Friday, April 2, 2010
Monday, March 29, 2010
FOC/mSCR PPT FTW
Now that it's been running for a few months (not continuously O_o) and seems to work with at least four different motors (scooter, scooter, LEAF, RC car), I'm more confident in pitching the simplified/streamlined field-oriented control setup at work in the 3ph Duo controller. But I totally understand that very few people will want to read the poorly-written, incomplete-and-yet-overly-detailed 78-page draft documentation for the controller itself.
For a quick intro, you can check out these two posts:
3ph Duo Wrap-Up Part 1: Field Oriented
3ph Duo Wrap-Up Part 2: Control
Recently, I consolidated the important part, plus some new data, into a set of much more palatable PowerPoint slides (a la Balance Filter) which I'm hoping will find their way through the internet and into the hands of some motor control-types who would rather see the big picture than a detailed discussion of one particular implementation. It covers field-oriented control in general, the "standard" method, called the synchronous current regulator, and some modifications to the standard method that make it possible to run with Hall-effect sensored motors and on relatively slow fixed-point processors. It doesn't go into any detail on the software, just lays out enough that a competent embedded controls person could probably pull it off on an Arduino or something.
Consider that a challenge.
For a quick intro, you can check out these two posts:
3ph Duo Wrap-Up Part 1: Field Oriented
3ph Duo Wrap-Up Part 2: Control
Recently, I consolidated the important part, plus some new data, into a set of much more palatable PowerPoint slides (a la Balance Filter) which I'm hoping will find their way through the internet and into the hands of some motor control-types who would rather see the big picture than a detailed discussion of one particular implementation. It covers field-oriented control in general, the "standard" method, called the synchronous current regulator, and some modifications to the standard method that make it possible to run with Hall-effect sensored motors and on relatively slow fixed-point processors. It doesn't go into any detail on the software, just lays out enough that a competent embedded controls person could probably pull it off on an Arduino or something.
Consider that a challenge.
Friday, March 19, 2010
Pirate Radio Control
I have a new favorite thing ever:
The string of unbelievably nice weather (following four days of hostile rain) motivated me to take the pile of parts that had accumulated on my desk all winter and finish turning it into a brushless-powered RC car. Now the original car (a Team Associated TC4 RTR) was already pretty good, and a lot of fun to drive. But I wanted to add some of my own style to it. Here's the outline:
I love this motor. Sure, it doesn't quite fit in the motor mount that came with the car, but that's what we have machine tools for:
The string of unbelievably nice weather (following four days of hostile rain) motivated me to take the pile of parts that had accumulated on my desk all winter and finish turning it into a brushless-powered RC car. Now the original car (a Team Associated TC4 RTR) was already pretty good, and a lot of fun to drive. But I wanted to add some of my own style to it. Here's the outline:
- Switch the brushed, 7.2V motor out for a brushless 14.8V motor.
- Add a 19.8V LiFePO4 battery.
- Add my own brushless controller and 2.4GHz RC system.
I love this motor. Sure, it doesn't quite fit in the motor mount that came with the car, but that's what we have machine tools for:
Operation 1: Carefully turn down bearing cap and...oh crap it broke off.
Operation B: Luckily the bearing itself is 0.500" so it fits
in the largest bore you could possibly make in the motor mounting
block anyway. Mounting block becomes new bearing cap. Win.
Machining disaster aside, the motor fits nicely into the existing mounting structure and, when combined with comically-oversized wire, looks pretty intimidating in-system. In addition to having a much lower resistance than the brushed motor, this motor is rated for 4S LiPo (14.8V) operation. So I'll go ahead and run it with 6S A123 (19.8V). What? It'll be fine.
The new battery pack, which you can see in the image at the top of this post, is rated for 70A continuous discharge. (That's 1,386W available...) Since I don't expect to ever need that much, I have a 30A fuse on the whole thing right now. I chose to run 19.8V instead of 13.2V or 16.5V for a couple of reasons. One, my controller won't even turn on with anything less than 18V. Two, I think the motor Kv is actually lower than it says (which I argue is a good thing). I'll also be running sinusoidal control. These both mean less speed per Volt of battery, so to make up for it I'm using a higher voltage.
The controller is the 3ph Duo, except with just one side built (so, 3ph Uno?). But it's still got all the bells and whistles of the Duo, including field-oriented sinusoidal control, current (torque) limiting, and built-in two-way 2.4GHz digital radio communication, courtesy of XBees. The Duo was designed for a 33V/20A motor (two, actually). This system is lower voltage, but potentially higher current. So, I made some modifications:
Specifically, the traces got some reinforcement, the capacitors have more capacitance, and the current sensors have been bypassed by an equivalent resistor to effectively halve their gain. I also used a different IXYS six-FET module, this one rated at 40V/180A instead of 100V/90A as in the original design. As it turns out, none of this was probably necessary because the car spins its wheels at 20A anyway. But...overkill isn't necessarily a bad thing.
