29 June 2013

EtekChopper: Drivetrain Design and Parts Selection

Time to get down to the nitty gritty and make some drivetrain decisions. I need to shuffle around the basic variables of my system in order to finalize the specifications and start buying parts. Decisions will be bold and underlined.


So far all I know is I am using a Sevcon Gen 4 motor controller  but I still need to choose a battery voltage, design or pick a battery pack, choose the motor, choose the gear ratio, etc. The constraints I'm working with are those of the Sevcon Gen4 controller, the space in my motorcycle, and the tug-of-war between performance and my wallet, among other things. 

The Sevcon Gen 4 wants a 72-80 Volt system. 72 Volts seems to be a standard voltage for things, so I will run a 72-Volt system. 
What does this mean? My battery will be a 72V  nominal. Assuming I'm going with 3.3 volt LiFePO4 cells, I am running a "24S" system, meaning 24 3.3V cells in series. Battery systems made up of smaller cells are usually defined by "XsYp", or X in series, Y in parallel. Parallel batteries allow for increased charge capacity and greater allowable output current. The more cells your have in parallel, the less electrical stress there is on one cell. 


Here is a CB750 frame solidworks model I found online, a motor for show, and four  A123 12S8P battery packs with included Battery Management Systems (BMS) that my friend Dane has access to and may give me. The BMS balances the charges of the many small 3.3V batteries to ensure the entire pack is healthy. Issue is, they are HUGE. I don't think I can fit more than two of them (for 12S8P) comfortably. I can probably fit the other two (12S16P) on sides, or as saddlebags. Maybe? Hmm. 

Or I can make my own custom 24S battery pack with some of the infinite 3.3V A123 cells, maybe 10P? That's 240 cells. Adding more cells in parallel will increase my maximum range, but how far exactly will that take me? 

ElectricMotion.org, the website build log for Lennon Rodgers' eMoto has a nifty spreadsheet available for download in the Specifications section where you can modify characteristics such as weight, battery size, voltage, gear ratio, etc and see projected top speed, power dissipation, range, etc. 

I took the liberty of uploading my own version of the Spreadsheet onto Google Docs. The most important parts of the document (to me) are the top speed and range. By changing the cruise speed (I usually have it set to 55mph, which is not energy efficient), drag coefficient, and other parameters, the spreadsheet spits out a maximum range estimate. I should be getting 15 to 30 miles of range on 24S8P of 2.5Ah cells, double that if I can fit 24S16P. By fiddling with the max voltage, the max motor RPM at 72 volts, and the gear ratio, I can find an achieve a max velocity that can keep me comfortable on a highway, so around 65mph. 

But before I can finalize all that, I need to find a motor that is happy running at 72 Volts (electric motor speed is proportional to input voltage).  For Cruscooter's Kv=190RPM/Volt motor to be running at 72 Volts, it will be running over 13000 RPM, which is REALLY FREAKING FAST. A quick hop over to McMaster shows that only their highest quality bearings have a maximum RPM that high. And they each cost half as much as Cruscooter's motor. 


So I need a low-Kv motor (Which translates to high torque constant) or one which can deal with being driven at many thousand RPM. I can go the way of Bayley and get myself a hybrid vehicle motor, like his Ford one. The issue is I would have to design my own case for it, like his, and would need to find one, which I can't seem to be able to do in five minutes of scanning Ebay. 



There's a HUGE Baldor Induction Motor Bayley was going to use in his motorcycle before he decided to use the Ford hybrid motor. Apparently Adam Bercu, another MITERS-goer, had gotten it to spin with the Sevcon Gen4 controller, albeit not efficiently. It also weighs like 100 lbs. 

Everyone around MITERS keeps praising the Etek motor, and there's apparently a few lying around up for grabs. 


Here's one I dug off a shelf at MITERS, though Charles tells me (and there's a sharpie scrawl on it) that the built-in hall sensors are dead. A quick check with an oscilloscope confirms this. I'm going to have to replace them. While this particular Etek-style motor was a custom order, not one that was ever available for retail, I am told it is almost identical to the ME0907, though a little bit more powerful

Looking at the datasheet of the ME0907 (Remember, mine is more powerful), it seems to be a decent-enough motor, but not quite taking full advantage of the Sevcon controller's 25kW possible power output. It has a Kv of less than 70 rpm/volt, so at 72 Volts it will spin at about 5000 RPM. It's 90% efficient at voltages from 24-48 (But I'm running at 72). It has a peak current of 250A, but the Sevcon can deliver another hundred.

Using these parameters I can get a ballpark for the absolute ideal peak power output of the ME0907 motor. A fully-charged A123 cell will be at 3.6 Volts. 


3.6V * 24 * 250A = 21.6 kW or 29 horsepower

The original CB750 engine was 68 horsepower peak, so at absolute peak ideal conditions this Etek motor can put out over 42% of the original engine's power. But the original CB750 weighed 500lbs, while I'm looking to cut it down to about 350lbs after gutting the 250-lb engine and adding 100lbs worth of batteries, controller, and motor. 

Power to weight ratio (PWR) is a thing, right? Assuming I'm 185 lbs (Well, I am), the PWR of the original CB750 (my weight included) would be 0.0992 hp/lb. EtekChopper would be 0.054 hp/lb. While I'm not even sure those are the right units (They seem to be inverted according to Wikipedia), it appears the ME0907 motor has over half the normalized power of the original engine. 

Considering the CB750 was dubbed a superbike, half the normalized power of that is not too shabby! Because it's direct drive electric, the torque to the ground is instant. Plus the ME0907 is free, but costs about $500 online. All I have to do is replace the sensors. I am using the ME0907 Etek Motor. (But you already knew that, didn't you! I have been calling this thing EtekChopper after all...)

