwards from it's circumference and the magnet wires will be on a stationary device that will resemble something like a brake disc caliper.

    Here are the components that I will begin with:



    ...looks like tomorrow might be warm enough to do some work on this.

    Attached Files:

    Last edited: Nov 18, 2009

  2. Darrell_H

    Darrell_H New Member

    I just looked through several of the 143 posts on the Halbach Axial Flux Motor thread and several red flags came up. I didn't have time to read all 143 but, since the I saw the same assumptions in post #143 as post #1 I have to wonder if any sanity check on this "theory" has been performed.
    Safe, can you address these issues:
    1. The 2nd photo on post number 1 shows what looks like copper coils around air, no iron. For a fixed Gauss level (current times turns) the flux in air will be thousands of time less than the flux in iron, so the flux coupled to the magnets will be much less for a given amount of current. Is there iron inside these coils?
    2. The article about Halbach states that all experts and CAD models are all wrong. This raises a big red flag because it sounds just like new-age logic. All CAD is incorrect....yet you use what looks like a CAD model in post #7. Contrary to this idea that all CAD is wrong, most 3D CAD analysis I have worked with shows field lines around a bar magnet very close to those seen in real life when you sprinkle iron filings around it. What is this CAD which is all wrong? Who are these "experts" who are all wrong?
    3. Also about post #7, what I first noticed in the picture is that the vertical field lines on the top and bottom of the vertical magnets are not mirror images of each other. What material do you have in your model besides air which would cause this? What is the "backing" material made of? What is the purpose of the horizontal magnets in the Halbach scheme. They don't seem to bend the field in any usable direction.
    3. Throughout the posts I see several times the phrase, "only copper losses". Copper losses contribute a huge portion of the total loss in motors. Why use the word, "only"?
    4. One picture (I forget which post number) shows a 3 phase stator which is surrounded by an array of magnets (looks like a typical wash machine PM configuration). The author shows all coils to have a North field pointing outward, then he goes on to diss that motor. This is simply not how 3 phase waveforms work. What gives with that picture?

    You're obviously very interested and working hard to experiment with this Halbach motor but, I'm worried that you'll spend a lot of time guided by this Halbach article which has a lot of misleading guesses which borders on pseudo-science. Some real 3D CAD analysis before spending days with trial and error could save a lot of time.
  3. spad4me

    spad4me Member

    Sanity Check he don't need no stinking Sanity Check.

    You missed the sixty plus pages on rewiring a brushed motor.
    The misleading article on a dishwasher motor.
    There are thousands of page's of odd pictures , that safe has authored.
    No real information just advertising on someone else's threads when he can.
  4. safe

    safe Active Member

    Halbach Uses No Iron

    I too was amazed when I learned of this because it seems to contradict everything we have thought "normal" about electric motors.

    Start with Wikipedia:

    A Halbach array is a special arrangement of permanent magnets that augments the magnetic field on one side of the array while cancelling the field to near zero on the other side. In the diagram, the magnetic field is enhanced on the bottom side and cancelled on the top side (a one-sided flux).


    In order to visualize what the flux lines look like I found a pdf that had these images that represent the two options.

    First there is a "standard motor" with alternating poles:


    Now with the Halbach motor the alternating poles also have the rotated magnets that cancel the magnetic force on one side and increase it on the other. The effect acts like a "short circuit" of the magnetic flux lines and this allows you to design without the need for iron. The only reason that iron rotors and stators are used is because the flux lines of a "standard motor" are so spread out that you need to "assist" them with the iron because iron holds magnetic flux many times better than air:


    ...there still is the issue of the "air gap" and just like with "standard motors" if the gap is too large it effects performance negatively.

    Can you see how those flux lines are different?

    The intensity of the flux lines are much greater (darker) with the Halbach and there is no need to "loop around" through some iron backing plate. The flux loops become small rather than large.
    Last edited: Nov 18, 2009
  5. spad4me

    spad4me Member

    I believe a halbach array doubles the flux in AIR on one side and reduces the flux in AIR on the other side.
    Proper selection of a laminated steel core concentrates the flux..
    Lots of people have tried to make an efficient halbach motor.
    You safe could be the first.
  6. safe

    safe Active Member

    No, I won't be the first... the solar racers in Australia use Halbach motors to get 95% - 98% efficiency already.

    However, I might be the first to develop a "practical" implementation of a theory that has already been used to win contests. Ebikes need a practical design... so that's what I'm doing... being practical.

    The winning solar racers use Halbach (axial, not radial) motors:

    ...or read more about the CSIRO Halbach motor. (pdf)

    Attached Files:

    Last edited: Nov 18, 2009
  7. safe

    safe Active Member

    In a "standard motor" the magnetic flux needs to follow a very long looping path before it finally returns to the other end of the magnet. Having to travel so far (in distance) means that the field strength will drop even with iron there to assist in keeping the flux strength high.

    With the Halbach motor they solve the flux strength problem by making the path shorter. The flux only needs to loop as deeply as the magnets themselves and then come right back.

