In the last RazEr episode, I whipped up the mechanical design for the monster truck wheel that will be the new scooter motor.The giant ass-magnets came in last week, as did the aluminum pipe that I purchased for the can, and the new bearings from my favorite shady eBay bearing house. So now, I’m going to build it.

Additionally, with the advent of the DEC modules and the chance to start completely from scratch (as opposed to rebuilding the old RazEr motor over and over), there is one experiment that I have been meaning to try on a motor like this. But first, some pictures.

The can of the motor will be made from a 4″ OD, 1/2″ wall aluminum tube. Here, I’m turning and boring the can to bounding dimensions on the Old Mercedes. Boring this thing out was an entertaining process, since the longest boring cutter I had only cut to 2.25″ depth, and the motor tube interior had to be a minimum of 2.8″ deep. As such, I had to hang the bit way out for extra depth and angle it counterclockwise slightly so the tube wouldn’t hit the shank. This resulted in a plenum of squeeeeeeeeeeeeeeeeeeeeeeeeeal that probably lost everyone in MITERS at the time a few years of audial health.

Luckily, the finishing pass on the bore cleaned out all the chatter, and the resulting can is the single shiniest thing I have ever made at MITERS. I adore fresh carbide inserts.

The tube handled threading very well, which was surprising, but is probably a result of the large diameter.  The thread itself is a 3.5″-24 thread, which isn’t any sort of standard as far as I can tell.

For the magnet can, I decided to save the trouble of machining a long steel part on our machine, and instead tripped over to the heävy \m/etäl machine in the automotive shop. Yes, the same one I was trimming down little plastic propellers with. While I’m generally not a fan of the larger lathe because of its impersonal feel and lack of cross slide locks, there are some times when there is no substitute for several tons of cast iron – namely, when you’re deep-boring in steel like I was with the motor cans. There was absolutely no chatter, skipping, or unsteadiness whatsoever.

I finished the inner diameter slightly larger than designed to loosen the very optimistic air gap of 0.3 millimeters.

Complementary shinies. The magnet can will be pressed into the aluminum outer shell after the magnets are installed.

Speaking of the magnets, here they are. 2″ long, 1/8″ thick, and 1/2″ wide. I purchased enough for two motors just in case I ever want to make RazErWD.

After finishing the major machining, I began harvesting the stators out of the copier motors. These are motors from a Xerox Phaser 4400 laser printer (Okay, so it’s technically not a copier motor, but copiers have this stuff too) that I snatched off Ebay in a sleep-induced motor binge.

These motors come apart reasonably easily – the can comes off after the front retaining ring is removed, then the stators just unscrews from its mounting post. So convenient compared to the pressed-and-Loc’dtite motors that I’ve had to deal with for RazEr in the past.

Cleaning the carcasses was easy, if not a bit repetitive. Here, I’ve lined them up inside the can to check for basic clearances.

And now the magnet gluing process begins. Again, I printed out the Handy Dandy Gobrushless Magnet Placement Guide. The seven magnets in there now are all the same pole orientation – they’re far enough away such that they do not affect each other, so epoxying them all down was easy.

MITERS received a shipment of legit extra-long set fiberglass lamination epoxy, so it meant that I didn’t have to deal with my adhesives setting on me as I was trying to maneuver the magnets in place. RazErmotors of the past have just used cheap hardware store hour-epoxy, or even worse, 5 minute epoxy.

After leaving these placeholder magnets to bake for a day, I returned to fill in the opposite pole. I found some +14 Balsa Wood Strips of Convenience that tucked tight into the gaps between the magnets. This was absolutely phenomenal, since that meant I didn’t have to play the (here physically impossible) game of inter-magnet balancing.

I found some glass epoxy filler hiding in the chemical supply shelf, so I mixed some of it up for placing the last magnets. I also filled all the gaps and edges with the stuff, so hopefully this can is much more structural than ones I’ve made in the past.

dual interleaved razermotor

The whole reason why I titled this post. I’m actually going to make two essentially independent motors inside this one motor. Why? Because I have enough volume inside the monster truck tire to put two motors, duh.

Well, that and the fact that the motor architecture is amenable to such modifications. The end result should let me push 4 times the amount of power I could otherwise flow through the windings, with the added bonus of double redundancy.