Just a few more modifications to make! An antenna mount:
The controller is the 3ph Duo, except with just one side built (so, 3ph Uno?). But it's still got all the bells and whistles of the Duo, including field-oriented sinusoidal control, current (torque) limiting, and built-in two-way 2.4GHz digital radio communication, courtesy of XBees. The Duo was designed for a 33V/20A motor (two, actually). This system is lower voltage, but potentially higher current. So, I made some modifications:
Specifically, the traces got some reinforcement, the capacitors have more capacitance, and the current sensors have been bypassed by an equivalent resistor to effectively halve their gain. I also used a different IXYS six-FET module, this one rated at 40V/180A instead of 100V/90A as in the original design. As it turns out, none of this was probably necessary because the car spins its wheels at 20A anyway. But...overkill isn't necessarily a bad thing.
Just a few more modifications to make! An antenna mount:
Can't screw that up.
And finally I need to make a heat sink / controller mount. Oh wait, I found this random block of aluminum that happens to already by exactly the right size. It even has a relief for the sensor wires in exactly the right place:
See how well it fits?
This never happens.
A bit of programming later, and I have an RC car again. Time to take it for a test drive at my favorite RC test drive site: the top of the MIT North Garage:
It's absurdly nice out and there's something about an empty parking garage roof that makes me happy inside. However, the North Garage also offers one of the most shocking opportunities for epic RC fail:
(Edit: With the motor current set to 27A, it now does 0-30mph in 2.2 seconds...)
See the ledge there? Yeah, the one with no railing at the ground?
This is what's on the other side.
But luckily the RC system works reliably and the XBee radios get plenty of range with the nice antennas. Using the built-in data acquisition system, I was able to record a top speed of 35mph on the garage. I'd like to say that was limited by my fear of driving off the side, but based on the data I think that is very close to the top speed with this setup. (I would guess 37-40mph.) It's not power-limited, though, so I can tweak either the gear ratio or the motor timing to go even faster. As for the acceleration, it goes 0-30mph in 3 seconds:
The motor current is set conservatively to 20A now. (It's current/torque controlled, so it has a very nice TCS/launch control feel to it.) But for utter minimum 0-whatever time, I could definitely afford to up the current a bit. After 15 minutes of driving with the current settings, I would call the motor "warm" and the controller "not even a little warm." I am very pleased with all the components and how well they work together. Things aren't supposed to work this well.
(Edit: With the motor current set to 27A, it now does 0-30mph in 2.2 seconds...)
Lastly, some video! The garage video is a bit boring so I clipped in some messing around at street level as well. If it looks like fun...it is.
Tuesday, March 16, 2010
LEAF Motor: Spin-Up
The Less Epic Axial Flux motor, a thought exercise a bit over five weeks ago, is now a spinning physical prototype. (Okay, I cheated a little since the rotor disks were already made for the original Epic Axial Flux motor.) Most of what remained to be fabricated was the new coreless stator. Although the entire motor did get a bit of a redesign. Here is the exploded view of the latest design:
The only new thing since last post is the "outrunner outrigger." It connects the mounting structure to one side of the motor shaft, which is not rotating. This completes a structural loop, giving the shaft more stiffness than if it were just connected to the central stator structure. The extra weight it adds is offset by the ability to take significantly more abuse from the chain load on that side of the rotor. This is a minor design change, though. What about the windings?
This test went to 1,500rpm. (Not 3,000 as the tachometer seems to say.) That's at 22V using BLDC (square wave) control. This falls right into the expected range for the delta-wound, 40-turn, 4-in-parallel winding set. It seems to draw a good deal of no-load current at this speed (15A). I think it's too early to say whether this is a function of circulating current in the delta winding, eddy currents, bearing losses (and in what proportions). The next step will be to set it up on my sinusoidal controller with rotor position sensors and see how it fares then. Since this is the target control scheme anyway, I will defer judgment for now and move on to smaller and better things...
Click for full-res.
And, the collapsed version:
Click for full-res.
The only new thing since last post is the "outrunner outrigger." It connects the mounting structure to one side of the motor shaft, which is not rotating. This completes a structural loop, giving the shaft more stiffness than if it were just connected to the central stator structure. The extra weight it adds is offset by the ability to take significantly more abuse from the chain load on that side of the rotor. This is a minor design change, though. What about the windings?
There's the first six, done in about three hours.
