Twist and Grind

It's been a while.  For about a week after my last post I didn't even think about the electric part of that infernal wheel.  I rode the bike with good old fashioned pedal power.  Here's Zandrevel after picking up some groceries:

Still on the non-electrical side, I've broken two of the drive spokes so far, so I'm a little unhappy with the wheel build.  If you remember, I pointed the extreme spoke angle the day I got the wheel.  I'm still debating with the wheel builder whether or not the lacing angle is too extreme.  The current spokes are 10.75 inches long; I propose we move to a slightly tighter double lace pattern by dropping down to 9.5 inch spokes and rotating all the spokes one click to the right.  This would greatly reduce the torque that's causing the nipples and spokes to fail:

 

So the next thing I did while waiting to come up with a good idea for the wheel was test the power storage of the batteries.  Since I didn't have a motor,  I decided to set up a few lights through a power inverter and left them on until they ran out... 

After over two hours, I still hadn't killed the batteries, but had clocked 50 amp hours at 24 volts.  That's 1.2 KWh out of the rated 1.5 KWh of the battery back.  Since you're not supposed to run lithium iron phosphate batteries all the way down empty until they've been cycled a few dozen times, I called it quits here.  The battery voltage was just fine, which is typical of this battery chemistry (falls off at the end).

Then I plugged the batteries into my awesome Anderson plug, which allows me to connect and disconnect in seconds.  All connections interface at this connector so I simply plug it in and then we're charging.  The power-bearing positive and negative leads are 10 gauge, while the other connections are 18 or so.

So then for a while nothing happened.  I bought another internal 8 speed Nexus hub for the commuter bike (Qimikom) off of craigslist and have been riding it instead.  Once I get the ebike up and running, I'll take apart the new Nexus hub so that I can see how to put my old one back together.  Hopefully, I don't end up with two dead hubs :).

Some time last week I took apart the motor for a better assessment of the situation:

As I mentioned previously, the nylon shield was not fully inserted into the hole and therefore the cables rubbed enough to wear through their insulators and onto the sharp edge of the hole.  Everytime we made contact, the motor controller would shut off; I definitely need to add a breaker to keep things safe.

Another quality issue I found with this wheel is the solder connections are poor:

The assembler obviously used an iron with too low a wattage to sufficiently heat up the solder and wire. I'm simply amazed at both the poor craftsmanship and the fact that these joints didn't overheat or break off.

Next I pulled off the coils:

At the time, I believed the coils were pressed onto the axle and therefore used a puller with 10 gauge wire wrapped around the base.  It turns out that once you remove the locking washer you can push the coils out from the permanent magnets.  The only force holding the coils in is the VERY strong magnetic force between the iron core and magnets.  You will need to push from the bottom very hard, at which point the coils will fly off, so make sure you have a helping hand to keep them from flying away.  Or you can use a puller like I did and slowly overcome the magnetic force.

Coils from the bottom.

Remaining junk.  There's no need to take the motor apart more than this unless the bearing is destroyed.  Fortunately, I didn't actually damage the bearing as I had previously thought.  Thankfully, the hardened steel directed my energy elsewhere.

I had previously started with a wire hole this big:

and widened it out to this:

And then I put this project aside for a week+... until tonight, when my room mate from college sent me an email asking how it went.  At about the same time, by current room mate showed an interest in helping me with the hub, so I decided I'd give it another go.  The first thing I did was clean up my prior work on the coil section hole with a Dremel:

Note the 1cm crack in the aluminum right next to the hole.  Keep in mind I've ridden 0 miles on this wheel since I started drilling into it.  This is very concerning and indicates low cycle stress fatigue.  I'm hoping I can get a new set of coils if needed from the seller down the road if this crack continues to spread (which it will until the wheel breaks).  

And then I didn't take a lot of pictures for a while because I was trying to get all kinds of different things to work.  I ended up widening out the hole in the axle that breaks the wires out into the electrical housing.  I used a Dremel and a nice conical cutting bit to smooth the entire hole out, made it much wider, and rounded out everything to prevent any sharp points from nicking my wire.  As for the wire, I ended up being able to get three 10 gauge enamel coated wires and 5 encoder wires through the axle!!!!!!  This is a huge achievement and I believe I'm the first person to do this.

Note that the wires are not yet connected.  I bent them into place and removed their insulation, but I need to get some epoxy tomorrow to attach the wires so that they don't rub on each other and the axle.  Since the enamel is so thin, I wrapped the wires in 2 layers of electrical tape before carfully bending and working them through the axle.  I guarantee you will not be able to route 10 gauge, solid core, enamel-coated wire through an axle that hasn't been widened out substantially.  Note that I didn't widen out the long hole through the axle, just the small hole that taps into the long hole.  This allows for a much more gradual bend while working the wire through the housing.

So what are the perks of having 10 gauge wire instead of the previous 14 gauge wire?  For starters, 10 gauge wire has more than twice the cross-sectional surface area than 14 gauge.  This means I can put much more current through the motor for much more time before things start to heat up.

One minor concern I had with 10 gauge wire was skin depth.  For a given diameter solid core wire, the skin frequency is the highest frequency you can push through a wire without causing the rapid electron flux to prevent current from flowing in the center of the conductor.  This obviously reduces the maximum current we can push through a wire if the frequency becomes too high.  At 40mph, we're traveling at 704 in/sec, which equates to 463 wheel rotations per minute, or 7.72 rotations per second.  Since the motor has 36 poles and is three phases, each wire will see 12 Hz per revolution.  7.72rps*12cpr= 92.6 Hz, or 5556 cpm.  This is nowhere near the skin depth for 10 gauge solid conductor copper (2600Hz), so we're guaranteed 100% penetration.  This means we can use the wire at its full rated current, which is 40 amps per NEC ratings.  Since the motor controller is 50 amps max and this is a three phase motor, everything's peachy.

A summary of the issues I've seen with this wheel thus far:
1. Two broken spokes (1 nipple, 1 spoke)
2. 1cm crack in coil assembly
3. Incorrectly installed sheath caused rubbing
4. Wires too small for rated power of motor
5. Wire-through-axle design is subpar and could be corrected as discussed in my previous post
6. Poor solder connections from wire to coils.
7. Rust in new axle

Overall, I'm less than pleased with this wheel, but I was able to get the wires routed correctly, so I'm pleased until the next thing breaks.  If that happens, I'll find another wheel hub.  I'm done wasting time with this wheel.