The average 12-tooth 14-pole LRK motor is generally wound in a “skip-tooth” configuration. Below is a diagram of a typical LRK winding (from the Crazy German R/C Aircraft Guy of all CGRAGs) and what a  motor wound like this actually looks like (from the BWD scooter, again).

I have historically wound my motors in the “distributed LRK” style, which spreads the single winding out to two adjacent teeth, with coils facing opposite directions. This is diagrammed out on the same site. The first observation that can be made is that the two (or four, if you count the other side of the motor) windings have different chiralities. This is something that has bitten me in the ass before.

The LRK winding in conventional Crazy International R/C Airplane Guy notation is A-b-C-a-B-c where the dashes indicate unwound teeth. The dLRK winding is AabBCcaABbcC. If you look carefully, there’s two full conventional LRK motors hidden in the winding, facing opposite directions.

• AabBCcaABbcC
• AabBCcaABbcC

Therefore, the second observation is that two interleaved motors can be wound on the same stator. Optimally, they would share the same Hall sensor commutation points, which are identical to that of a straight dLRK motor.

The consequences of this dual motor setup should be four times the maximum power of said standard dLRK motor. This is because each winding will be half the length, produce half the back-EMF, yet have half the phase resistance. Twice the current can flow, but the torque constant will be halved, and there are also twice as many windings now. However, because the back-EMF is also halved, the motor will tend to spin twice as fast.

Roughly speaking.

To effect these ends into the new RazEr motor, I have prepared the “RazEr Double DEC’er” board, a variant of the skate controller boards that I specially bred (designed) to fit into the available space in the scooter, which is more square.

Notice how the Hall sensor inputs are common to both DECs, as is the throttle and other signal pins.

I actually designed these boards a while back (before DIR became a legitimate idea of its own, and when I was just seeking to make a 2WD scooter), but with the above modifications made so the DECs are chained to eachother, I sent the boards to be fabbed.

…Yeah, I was a cheapskate again and went with the unmasked, unscreened board option. Hey, at least they conduct in the places they should.

Steps left on RazEr rEVolution:

• Finish making the motor! I need to machine the center axle & stator mount, then fit the stator. The endcaps also need to be machined.
• Cut the frame pieces once I get my aluminom slabs in.
• Wind the motor with the dual interleaved LRK winding (which will most likely fuck with my mind at least once).
• Assemble the RDD board and see if my crazy idea actually works
• SCOOTER PARTY

One of the compliments that RazEr always gets is that the vehicle is so freakin’ small. Its outline is the very image of a stock Razor (™,®,©, what-have-you) scooter, and most people do not notice the hub motor until I show them. They tend to try to look under it or behind the rear wheel to see where the motor is. The other reaction I get is more along the lines of what the hell was that, aroused when I fly past a group of unsuspecting and well-meaning pedestrians.

Novelty value aside, the size of RazEr is also one of its worst flaws, and the biggest engineering headache. The stock Razor A3 frame is just small enough to not really fit anything. The latest iteration of the scooter actually featured a fully custom underside so I could put meaningful amounts of battery in. The addition of this drop-frame compromised the stuctural integrity of the scooter significantly. The motor width is constrained to fit between the existing forks, which puts a limit on how much torque it can physically make, as well as makes it a pain to service or remove.

Given that, what would I change if I could start from a blank Inventor file and design the scooter frame from the ground up, while maintaining the pudgy kick scooter look as much as I could?

Before I answer that, let’s go back in time a bit to see what necessitates this total redesign. In April, I “rebooted” RazEr from a state of suspended animation (and total motor wreckage). The 2nd iteration hub motor was pretty much totally destroyed, save for the stator, which I recycled back into version 3, the present one.

During what was supposed to be a routine test ride just a few days later, this happened.

D’ooooooohhhh.

At the time, I was purposely riding over rough cobblestone to see what would break first. I had expected that perhaps the bearings would crunch again, or I’d vibrate a magnet loose. It turns out that the whole motor shifted on its bearings (which means I did an excellent job making sure that bore was a press fit…), and the windings started running into the conductive aluminum side plates. This grinding caused several windings to short and burn out.

The arcing marks and smoke residue on the side plate is a giveaway as to the failure mode.