Compared to the ones in the scooter motors, these windings are a breeze to make, for a number of reasons. First, the flat magnet wire is absolutely wonderful. It packs nicely and stays where you put it. But, it only really works because I can remove individual winding inserts and wind in free space. This means instead of looping wire through slots, I simply spin the entire insert, spooling wire onto it under tension. It's not trivial, but it's not very hard either. I would place that in the "win" category for this motor.
All the windings, a two-day job.
With all 12 windings in place, and plenty of room in the center of the stator for interconnections, the job is easily finished. I decided to connect all four winding sets (12 divided by 3-phase) in parallel, and to wire the parallel sets in delta. This will give the lowest-voltage, highest-current motor possible with these windings. Before you yell at me for that being the most inefficient configuration: To first order, it can produce the same power per unit dissipation in the copper. And even in this lowest-voltage configuration, the motor still takes about 50V to get to 3,000RPM. When I get a controller that can handle more than 48V, I'll re-wire it. For now, I'm eager to get on with testing, so I put it together:
Kit Bot FTW. I'll replace the outrigger later.
It's not set up for sensored control yet, so I needed to borrow a high-current sensorless controller. Luckily, I know exactly where to find one. Cold Arbor, a combat robot with a brushless cold saw, happens to live nearby and also happened to have pre-cut and stripped wires courtesy of a particularly violent match. So, with a kick-start to get the sensorless controller going, it spins up:
This test went to 1,500rpm. (Not 3,000 as the tachometer seems to say.) That's at 22V using BLDC (square wave) control. This falls right into the expected range for the delta-wound, 40-turn, 4-in-parallel winding set. It seems to draw a good deal of no-load current at this speed (15A). I think it's too early to say whether this is a function of circulating current in the delta winding, eddy currents, bearing losses (and in what proportions). The next step will be to set it up on my sinusoidal controller with rotor position sensors and see how it fares then. Since this is the target control scheme anyway, I will defer judgment for now and move on to smaller and better things...
Thursday, March 4, 2010
LEAF Motor: Assembly Box of Doom + Back EMF
If you're just tuning in, the LEAF (Less Epic Axial Flux) Motor is the end result of a long and unnecessarily complicated process that started when I decided I would buy a set of 16 neodymium iron boron wedge magnets for $74, figuring I would make a motor out of them some day.
Well now I have two sets of 16 magnets, and I want to put them together with less than 3/4" between them. This translates to roughly 500lbs of force trying to smash them, and anything between them, together. This calls for the fabrication of a very strong box from which to make a giant motor puller. Here's the bottom of the box:
It's only purpose in this world is to hold the lower rotor disk securely. The top of the box is much more interesting:
Well now I have two sets of 16 magnets, and I want to put them together with less than 3/4" between them. This translates to roughly 500lbs of force trying to smash them, and anything between them, together. This calls for the fabrication of a very strong box from which to make a giant motor puller. Here's the bottom of the box:
It's only purpose in this world is to hold the lower rotor disk securely. The top of the box is much more interesting:
This is the "puller," with three 5/16"-18 threaded rods that screw into the top rotor hub. The rods in tension do the work of lifting or holding against the axial force, sending it to the side of the box in compression. All together, it looks something like this:
The motor puller box is designed with about a 3x safety factor on all the forces, but along with the box, there are some rules. Rule Number One is never put appendages between the disks. They are always separated by the puller and then the entire top of the box is removed by lifting from the sides before any work is done on either disk. Rule Number Two: whenever the disks are separated, a block of wood between them acts as a failsafe in case the box or rotor disk breaks.
So that takes care of one of the things over which I was losing sleep. The other, winding, turns out to be not that hard at all with the flat magnet wire I picked up from Alpha-Core. Now all I need is a few solid free hours in which to do it. (Actually, probably like a full free day.) But anyway, I managed to do a single winding. I basically half-ass the 40-turn winding, then force it to be the right shape with my winding press:
So that takes care of one of the things over which I was losing sleep. The other, winding, turns out to be not that hard at all with the flat magnet wire I picked up from Alpha-Core. Now all I need is a few solid free hours in which to do it. (Actually, probably like a full free day.) But anyway, I managed to do a single winding. I basically half-ass the 40-turn winding, then force it to be the right shape with my winding press:
Patent Pending.
Okay, I will put a little more effort into the real winding. But for now I just wanted to get to the magical single-winding back EMF test that I have so many times claimed is actually a useful thing to do. This time, I'm sure it's legit. With a coreless motor, there is no incomplete magnetic circuit to worry about. The no-load B-field between the disk looks the same with or without the windings. So with one winding's measured back EMF, I can extrapolate the performance of the entire motor. (You probably still don't believe me...I don't blame you.)
Anyway, I put the winding in the stator and put the stator in the box:
(Notice the nice new countersunk non-magnetic stainless screws?)
