The Halbach Disc Motor Ebike Project

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Condensed Version of this Thread
http://www.ebikehub.com/forum/viewtopic.php?f=4&t=2452&sid=b4d3facc91ae9a71b85fb7b86c8f0bf7&start=0

...would look trick Safe if you got some carbon fiber and laid it on as the last layer to mate 'carbon fiber' gives a "racey look" to things...

Both the CSIRO and the LaunchPoint motors are designed with embedded carbon fiber. My plan is to make this first one and learn from it and then build a female mold of it. From the female mold I will be able to then create new discs much more rapidly and it's at that time that I might drop the aluminum core and do the whole thing in carbon fiber. It's the aluminum chainring that is providing all the structural strength on this one. (not the fiberglass)

Fiberglass cloth is pretty strong... and it's cheaper and easier to work with. Too much resin in a build will tend to produce an overly rigid item that will crack easily. The more layers of fiberglass you use the more flexible and durable the result. So far I have three layers on each side, mat, cloth, then mat again. I will continue to build up the disc until it's indestructable. The fiberglass is a very low weight material for it's strength. (though carbon fiber takes it to the next level of course)

The "Second Half"

The "Second Half" of this project will be to create the stator side which is the part that will actually experience the heat. The disc side is essentially unaffected by motor activity and as long as it's physically strong enough to handle the load it will operate without fail. The stator side will need to have magnet wire built into it with coils and they are going to heat based on the level of current.

I'm more worried about the "Second Half" than the first... the disc part is actually the easier part of this project.

Bummer... as far as the forecast can see around here we will not see another day of 50 degrees for a long time. So on this side of the world we are in winter. Three months of cold before spring.

It's 23 degrees at the moment.

Friday temp update... 17 degrees.... brrrrrr....

 

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Theory, Theory and More Theory...

The weather is freezing... the design needs figuring... it's time to get to the math. (the homework)

I was fiddling with the Simulation program and was able to evaluate based on their "Contour Tool" that across a 20 turn 20 AWG coil (I just made up the numbers for testing purposes) I "seem" to be getting 2 Newtons of force when 100 amps is applied.

So the question becomes:

What does that translate to as far as rear wheel power?

The "lazy math" approach is to look at what my other bike was doing and work backwards. From what I can tell my old bike puts out about 50 Nm when at it's running speed in first gear and more like 80 Nm off the line. This means that in order to achieve 50 - 80 Nm we need to do the math backwards. Let's just assume 75 Nm as the goal.

First we factor out the radius of the disc:

4 inch = 0.1 meter

75 N/m * 0.1 m = 7.5 N "required" (say 8 Newton)

...if each coil is able to achieve 2 N we will need a number of coil sets:

8 N / 2 N = 4 coils ...we need coverage of 4 coils times Three Phases or a total of at least 12 coils in total.

So the "bottom line" is that it looks like I'm going to want to use "at least" a total of 12 coils which are set up as Three Phase power.

So we have:

192 magnets in total

96 magnets per side

Halbach "Double Block" pattern means 48 sets of polarities per side

NN >> SS << NN >> SS << etc...
NN << SS >> NN << SS >> etc...

48 Halbach rotations means 24 actual poles per side

N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S
N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S

For every two poles there needs to be three coils sandwiched in the middle, so (24 / 2) * 3 = 36 so if we want to cover the entire disc we need 12 sets of three pairs.

N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S
123-123-123-123-123-123-123-123-123-123-123-123
N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S-N-S

25 Hz = 120 * 25 / 12 = 250 rpm -> 17.8 mph (24" wheel)
50 Hz = 120 * 50 / 12 = 500 rpm -> 35.7 mph
75 Hz = 120 * 75 / 12 = 750 rpm -> 53.5 mph

One quarter of the disc is 12/4 = 3 sets of three pairs or 9 coils in the stator. (not adequate... need more)

123-123-123

One third of the disc is 12/3 = 4 sets of three pairs or 12 coils in the stator. (the minimum)

123-123-123-123

One half of the disc is 12/2 = 6 sets of three pairs or 18 coils in the stator. (plenty... 50% surplus)

123-123-123-123-123-123


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This is not well thought out logic (sloppy)... but it's sort of a "ballpark estimate" that will be examined over and over (to many peoples distress) all winter long.