Afterwards, I hastily rebuilt the core using a spare harvested copier motor that had a stack 5mm shorter than the magnets in the can (-20% torque). I was out of 22 gauge wire, and only had 20 gauge, so I rewound the core using dual 20 gauge wire. The problem was that I could only fit maybe 18 turns on each tooth, which was significantly less than the 25 turns per tooth before – another loss of 33% or so. All things considered, this rebuilt core was complete shit.

Then I discovered during testing that I wired up the B phase backwards. So, I had to cut the termination and jump wires around until that phase was correctly hooked up again. The result after all this turmoil was a motor which looked like this:

AAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHH

Okay, so it actually worked just fine, despite being terrifying. The loss of torque was definitely felt during rides, when acceleration was weak and current consumption was much higher than before. The controller’s runup to full motor start took longer and was less consistent. Things got hotter.

I was pretty unsatisfied, but that’s how RazEr’s been for all of May and June.

RazEr rEVolution

Enough was enough. I’ve been brooding over the build of a fully custom scooter for a while now. It would address all the volume and rigidity issues of stock kick scooters, while fitting my custom designed components with no compromises. Specifically, tt would carry a much torquier hub motor and enough A123 DeWalt drill cells to match or exceed the watt hour capacity of the existing 5Ah lithium polymer packs. It would be more heavily built and stiffer.

Yet I’ve always put the idea aside when it came up because I figured it was more building than I actually wanted to do.

Well, forget that now, because I’m sick of bumming around with toys. And after building the \M/ETALPAXXX, I’m a little less averse to the idea of making more battery packs.

Let’s begin. I’ve titled this rebuild RazEr rEVolution because….

Because.

I always start with a very basic flat geometry study. Here’s the new frame at a pretty early stage. It’s still box shaped like the stock Razor A3 frame, and even has the front chamfer.

But what’s less visible just from a screenshot is the increased dimensions. The underbody is a full inch wider than a stock A3. The whole body, deck included is 2 inches thick (compared to around 0.75″ stock, and the 1.75″ depth of the appended underframe of RazEr).  This whole frame section measure 2 feet long from front to back, too.

Oh, and the wheelie bar is taller and more integrated. That’s important too, but basically, it’s what the A3 would be if it actually filled its outline instead of shrinking away from it in a cost-cutting induced anorexia.

Note the cutout in the top plate of the frame – this is to ensure available space to fit the 12S2P LiNP battery pack, which will clock in at a cool 38 volts and 4.6Ah.

Now I get to the fun part – laying out where the joints of the puzzle will go. I’m going to continue on my spree of tabs-slots-and-T-nuts construction, which ought to make this thing come off fast on an Awesomejet. The t-nut slots haven’t been placed yet in this picture.

I’m only designing the business half of the scooter. The front end will be a stock A3 steering neck and folding joint, since that has worked fine in RazEr from the very start.

One of the issues with mating your own back ends to existing front ends is the rigidity of the joint in the middle. Razor scooters are generally pretty well designed in this area, but once you append your own design to it, all bets are off.

To promote structural integrity, I designed in this 1/4″ nut plate in the dead space in front of the batteries. It has the stock folding joint bolt pattern in it, and is otherwise physically interlocked to the frame.

To address the whole “Hey, you’ll be stepping on your battery packs with that big cutout in the middle there!” problem, I’m going to overlay the frame with a nonstructural cover. The cover is shown rendered in carbon fiber, but that’s alot of fucking carbon fiber and I’ll most likely take the cheap way out of either 1) Garolite or 2) polycarbonate. Or the same sheet of 1/8″ aluminum if I’m that lazy.

Here’s the  side text, modified after insertion to be “waterjet friendly”. Closed R’s, A’s, and O’s just make for a n00bly mess when cut out.

The “EV” in “revolution” is cleverly capitalized to be clever.

new rimz… again

So the version 3 hub motor looks kind of cheesy in the screenshot of the new frame. It appears atrophied and undersized, kind of like putting 15″ wheels on a luxury SUV.

With the new frame width (three whole inches of between-the-forks space!), I’m definitely going to design a new motor to use it.

Here’s the tire!

…

It’s a 5 inch diameter, 2 inch wide polyurethane-tread, aluminum-core fork truck wheel from McMaster-Carr. Did I mention it’s 2 inches wide?