Anyway, I put the winding in the stator and put the stator in the box:
(Notice the nice new countersunk non-magnetic stainless screws?)And stupidly I did not take any pictures of the whole thing together. But to my great relief it actually spins, the magnets no longer grind against the screws, it is centered, and the gap is...well it's not very uniform. It sort-of favors one side. I think it might need a little persuasion toward the other side. But it doesn't seem like the 1/r^2 problem...the dual bearings on each rotor disk seem to take care of that. But anyway mechanical stuff can be poked and shimmed and squeezed into compliance. What about the fundamental premise that this thing can actually transform power?
This time, I didn't need the go-kart to spin it up. (No cogging torque, and no violent shaking of the entire test bench.) I used a smaller but equally unorthodox prime mover: a cordless drill with a hole saw attached to it. The hole saw dug into the rotor hub nicely and got it up to about 600rpm, plenty fast enough to capture a back EMF waveform on the scope:
More winding ahead!
This time, I didn't need the go-kart to spin it up. (No cogging torque, and no violent shaking of the entire test bench.) I used a smaller but equally unorthodox prime mover: a cordless drill with a hole saw attached to it. The hole saw dug into the rotor hub nicely and got it up to about 600rpm, plenty fast enough to capture a back EMF waveform on the scope:
Oooooooooo...shiny.
Lastly, I measured the winding resistance for a single 40-turn winding to be 0.127Ω. Since there would be four in parallel, the phase resistance is actually one quarter of that, or 0.032Ω. This starts to give me a feel for the thermal performance of the motor. Altogether, the motor constant and resistance are fairly similar to the array of small EV motors (Etek, Mars PMAC, PERM, etc.). The LEAF, which is the only coreless motor in this lot, will likely have a slightly higher torque constant, but also a higher resistance and lower thermal mass for heat sinking. So we'll see how it compares in continuous and peak ratings. As always, it will depend a lot on the cooling solution.That is much more sinusoidal compared to the steel core version. Why? It's still a concentrated winding with non-skewed magnets. Well, probably a few reasons: The magnetic interaction is more distributed, not focused to the edges of the stator teeth by steel. It's also interacting with field that is relatively far away from the magnets, where it starts to blend together smoothly from North to South instead of having sharp transitions.
As for the fundamental motor constant, the amplitude of the back EMF as a function of frequency (speed), I get about 9.5V(rms) per 1,000RPM. Since four of these windings would be in parallel, that is essentially the phase voltage of the motor. That's a fairly nice number, since in the practical speed range for a home-built motor (3,000RPM?) it falls into the 72V battery category. Through the magic of power conservation, I can come up with a torque constant for the motor as well: 0.27Nm/A(rms). This is...well it's all pretty much dead on with the predictions. (Wow, even the shape of the back EMF is correct...)
As for the fundamental motor constant, the amplitude of the back EMF as a function of frequency (speed), I get about 9.5V(rms) per 1,000RPM. Since four of these windings would be in parallel, that is essentially the phase voltage of the motor. That's a fairly nice number, since in the practical speed range for a home-built motor (3,000RPM?) it falls into the 72V battery category. Through the magic of power conservation, I can come up with a torque constant for the motor as well: 0.27Nm/A(rms). This is...well it's all pretty much dead on with the predictions. (Wow, even the shape of the back EMF is correct...)
More winding ahead!
Friday, February 19, 2010
LEAF Motor Blitz: Everything but the Windings.
One of the most important lessons I've learned in the last six years of engineering study is how to slow down, organize, and carefully execute a big project.
But screw it, sometimes you just have to put the gas down.
Since my Feb 4 post, I've been blitzing the LEAF motor project, refining the design and building all the new components in two weeks. Everything but the windings. Here's the latest design:
But screw it, sometimes you just have to put the gas down.
Since my Feb 4 post, I've been blitzing the LEAF motor project, refining the design and building all the new components in two weeks. Everything but the windings. Here's the latest design:
It's gone through significant refinement from the Feb 4 concept mostly as I realized the particular characteristics of a coreless stator that made the design more flexible. The most obvious change is in the mounting and loading: One benefit of the coreless design that I didn't mention in the last post is that, at least in open-circuit, there are no axial forces between the rotor and stator. In fact, in open-circuit, there are no magnetic forces at all between the rotor and the stator. No cogging. The only forces paths are between the two rotor disks.
There is still a tremendous axial force, almost 500lbf, trying to squeeze the two disks together (and crush your fingers in the process) but the coreless version doesn't suffer from the air gap instability I mentioned here, where one rotor disk or the other would rather snap to the stator than maintain an equal air gap thanks to the 1/r^2 law. There's also a very large torque holding the two magnet disks in alignment, much larger than the torque the motor itself is usually producing. All this means that the outer can is, in fact, optional. It does provide a degree of stiffness, but with 1/4" steel-backed magnets, rotor stiffness is the least of our problems.