Now it's time to choose the single-stage gear reduction. After tuning the gear ratio from 3.5 to 6, I settled on 5.8 (10 teeth at motor, 58 teeth at rear wheel) for the following reasons. 

I want decent wheel torque so I'm not sad when an intersection turns green. Most vehicles are defined by a 0 to 60 mph time. My 2005 Mazda 3 sedan goes 0 to 60 in about 8 seconds. The original '79 CB750 went 0 to 60 in about 5 seconds. Given the peak current (Charles claims this etek motor ), Torque constant, gear ratio, wheel radius (13" tire radius! I almost messed this up), loaded vehicle weight, a rough 0-60 time for my system can be found: 


60mph/(300A*0.13 NewtonMeters/A *(58/10) /(13") /((350+185)lbs)) = 9.5 seconds

So realistically it'll be more like 12 to 20 seconds because the above calculation doesn't take into account wind drag. As for city driving I can get to 20 mph in an ideal 3.17 seconds, so I'm content with this. 

As for top speed, I want to be able to drive on a highway if necessary, which is usually 55 mph minimum. I also want some wiggle room to be able to pass/dodge cars if necessary, to I shot for a 65mph top speed. Plugging into Wolframalpha the Kv, nominal battery voltage, gear ratio and wheel circumference yields: 

(68 RPM/Volt)*3.3V*24*(10/58)/Revolution*2*pi*13" = 71.8 mph

Sweet. Now, I know what you're thinking. But this time my system can put out 100A continuous. Just check out the following calculation of max motor force minus wind force at 65mph: 

0.13 Newton meters / Amp * 100 Amps * (58/10) /(13")
- (.5 *(1.275 kg/(m^3))* (65 mph) ^2 * 0.8 * .5 m^2) = 13.04 N 

I also need to decide what size chain to use. Too small and I'll be breaking the chain like Straight Razer ate through #25 chain. The original CB750 had #530 chain, which is quite a bit bigger according to gizmology.net's page on sprocket and chain sizes. I've heard of people using #40 and its vehicle derivatives on electric motorcycles, but this post of a broken 420 sprocket convinced me to stick with the larger #50-series of sprocket. 

But which one? What is the difference between #50, 520, and 530 chain/sprockets? This post has the answer, as does the gizmology page. #50 and 530 are nearly identical. Let's go with #50 chain! 

For the small 10-tooth sprocket, I looked no further than Surplus Center for an inexpensive steel one whose keyway and bore diameter was compatible with the 7/8" output shaft of the Etek motor. 

Here I introduce to you bikebandit.com. On this site you can specify the year, make, and model of your motorcycle and look through pages of the manual for the exact part you need. They also keep track of compatible aftermarket or universal parts and ofer those options as well. They SHOULD have the right rear sprocket, right? 

Wrong. Apparently 58 teeth is not available for the CB750. The highest rear sprocket available is 48 teeth, which simply wouldn't be sufficient for my torque/speed needs based on my rear wheel size. I guess I'm gonna have to make my own then! (Which I've done before for MelonChopper, I just have been too lazy to blog about it yet). 


Some quick searches on Google led me to Sprocketeer Online! It takes in the desired pitch, roller diameter, and teeth, and generates both a DXF file and generic CNC code for milling out a sprocket. I then simply imported the resultant DXF into Solidworks and extruded it to the right thickness (0.343"). 


However, In order to make the rear 58-tooth 530 sprocket compatible with the CB750's rear wheel mounting holes, I had to either measure the current one or find a drawing with the proper dimensions. I found a drawing at Dime City Cycles (third pic), and added it to the sprocket. I'll waterjet it out of steel or high-strength aluminum and file the edges until the chain fits. Trust me. It'll work. 



They also had 10 feet of #50 chain (Solidworks estimates I need about 5' 7") and master links/extender links for pretty cheap. I freakin' love SurplusCenter. 

Is that it? I think so. For now. Time to start buying stuff. 

Final Specifications:
  • Overall Estimated Specs: 
    • Weight: ~350 lbs (no driver)
    • Maximum Speed: >65 mph
    • Ideal Maximum Power: ~19.8 KW (~26.5 horsepower)
    • 0 to 60 mph: >10 seconds
    • Range: ~15 miles @55mph, ~30 miles @ 30mph 
      • (double that if I can fit the other two battery packs)
  • Motor controller: Sevcon Gen4
    • Free, Estimated cost $925.00
    • 350A peak, 140A continuous
    • 72-80 nominal VDC, (39-116VDC Absolute limits)
  • Motor: Custom Etek-style, Almost an ME0907. 
    • Free, Estimated cost $500.00
    • Happy at 24-48 Volts (I will drive it to 72)
    • Over half the normalized power output of the original CB750 engine
    • 300 Amp peak, 100 Amp continuous
  • Batteries: 2x or 4x 12S8P A123 Packs, 
    • Free (Thanks, Dane!), Estimated cost $936 each pack
    • 72 Volts nominal
    • Integrated Battery Management System
    • 2.5Ah per cell, 2x or 4x (12 x 8) cells. 
  • Charger: I'm liking this one so far: http://www.cloudelectric.com/product-p/bc-sco7220.htm
    • $425.00, but I'm sure I can find a high current one for even cheaper. 
  • Transmission: Single-Stage 5.8:1 gear ratio #50 (530) chain
    • Motor Sprocket: 58 teeth, custom made
    • Rear Sprocket: 10 teeth, from surpluscenter.com
    • Chain: about 105 teeth, from surpluscenter.com

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