And it actually gets more complicated because there are two ways to do the coils, either overlapped or not. (so the coil count could be higher)

Much more thinking is required...

(however ~1000 watts does look "likely" with this motor... the goal)

 

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Old Man's Logic

The young tend to see their lives in a way were they think they live forever. The old tend to see their lives as more finite. The old accept life as being limited. The young strain to rebel against limits.

Applying the "Old Man's Logic" to the ebike I'm trying to build a motor that is for a specific limit, the Federal Ebike Law, so in everything I do I need to focus on how to operate best within the limits I have.

The "Old Man's Logic" seeks "Ideal Power". (~1000 watts in this case)

The "Young Man's Logic" seeks "Infinite Power".

...the laws are made by the old.

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I've used the basic formulas:

Rpm = 120 * Frequency / Poles

Power = Torque * Rpm

...and have plotted the results of different pole count options. Also, I added the effect the motor rpm has on speed by basing the mph on a 24" wheel, which is the size I will be using.

Generic Charts of Power, Torque and Frequency (mph as extra)

In all three charts the BLUE line is the frequency and it's always from 10 Hz to 100 Hz, which are the reasonable frequencies to use before there are problems.

For the 4 pole motor we see that the speed required of the motor is so high that you just can't achieve much torque until you are up to 20 mph. It's not an optimal choice.

For the 24 pole motor the torque at really low speed is excellent, but the speeds are only from 0-20 mph and then you run out of frequency range. It might be possible to drive at much higher frequencies and be able to use this motor, but you are likely to begin to encounter problems.

For the 12 pole motor the usable power starts at about 10 mph and goes up to 50 mph and that's the speed range I want to have. It appears that the 12 pole motor is just about right and this is a GENERIC answer because it only considers power and frequency. (the specifics of the motor itself are not included)

The torque numbers do look correct as discussed in the previous posting... it looks like 75 Nm is about all I need for peak torque. Speeds below 10 mph can be assisted by pedaling, so the very low end torque is of less importance than the "sweet spot" in the middle.

Finally, the case of the "speed limited" street version of the bike would have a frequency limit of ~35 Hz which would limit the speed to 20 mph while still providing high levels of torque within the speed range. For 30 mph speed limited states you could use a speed limit of ~45 Hz.

It's currently 25 degrees outside.

 

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Reinventing The Wheel

During these cold winter months it's good to go back through the formulas and rework them in order to make sure that everything "makes sense".

One of the questions I had was if you have a motor like the Halbach Disc motor and it is more or less a "constant torque" device (given you have a predefined current limit) then at what point is the "required" torque going to cross the "supplied" torque?

Think for a second... at zero rpm the torque "required" is infinite... so you can never really supply enough torque to achieve perfection. But at some speed the motor starts to multiply it's torque by it's speed to get more and more power. At some point the two curves cross.

From what I can tell the Halbach Disc motor as I envision it right now will be very near perfect when run at 1000 watts because the full realization of power will occur as early as 9 mph:

For the 2000 watt limit the power is slow to build from 0 to 17 mph, but past that point the motor should be adequate to deliver 2000 watts at speeds above:

A 5000 watt limit would require a very strict current limiting regime below 43 mph, but when you get beyond 43 mph the motor will be delivering good power:

The summary on this is that some things sort of pop themselves out at you depending on how you look at it. Every once and a while it's good to just throw away how you calculated something previously and try it from a different perspective. With luck your different angle arrives at the same solution and that is comforting.

I can relax and say:

"Yes... the wheel... it's a concept that works from all angles."

The world is at peace.