Ditching the outer can opens up a whole new range of possibilities. For one, wire entry no longer has to be accomplished through the shaft itself, as it would in a hub motor. In fact, the shaft doesn't have to be the mounting point, either. Hence, the blade-like design you see above with a hollow mounting tab that allows wire entry through the side. It's not hard to imagine forcing air through the side as well, for cooling. Here's what it looks like IRL:
The shaft is actually integrated with two machined aluminum hubs. The structure is essentially a sandwich of ABS plastic winding inserts (the black triangles) in 1/32" carbon fiber, with a bit of aluminum thrown in outside the magnet area where extra stiffness is required, such as at the mounting points. The inside thickness of the sandwich is 1/2".
Myth: Carbon fiber is difficult to machine. Actually, it's difficult to make. But if you buy plates of it and just need to drill a few holes, it's actually very easy to do. I didn't even need a mill: I made a nice aluminum template and rotated it around to locate all the holes:
There is still a tremendous axial force, almost 500lbf, trying to squeeze the two disks together (and crush your fingers in the process) but the coreless version doesn't suffer from the air gap instability I mentioned here, where one rotor disk or the other would rather snap to the stator than maintain an equal air gap thanks to the 1/r^2 law. There's also a very large torque holding the two magnet disks in alignment, much larger than the torque the motor itself is usually producing. All this means that the outer can is, in fact, optional. It does provide a degree of stiffness, but with 1/4" steel-backed magnets, rotor stiffness is the least of our problems.
Ditching the outer can opens up a whole new range of possibilities. For one, wire entry no longer has to be accomplished through the shaft itself, as it would in a hub motor. In fact, the shaft doesn't have to be the mounting point, either. Hence, the blade-like design you see above with a hollow mounting tab that allows wire entry through the side. It's not hard to imagine forcing air through the side as well, for cooling. Here's what it looks like IRL:
The shaft is actually integrated with two machined aluminum hubs. The structure is essentially a sandwich of ABS plastic winding inserts (the black triangles) in 1/32" carbon fiber, with a bit of aluminum thrown in outside the magnet area where extra stiffness is required, such as at the mounting points. The inside thickness of the sandwich is 1/2".
Myth: Carbon fiber is difficult to machine. Actually, it's difficult to make. But if you buy plates of it and just need to drill a few holes, it's actually very easy to do. I didn't even need a mill: I made a nice aluminum template and rotated it around to locate all the holes:
And for the 1-1/2" center hole:
Yep, a wood-cutting spade drill. (Wood is made of carbon too, right?) The pointy edges cut through the thin carbon fiber sheet very nicely just before the flat spot hits the surface.
The result is a remarkably stiff stator sandwich that weighs about 750g and looks really cool. The windings will add another 1.25kg or so to the stator mass, but more about those later.
Even though the rotor disks are no longer attracted to the stator, they still suffer from the 1/r^2 problem with each other. Meaning, they would potentially tend to close the air gap on one side of the circumference while opening it wider on the other side, rather than sharing it evenly all the way around. Furthermore, there will be large forces and torques on the rotor disks from the load as well, coming in through the chain and sprocket. All this suggests that the single conical bearing, even magnetically preloaded, is not at all sufficient to hold the air gap. Without an outer can linking the structural loop, they are under-constrained.
So I did what I should have done before anyway and modified the rotor halves to each be completely supported. Meaning, they each have their own thrust bearing and radial bearing, separated by a large enough distance to well-constrain the rotor disk on the stator shaft. This required an array of new round parts...
...which fit together as in the exploded view above. Also in the category of things I should have done before anyway, the sprocket spacers now have centering features that capture the sprocket. The bolts are no longer used for alignment, they simply hold things together. The sprockets are captured between the two bearings, providing a well-supported reaction to the chain loads. Minus the bolts, it looks like this:
And it spins quite nicely. There will be sprockets on both sides, mostly for symmetry, but it could potentially drive a split axle. I'd like to believe it would behave like a limited-slip differential, but honestly don't know what this would do.
So that leaves the winding. And here I have one last trick: flat magnet wire!
Processed by Alpha-Core, this stuff is rectangular cross-section enamel-coated magnet wire. While it may actually be more difficult to wrap a normal motor with this wire, because it can't twist, it's significantly easier in this case. I can take off an individual winding insert, spin it around while pulling some wire off the spool, put a bit of epoxy to hold it in place, and then screw the insert back in place. (Okay, that makes it sound easy... It's still a time consuming process, which is why I haven't actually done it yet.) The result is a relatively tightly-packed winding that also has a thinner cross-section, cutting down on eddy current loss.