    So the answer is that there is less "work" that needs to be done, so it's not as high a priority to increase the magnetic strength with iron cores. The CSIRO motor only uses four turns of wire to achieve what they need. (it's amazing)

    Also, once you start to reintroduce the iron you then have to deal with the losses associated with the iron and those are unnecessary losses. If you need more magnetic strength you just use more copper in the windings.


    The "Bottom Line"

    The Halbach motor "exists", has proven itself in competition and comes out on top. Whether I can achieve equal results is more a statement about my own fabrication capabilities in my garage than the theory itself. It would be nice to have machine tools and access to high tech manufacturing, but for now it's being done on a shoestring budget. If things go well in the first version then I'll try another and another...
    Last edited: Nov 18, 2009
  8. safe

    safe Active Member

    Halbach Disc Mounting

    The first thing I've done (other than lace up the double sided disc hub to the rim) is to create this chainring "spider" that attaches to the six bolt hub.

    The magnets will be attached to the chainring (eventually) in a Halbach pattern where the magnetic forces are directed outwards. (radially) Then outside the chainring will be the actual magnet wires (coils) in a sort of disc brake caliper configuration covering only about 1/4 of the chainring. The idea is that it's easier to use lots of magnets in a big chainring and get the leverage advantage that way than to use a full wrapping around set of coils on a smaller disc. However, if the power turns out far less than I'm hoping for I might end up having to do the full wrap. (so future upgrades are likely)

    This was about a 3-4 hour project for the mount... not too bad...


    ...tomorrow I'll be grinding down a steel rear hub so that I can extract the threaded portion for a freewheel attachment. It looks like it's going to work out really easily. (easier than I thought)

    Attached Files:

    Last edited: Nov 19, 2009
  9. safe

    safe Active Member

    I'm kind of pondering this issue myself...

    The Halbach magnets take care of just one side of the flux path problem. The other side of the flux path is the magnet wires (coils) and the presence of the Halbach array on one side does not do anything to help on the other. The dual Halbach axial flux motor designs concentrate everything into the center and then the rotor (which holds the coils) only has to pass through the center of the magnetic field that is already there. The dual Halbach is the superior design choice if you can do it.

    In my design the chainring will hold the Halbach array and it will face outwards in the radial direction. However, the magnet wires (coils) have no iron core backing to them, so they might not be very efficient. In a sense I'm going backwards in the design for practical reasons and that might mean that I'll have to reintroduce a small amount of iron into the magnet wire (coils) in order to get decent performance.

    So in a sense this might NOT actually end up without iron... :beta1:

    (I'm just hoping to minimize it as much as possible or maybe add excess copper as a substitute)
    Last edited: Nov 19, 2009
  10. safe

    safe Active Member

    Good Enough For NASA?

    In a rather large (8 mb) pdf file from NASA they go into the use of Halbach motors and it looks like I've found the preferred way to do the windings:


    ...this looks to be the proper way to do the winding. (no mention of iron)

    Technically speaking I think this is a "magnetic bearing" so it might not be right for this application.

    Rotating the coils the other way (like in the first motor I saw) doesn't look to be the optimal way to go, but I'd like to get a Simulation of all this before I finalize on the design. (it might be some time before I get to the coils anyway, so no hurry) I'm pretty sure about the magnet design for the chainring, but this magnet wire coil stator is still being thought out as I go.

    What would be pretty interesting is to have the center of this stator hollow for either air cooling or even water cooling. Being able to pass water past the coils (which is the only source of heat) would allow me to experiment with overloading the motor to see how powerful I can go with it. At this point I have no idea how much power will be possible with this motor.

    Of course this is all assuming that the idea is correct... :goofy:

    Just by observation it's pretty obvious that 3/4 of the copper in the coils is wasted because the only part that is going to produce a "useful" magnetic field is next to the magnets. The loop is closed (so that's good) but it means there's some wasted copper. A little wasted copper seems a tradeoff that you have to make in order to avoid the use of iron.

    Attached Files:

    Last edited: Nov 19, 2009
  11. Safe,

    I am stoked to see you are actually building this thing. This will be one of the most innovative efforts in motor biking. Go for it man. Follow your dreams.
    To the critics: Whats he got to lose? This is a hobby. He is enjoying himself, researching, learning, designing. It's a great creative outlet regardless of success or failure.
    Do it man.
  12. safe

    safe Active Member

    Thanks KeepOnKeepnOn it sometimes feels like I need "permission" to simply want to try something new for ebikes.

    Having found this image from the NASA Halbach projects it does tend to give me greater optimism that I'm going to be able to pull this off:

    Last edited: Nov 20, 2009
  13. safe

    safe Active Member

    To Cog, or Not To Cog?

    Generally speaking the idea of having cogging in your motor is considered a bad thing. It will set limits on the motor that otherwise might not be there.

    However... I have my doubts about the abilities of an RC motor ESC to properly "sense" the backEMF on a Halbach motor when it is made so perfectly as to not cog at all... so one strategy to guarantee that the RC ESC's would work is to reintroduce the cogging if possible. As it turns out it's possible to have cogging with the Halbach and not need iron to do it.