Note: This is not including anything about the motor efficiency, backemf, or heating and is not to suggest that you can actually fully realize this in practice, but it does give the rough outline of what the limits are. If you really wanted to build a 5000 watt motor my design at present is inadequate for that purpose. For 1000 or even 2000 watts it's probably going to be fine.

It's the thermal modeling (cooling) that is the ultimate limiting factor in the ability of the motor to use current and therefore create power. That is controlled by the construction of the stator... more copper means less heat.

 

[AussieJester]

Still think you should get some form of heating in your garage Safe DIY setup might be a good project too look into for next winter!
I would gladly give you 20 degrees in exchange for -20 hahaha

enoob on ES has been making good progress on his Halbach motor shall post some pics in your other thread in a day or so, he has aluminium disk with all those round magnets now mounted (i linked you in other thread IIRC?). He has offered to send me one for thrashing if they work out! he is also looking at coming to OZ for a few months next year when he does we will be doing a joint build (me making frame he the electrics side) ES are in the process of organising a DIY motor comp on some nice ideas and some clever chaps in for it.

Hope you get a break in the weather anywayz...

KiM

 

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At the moment it's 6 degrees outside and under 40 degrees in the garage. Once winter sets in the cement of the floor gets cold and it's really hard to overcome all that thermal mass with heating. Something like a small electric space heater (all I have now) just wouldn't have any effect. Also, it's a four car garage (2x2) so it's a massive space to heat. Realistically it will be a few months before I do any more work on the motor.

I've followed the ES thread and the guy over there is still building a "motor" in the old fashioned sense. My project is different because I'm building a "disc" that attaches to the rear wheel and the stator will be bolted to the frame. What I'm doing is radically different from anything I've seen.

Has anyone seen a true "Halbach Disc Motor" being built? (that attaches to the rear wheel)

There are lot's of "axial flux" motors of either Standard or Halbach design (both one sided and double sided), but no one seems to have made the jump to a pure ebike solution. Unless someone beats me to it I'll be the first to create the Halbach Disc "ebike specific" motor.

 

[AussieJester]

At the moment it's 6 degrees outside and under 40 degrees in the garage. Once winter sets in the cement of the floor gets cold and it's really hard to overcome all that thermal mass with heating. Something like a small electric space heater (all I have now) just wouldn't have any effect. Also, it's a four car garage (2x2) so it's a massive space to heat. Realistically it will be a few months before I do any more work on the motor.

And you have 4 cars? ...NO yo dont....so why not section off a smaller area a "car and a half wide" for your workshop so that you CAN heat it instead of sulking in the house for 3 months of the year..?

KiM

 

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This coming year might be the last year I'm in this house. I spent a bunch of time a couple years ago making the garage look nice and I don't want to make it look "ghetto" with a sloppy partition in it. (it's a really nice garage)

There's nothing wrong with taking a break from things... there's certainly no hurry. I don't believe in man caused global warming so it's not like I'm helping to save the world or anything.

Spring will come...

 

[AussieJester]

This coming year might be the last year I'm in this house.

I thought i just heard a screech of joy..was that you JohnRobHolmes

KiM

 

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When I moved here late in 2002 I had a "Five Year Plan" to be here and after that to leave. It's soon to be 2010... so this summer will be three years longer than I had originally planned.

I've lived in San Diego, San Francisco, Lake Tahoe, Oakland, Orinda, Moraga, Lafayette, Concord and Walnut Creek which are all places in California.

Never did I expect to be here so long...

I'd rather live in a place like Lake Tahoe, so in the summer I might bike and then in the winter I would ski.

(my leading choice now is Colorado which has the mountains for that sort of thing)

Let's be honest... building bikes is nasty work. It has it's satisfactions, but it's not exactly my favorite thing in life. Taking breaks from the "nastiness" of bike building / fabrication helps (for me) to not get sick of it. I just can't imagine the people that have to do a job like fabrication all the time... I would burn out really quickly in something like that.

So breaks are good... (for me anyway)