The best part is that I can wind the whole motor. Meaning, it can actually run. It won't be the 30kW monster that this project originally targeted, but it will still fall into the high-single-digit horsepower category, and weighing in at just about 25lbs. It should get pretty good efficiency numbers as well. Testing to begin soon!
The result is a remarkably stiff stator sandwich that weighs about 750g and looks really cool. The windings will add another 1.25kg or so to the stator mass, but more about those later.
Even though the rotor disks are no longer attracted to the stator, they still suffer from the 1/r^2 problem with each other. Meaning, they would potentially tend to close the air gap on one side of the circumference while opening it wider on the other side, rather than sharing it evenly all the way around. Furthermore, there will be large forces and torques on the rotor disks from the load as well, coming in through the chain and sprocket. All this suggests that the single conical bearing, even magnetically preloaded, is not at all sufficient to hold the air gap. Without an outer can linking the structural loop, they are under-constrained.
So I did what I should have done before anyway and modified the rotor halves to each be completely supported. Meaning, they each have their own thrust bearing and radial bearing, separated by a large enough distance to well-constrain the rotor disk on the stator shaft. This required an array of new round parts...
...which fit together as in the exploded view above. Also in the category of things I should have done before anyway, the sprocket spacers now have centering features that capture the sprocket. The bolts are no longer used for alignment, they simply hold things together. The sprockets are captured between the two bearings, providing a well-supported reaction to the chain loads. Minus the bolts, it looks like this:
And it spins quite nicely. There will be sprockets on both sides, mostly for symmetry, but it could potentially drive a split axle. I'd like to believe it would behave like a limited-slip differential, but honestly don't know what this would do.
So that leaves the winding. And here I have one last trick: flat magnet wire!
Processed by Alpha-Core, this stuff is rectangular cross-section enamel-coated magnet wire. While it may actually be more difficult to wrap a normal motor with this wire, because it can't twist, it's significantly easier in this case. I can take off an individual winding insert, spin it around while pulling some wire off the spool, put a bit of epoxy to hold it in place, and then screw the insert back in place. (Okay, that makes it sound easy... It's still a time consuming process, which is why I haven't actually done it yet.) The result is a relatively tightly-packed winding that also has a thinner cross-section, cutting down on eddy current loss.
The best part is that I can wind the whole motor. Meaning, it can actually run. It won't be the 30kW monster that this project originally targeted, but it will still fall into the high-single-digit horsepower category, and weighing in at just about 25lbs. It should get pretty good efficiency numbers as well. Testing to begin soon!
Thursday, February 4, 2010
Thought Exercise: The LEAF Motor
Okay, I've been playing with SolidWorks a little too much.Here's the thought: Since I already have awesome rotors with conical bearings for my axial flux motor, maybe I can get something up and running quickly while I debate what to do about the problems with the full ("epic") version. Specifically, the cost in both dollars and hours of finishing the stator, the difficulty of assembly and testing, and the lack of a real use for it anymore are limiting my desire to push hard on the segmented stator core version, thus far the focus of the project.
Talking to some solar car types got me thinking about coreless motors. A coreless motor is a motor that has no steel core, just windings. In fact, for the first time this year M.E. sophomores at MIT got to make coreless DC motors as part of Mechanical Engineering Tools. They have some obvious benefits, such as lighter weight, no cogging torque, and high efficiency due to the elimination of core losses.
But aren't they, like, way underpowered? It's true that the steel core helps focus current from lots more windings up into a tiny air gap with very high fields. Coreless motors, by contrast, tend to have lower field strengths and less space for windings, both of which limit the maximum torque output of the motor. So this will not be the Epic Axial Motor. Instead, it's been named the
Talking to some solar car types got me thinking about coreless motors. A coreless motor is a motor that has no steel core, just windings. In fact, for the first time this year M.E. sophomores at MIT got to make coreless DC motors as part of Mechanical Engineering Tools. They have some obvious benefits, such as lighter weight, no cogging torque, and high efficiency due to the elimination of core losses.
But aren't they, like, way underpowered? It's true that the steel core helps focus current from lots more windings up into a tiny air gap with very high fields. Coreless motors, by contrast, tend to have lower field strengths and less space for windings, both of which limit the maximum torque output of the motor. So this will not be the Epic Axial Motor. Instead, it's been named the
Less Epic Axial Flux Motor
or, for short
LEAF Motor
or, for short
LEAF Motor
And I have no less than four ways of estimating the performance of such a motor as is pictured above, which would have a total thickness of 1.7" when collapsed. You know, like a leaf. First, a little more detail about this semi-arbitrarily-chosen test case:
Rotor: Same as before. These magnets on two low carbon steel backings. Not ideal for a coreless motor, but they already exist which gives them +1,000 points by my score keeping.