Originally a wheel investigation for BWD, it will now be used with the new new new! RazEr motor:

It looks like the V3 motor if I pulled on the ends really hard, and essentially it is. The one exception is the flanged endcap, which was a design investigation.

The stator will be 3 (count’em: THREE) stacked 67.5mm diameter, 17mm thick copier stators that I binged off Ebay a few months ago. The total lamination height is designed to be 46 +/- 1 mm, to account for lost laminations during the joining process.

That looks so much better. It fills the space like a giant custom hub motor should do. Now I’m talking like

Holy crap, what if this thing had HUB MOTORS in those wheels?

Of course, one of the hazards of designing your motor before you actually get ahold of the wheel to measure it is

…IT DON’T FIT.

The tread depth turned out to be shallow enough on the new wheel such that if I cored the thing out all the way to the rim (past the relatively thin web center), it would just fall right through my motor!

The wheel could be bored to a diameter of 3.6 inches, but the motor is only 3.25 inches at the threaded portion, and 3.5 inches at the flange. I designed it to use my existing Deathrunner and V3 motor stock, which is a 3.5″ steel pipe.

Great.

So here’s the 5AM hackaround… just shove that motor inside a plushy, shiny aluminum case. I ordered a 4″ diameter, 0.5″ wall aluminum tube to make the locking ring from anyway, and decided against ordering a huge 4″ steel pipe just to make the motor casing from. It would be unnecessarily heavy and would have taken very long to machine.

Instead, I could just chop a small section of the pipe I  had already and make it a magnet ring. From the 4″ diameter tubing would come the motor casing and side plates, as well as the threaded ring. The magnet assembly can then press or adhesive-fit into the casing.

The magnets were big enough this time (2 full inches wide, of course) such that it was out of the range of SuperdupermagnetGeorge‘s stock. I cruised the intertubules for a while, and settled on Applied Magnetics and their 2″ x 0.5″ x 0.125″ N42 chunks. A Gobrushless calculation shows that these are an optimal fit.

With the side plates, tire, and threaded ring loaded in, the whole thing looks ridiculous.

I went back to the cross-drilled holes method due to familiarity. I had 17 cap screw on that flange at one point in time (because more cap screws makes a project more hardcore), but fortunately returned to sanity just long enough to realize that I would only have the patience to machine 9 of them.

So there are nine.

And now the whole assembly slammed onto the chassis model. I’m essentially going to be riding a small steamroller.

preliminary calculation

Let’s plat the nibbler game with the most recent dimensions. It’s a crude, unsound, faulty, but convenient way to ballpark the torque production of a BLDC motor. The dimensions are:

• N, turns per tooth. I’ll use the (v2, working) RazErmotor number of 25 turns per tooth. Assuming I wind all the teeth, it means 8 are active at any point in time. Each tooth contributes two runs through the magnetic field, so the total number of “active passes” is 8 * 2 * N, or 400.
• B, the magnetic flux density in the motor, in Tesla. Essentially “strength of magnets”. N42 magnets have a surface Br rating of 13,000 Gauss (1.3T). Add in the airgap factor t / (t + g) where t is the magnet’s thickness (1/8″, i.e. 3mm) and g is the airgap length (designed to be 0.3mm, but I more realistically shoot for 0.5mm), and the reuslt is Br * [t / (t+g)] for 1.1T in the airgap.
• L, the active magnetic length of the motor, which is the stator height: 45mm, or 0.045 meters.
• R, the active radius of the stator, which is 33.75mm, or .03375 meters.

The motor constant Kt is then really roughly 400 * 1.1 * 0.045 * .03375 = 0.668 Nm/A. That’s almost absurd, and is probably 33% or so higher (via past experimentation) than what it will end up being.

Still, that’s insane. RazEr’s v2 stator had a measured (not roughed out) Kt of 0.25 Nm/A. By direct scaling of the stator length, i.e. if I stretched that stator out to 45mm and kept all other parameters the same, it would have a Kt of 0.45 Nm/A. I suspect this is a more realistic number.

when do i get to ride it???

I ordered everything yesterday, scheduled for easy-peasy laid-back ground shipping. This means I can probably start plugging on the motor right after Otakon concludes (and the August robot season begins). Look out for this thing at Dragon*Con in September, since it’s small enough to throw into the same box as the robots.

Knowing me, though, I’ll just blitz the entire frame on Monday and shove the existing RazEr motor in there.