    First we look at the Single Block Halbach which is the way you normally want to do it. You can see that the magnetic flux is really smooth and there is no cogging that is visible. Cogging is when the magnetic strength varies (color changes) but it's not related to the flux path direction:


    Now we look at a Double Block Halbach where each set of two blocks are pointed in the same direction, but following the Halbach array orientation. The configuration will cut the pole count in half, which may or may not be a good thing. This will give lot's of cogging and a very strong magnetic field:


    Lastly we look at the Standard magnet configuration with it's flux lines that loop both above and below:


    :idea: Thoughts...

    It's a tough decision to make... if I go with the Double Block Halbach it will make it easier to build the magnet wires (coils) because it doubles the spacing between poles. Also with the Double Block Halbach the cogging will be very significant and that will guarantee that the RC ESC's will recognize the motors behavior, but it will also double the motors base speed. One of the "gotcha's" that I saw with the RC ESC's is that they usually don't go above about 10-12 poles as a maximum in their configuration options. With the Single Block Halbach I'd end up with a 16 pole motor and that might freak out the ESC. By going with the Double Block Halbach the pole count is reduced to 8 pole and that's pretty conservative for the RC ESC's. (it would be a decision of practicality)

    I don't see any advantage to the Standard configuration because the magnetic flux strength is not larger and yet it has cogging.

    This is one of those times when it would be nice to build two discs, one with Single Blocks and another with Double Blocks and try them both out. However, for the short term I'm going to need to decide one way or the other. :thinking:

    Attached Files:

    Last edited: Nov 20, 2009
  14. safe

    safe Active Member

    That Was Easy

    Take a regular steel hub, grind off the lip near the threads where you mount the freewheel (inside) then grind the hub down until the side separates from the rest and you are pretty much there. Since the hub is it's own "lathe" you just spin the hub with the grinding wheel friction. So easy.

    On top of all that a 36 hole hub has spoke hole spacing that (in this case) perfectly matched the disc holes so that all I had to do was drill. That was sheer luck... and it was just because the steel hub was the right size.

    The entire part took an hour. :cool:

    It's only taken me about 5-6 hours in total... and if you bought the spider instead of a modified one it would be even quicker. With the right parts is just a little grinding, some drilling and then paint.


    ...this might also be a useful idea for people that just want to add a motor to the rear wheel through a separate chain.

    Attached Files:

    Last edited: Nov 20, 2009
  15. AussieJester

    AussieJester Member

    Recumpence makes an adapter that bolts directly onto the disk brake mounts of a hub to take a sprocket.

    pictured above the astro/reduction drive

    This might be another alternative if you want the motor on the left hand side of the wheel would be simple as bolting it on, and you know its accurate its cnc milled.

    Theres now 3 separate DIY motor threads on ES safe, one fella has already built the motor out of aluminium cnced on his work mill. Tiz smaller scale to test his ideas at present but he moves fast.

  16. spad4me

    spad4me Member

    Safe here is the original innovator's post and his idea.

    And some pictures of his rear sprocket mod.
    Check the date LOL.

    I am currently using a copy of treewks rear hub mod.
    It was a surprise to me when you invented it LOL
    Last edited: Nov 20, 2009
  17. AussieJester

    AussieJester Member

    A 'first' on this site possibly but not 'first' to do it, using the bottom bracket bearing housing for the freewheels is an old trick matey seen it done many a time on various forums, you can use the left hand threaded one with a SouthPaw freewheel on the left hand side of the rim too (check my worklog i have done exactly that on my cruiser) Interestingly, the left hand threaded bottom bracket housing must come in various thread pitches as i tried 4 before i found one that would screw in successfully!

    Safe often is the 'first' to do many things though...according too himself anywayz LoL

    Where you been Spad4me anywayz haven't seen you about for ages?

  18. safe

    safe Active Member

    Missing The Point Are We?

    You guys are acting like a bunch of girls jealous over being the most beautiful !!! :tt1:

    The point wasn't that it was the "first", but that it was easy and didn't take a lot of time. I've seen them before and AussieJester and I even talked about them before.

    When the Halbach Disc Motor is done I suspect that people will immediately jump in and say things like:

    "Oh, well that's nothing really new... all the ideas were already there... safe does not deserve any credit for getting it together... blah... blah... blah."

    ...oh human nature is always such a joke. :jester:
    Last edited: Nov 21, 2009
  19. Email

    Email Member

    Nothing to add, just posting to get into subscription updates. Also the NASA photo/drawing I do not belive needs an IRON core because it appears to be working off of the immediate boundary field. Iron would only focus the flux on the copper perpendicularly to the coil. I think it works off the outer boundary of the flux, though I could be wrong.
  20. Email

    Email Member
    The North and South poles that pulse, just use ND Magnets pointed sideways to prevent having to have the iron in your coils.
    See Video and imagine the following setup:
    > coil < coil >
    Last edited: Nov 23, 2009