Stator: 14AWG magnet wire, 28 turns per coil, 12 coils in the following winding pattern: ABCABCABCABC. For once, I'm not using a crazy fractional-slot configuration because cogging doesn't matter. As a reference value, I am using 20A as a reasonable amount of current to put through this wire, which equates to about 10A/mm^2, or 200 circular mil per amp. This is not conservative, but not insane.
Gap: The gap between magnets is 0.7". This means the peak flux density is about 0.5T, with these magnets. This is significantly lower than the 1T field density in the air gap of the steel-core motor. This, plus the fact that 0.7" is not a whole lot of space for copper, is what will make this motor less powerful.
So, what about those four methods?
Rotor: Same as before. These magnets on two low carbon steel backings. Not ideal for a coreless motor, but they already exist which gives them +1,000 points by my score keeping.
Stator: 14AWG magnet wire, 28 turns per coil, 12 coils in the following winding pattern: ABCABCABCABC. For once, I'm not using a crazy fractional-slot configuration because cogging doesn't matter. As a reference value, I am using 20A as a reasonable amount of current to put through this wire, which equates to about 10A/mm^2, or 200 circular mil per amp. This is not conservative, but not insane.
Gap: The gap between magnets is 0.7". This means the peak flux density is about 0.5T, with these magnets. This is significantly lower than the 1T field density in the air gap of the steel-core motor. This, plus the fact that 0.7" is not a whole lot of space for copper, is what will make this motor less powerful.
So, what about those four methods?
Method #1: Nibbler
Just an example.The force acting on the wire is just: F=IBL (fibble), and it obeys the right-hand rule. In a motor, there are many lengths of wire all carrying current, so we multiply in the total number of active wires, N. Also, to get to torque, we use a radius, R. That gives T=NIBLR (nibbler). In this motor, N=448, I=24.5A, B=0.5T, L=2in, and R=3in. (All roughly speaking...I can justify those numbers but it would take another page of maths.) This gives a torque of 21.2Nm (at this value for current, which gives the same power dissipation as 20A continuous in each wire...don't ask).
Method #2: FEMM Static
The trick I've been using for axial motors in FEMM is to unwrap them around the average radii of action. So, the magnets span from 2in to 4in. I unwrap the motor at the 3in radii, literally unrolling it into a straight line. This has worked pretty well for me so far, but it's a little trickier with a coreless motor, I think.
Anyway, FEMM allows you to set material properties, magnet orientation, turn counts, current, etc. It then can predict electromagnetic forces. It is inherently a statics program, meaning it will give you a force, but won't simulate motion or back EMFs without some coding (see below). Anyway, all I really care about for now is a ballpark torque estimate. So, I put the model into FEMM and put 24.5A into a coil sitting right between two magnets (max torque). It created a force on the coil of 32.6N. If 8 of 12 coils were generating this force, at a 3in center of action, it would create a torque of 19.9Nm.
Anyway, FEMM allows you to set material properties, magnet orientation, turn counts, current, etc. It then can predict electromagnetic forces. It is inherently a statics program, meaning it will give you a force, but won't simulate motion or back EMFs without some coding (see below). Anyway, all I really care about for now is a ballpark torque estimate. So, I put the model into FEMM and put 24.5A into a coil sitting right between two magnets (max torque). It created a force on the coil of 32.6N. If 8 of 12 coils were generating this force, at a 3in center of action, it would create a torque of 19.9Nm.
Method #3: FEMM Dynamic
Here's where it gets ugly. My recent adventures in sinusodial, field-oriented control have got me thinking about better ways to characterize motors. The NIBLR and FEMM Static methods give you torque as a function of current, which is like a DC motor torque constant. This is a pretty good approximation, but it leaves out the important details of the shape of the back EMF, which is related to the shape of the magnets and coils. This is especially true in a coreless motor, where the focusing effect of steel stator segments is not present. So, I've also played around with scripting FEMM to generate a back EMF plot. I won't go into details, but the basic steps are:
- Import geometry. Group coils so that they can be selected as one block.
- Loop through a set of angular displacements for the coils, moving the block between each one. Run static analysis each time and record flux linked by the circuit.
- You now have a graph of flux vs. electrical angle. Take the derivative [Wb/rad] and multiply by electrical frequency in rad/s. [Wb/rad x rad/s = Wb/s = V]
- That's back EMF.
The more points you take, the less noisy it will be.
How do you get from back EMF to torque though? Thanks to The Best Motor Reference Ever, I finally understand this link. The idea is simple: Take the back EMF and drive a current into it. That's power. On the other side, you get torque and speed. Speed is already set by the electrical frequency you choose in step 3 above (1,000RPM in the case of this example). So just...solve for torque. This all works very nicely when you assume that both the back EMF and the current driven are sinusoidal. You can then use RMS values to get power.
Doing this quick math for the FEMM dynamic back EMF nets a torque of 18.4Nm for 20A RMS.
Doing this quick math for the FEMM dynamic back EMF nets a torque of 18.4Nm for 20A RMS.
Method #4: What the FluxCutter?
Ok this isn't a real method. I just made it up. But it might have some appeal to the mechanically-inclined out there, since it's a very geometric method involving CAD. The problem I was trying to solve was: FEMM is 2D. It can't account for the strange shape of the coil, which occupies a different fraction of the circle at the inner radius than it does at the outer radius. How, then, to get a fourth method (there must be four, now that I said it) without switching to a 3D magnetics package?
Well, I can use a 3D CAD package...like SolidWorks. First, I made a visual representation of the flux density one might see in this motor. I made it sinusoidal, but really I guess you could make it whatever you want. FEMM suggests it might be sinusoidal in the middle, and closer to trapezoidal near the magnet faces. Anyway, a sinusoid seems like a decent place to start. I wrapped that around a cylinder with the correct inner and outer radius, in inches. Then, I scaled the z-axis features such that 1 inch was equal to 1 T. Thus, 1in^3 is really 1Tin^2 which can be converted to flux in Webers!
All that's left was to make the FluxCutter, an extruded cut with the profile of an individual turn of wire, that cuts volume off the bulk shape. By measuring this change in volume, I got the flux. The rest was the same as Method #3. Oh, except I could also account for the slight offset as coils get wrapped in multiple layers. I assume FEMM does this for spread-out coils, but not in 3D.
Well, I can use a 3D CAD package...like SolidWorks. First, I made a visual representation of the flux density one might see in this motor. I made it sinusoidal, but really I guess you could make it whatever you want. FEMM suggests it might be sinusoidal in the middle, and closer to trapezoidal near the magnet faces. Anyway, a sinusoid seems like a decent place to start. I wrapped that around a cylinder with the correct inner and outer radius, in inches. Then, I scaled the z-axis features such that 1 inch was equal to 1 T. Thus, 1in^3 is really 1Tin^2 which can be converted to flux in Webers!
All that's left was to make the FluxCutter, an extruded cut with the profile of an individual turn of wire, that cuts volume off the bulk shape. By measuring this change in volume, I got the flux. The rest was the same as Method #3. Oh, except I could also account for the slight offset as coils get wrapped in multiple layers. I assume FEMM does this for spread-out coils, but not in 3D.
With a chunk of flux cut out.
I will admit this is just plain silliness. You get more accuracy in the shape of the coils, but less accuracy in the shape of the field. (Unless you can model the 3D field more accurately than a sine wave...which if you can do that you probably already have a 3D magnetics simulator.) But it was fun and did yield a result. The torque predicted by this method was 15.8Nm at 20A RMS.
In summary, the four methods I use predicted reasonable torque values between 15.8Nm and 21.2Nm for 20A through 14AWG wire. I would call that pretty consistent. This says nothing about the operating voltage and current of the motor; that would depend on whether the coils are connected in series or parallel. (Probably, all four would be in parallel, giving an 80A motor...but the torque is the same.) Depending on the achievable speed, this is decent power (1.7-2.2kW per 1,000RPM). And this is not really a peak rating.
What about dissipation? Well, a quick estimate of the resistance of such a winding suggests that at 20A the motor would be dissipating a bit under 200W due to copper losses. So it has a chance to break into the mid-90% efficiency range by 2,000RPM, and even earlier at lower currents. Although I think with the existing bearings it won't.
So, not quite as epic as the full version, but probably worth a try. It's definitely easier to make, and might in fact make a very practical motor.
In summary, the four methods I use predicted reasonable torque values between 15.8Nm and 21.2Nm for 20A through 14AWG wire. I would call that pretty consistent. This says nothing about the operating voltage and current of the motor; that would depend on whether the coils are connected in series or parallel. (Probably, all four would be in parallel, giving an 80A motor...but the torque is the same.) Depending on the achievable speed, this is decent power (1.7-2.2kW per 1,000RPM). And this is not really a peak rating.
What about dissipation? Well, a quick estimate of the resistance of such a winding suggests that at 20A the motor would be dissipating a bit under 200W due to copper losses. So it has a chance to break into the mid-90% efficiency range by 2,000RPM, and even earlier at lower currents. Although I think with the existing bearings it won't.
So, not quite as epic as the full version, but probably worth a try. It's definitely easier to make, and might in fact make a very practical motor.
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