## Transmissions vs. Dual Motors

I’ve been messing around with motors lately, and stumbled on something that I think explains the slightly different nature of electric motors, power curves and transmissions.  This time I’m not using graphs, I’m using actual performance.  It works with three configurations of the same bike.

The first configuration was with the Motenergy ME0709 with gearing that gave me a top speed of around 70mph.  It was capable of throttle-blip wheelies.  If I geared it slightly taller, it would possibly do a higher top speed but about the fastest I think I could get it to go would be around 100mph.  The top RPM is around 3200.

I then put in an ETEK.  This motor is lighter, by something like 12lbs.  It was using the same gearing, but with a top RPM of over 5000, the top speed looked like it was around 110mph.  However, it didn’t have the low-end torque to do wheelies with this gearing.  (One of the tests I want to run is to compare the performance of the two motor geared for the same top speed, to see how the power distribution goes, and see if, first, the ME0709 can pull the top speed for high mph gearing, and see if the ETEK can pull wheelies at lower mph gearing. )

So, with the ETEK, I have the top speed I want, light weight, but not enough low-speed torque.  If I gear it down I may get the torque, but lose the top speed.  Perfect application for a simple two-speed transmission, right?  Maybe so, if there was such a beast.  But here’s where it gets interesting.

Take your ETEK, keep the gearing the same (for high top speed), and add another ETEK. What do you get?  You get the same top speed, but double the torque at any given RPM.

Running two motors in parallel keeps your voltage the same, but feeds twice the current (assuming you’re able to feed that kind of current).  It adds the weight of a second motor, in this case, 21lbs.  I suspect you couldn’t build a two-speed transmission that’s going to be lighter, so we’re even there.  It also puts less strain on each motor, than you’d have for a single motor set up with a transmission, basically being asked to pull hard at low speeds, and pull hard at high speeds.

Basically, you’re adding thermal mass and torque at the same time.  And that, my friends, is the secret behind getting your head around the concept of why adding motor capacity is different, and maybe better, than adding a transmission.

It gets back to the basic difference between gas motors and electric.  Gas motors represent an entire system that delivers power, similar to the entire electric drive system.  An electric motor is only a part of that entire system, and it’s basic function needs to be power handing capacity, rather than thinking about it as power delivering capacity. See what I’m doing here?  The system delivers the power to the motor, and it has to be able to handle it without blowing up, to be a balanced part of the system.  (…where gas motors ARE the system.  Think in terms of the piston, if you’re a gas-head.  The piston has to be able to handle the power delivery of the gas motor without melting, whether you’re running a normal motor or a turbocharged nitrous configuration.  The more power in the combustion chamber, the more stress the piston’s going to get.)

So adding a transmission to a motor is going to stress it more.  If the motor is big enough to handle the stress and the system can deliver the amps, then it will work.  There’s parasitic loss, but that’s probably not enough to worry about.  Adding another motor spreads the torque curve, by doubling it, and adds thermal mass.

Only by looking at the entire drivetrain system, can you really answer the question…  will additional motor capacity work better than a transmission.

The end result is only a little more weight- roughly 42 lbs of motor, not counting a few pounds for the mounting, and twice the torque.  The ME0709 is what, 34lbs?    Also, twice the current handling capacity.  The ME0709 handles 300amps peak.  The peak current for the ETEK is hard to find, and I don’t believe some of the numbers out there, but if you take a conservative 250amp peak and double it, then you’re looking at 500amps peak.  Well above the ME0709.

As I’m becoming fond of saying, it’s all about the system.  As remotecontact is fond of saying, it’s all about finding the bottleneck in the system.  Rather than simply adding juice to one component or another, you have to think in terms of the entire system and how all the components work together.  And one more thing, maybe the most important of all.  It all depends on what you’re designing the system to do.

## The Lipo Project: Balance Block and the Mini Bus Bullet Block

As is often the case when I’m in the middle of a project I put all this stuff together and kind of forgot to post it here.  Sorry.

Here’s what I ended up with for connections and wire management for the lipo packs.

First, with the help of David O’Brien, I got these little things together for paralleling the balance tabs:

After we got done, he found some boards that were set up as strips in the bottom of some drawer, but watching him solder this thing was like watching Leonardo paint.

Here’s what I came up with to parallel the 9mm bullet connectors on the Turnigy packs:

A simple chunk of copper, drilled with holes to accept the connectors.  Add a little heatshrink and you gotcha Mini Bullet Bus Block.

Connectors working loose is a huge issue, so I wanted to figure out some way to mount them, lock them, and help manage the cables.  I made up a simple ABS plate with some cutouts and ridges that does the trick.

Here’s what it all looks like in the bike:

## Dept of Watching Grass Grow: Testing the Turnigy, Part Two

So here’s the final strategy.  (Comment from RC” “ur doin it rong.  but not too rong”

The idea is to discharge the packs down to a safe level, (see additions below) so starting with topping up the charge is kind of a waste of time.  Instead, I decided to just hit them right off with a discharge for two hours…  which, coincidentally, brings them down to around 3.1-2.9V.  Their safe minimum.  Right away, 2 1/2 hours saved.

So, at the end of that, starting usually at around 3.8V, we’re ending up with, for example, cell voltages of 3.26 – 3.14 – 3.07 – 3.06.  Not bad.  And, they balance out to within .01V by the time they’re back up to 3.5V. Total elapsed time is about two hours for the discharge, and about an hour for the charge.

One comment.  One thing I learned early on with digital cameras, you’ve got to label (and number) your batteries.  There’s always one battery in a pack that’s bad, and, simple as it seems, if you don’t label them you have no way to track their performance, especially over their lifetime.  Simple housekeeping…

Update: OK, here’s where I’m at.

The goal here is to weed out bad packs.  The other goal, which I can’t do right now, is to try to match up packs based on the internal resistance of each cell, making everything more balanced.  This is something I think I’ll tackle later on, once I have more chargers and more packs.  For now, I just want to flag any packs with cells that are weak, which should show up as low voltage after a bit of a discharge, and uneven voltage after a top-off charge.

Bottom line, I’m continuing with the process.  After I run the bike gently a few times I’ll do balance charges of each module and see how that looks.  I’m going to save the fine-tuning for when RC is here…  and I can ply him with beer.

## Dept of Watching Grass Grow: Testing the Turnigy

The primary purpose of testing the Turnigy lipo packs, at least so I’ve been told, is to determine how each cell “drops off”, or, how the voltage decreases as the cell discharges, near the minimum charge level.  If a cell gets to a point and drops off too rapidly, you have a bad cell, and thus a bad pack.  That can make it build up heat, which, especially for lipo, is a very Bad Thing.

The charger I’m running is the Turnigy 150W unit shown above.  I’m setting it up to discharge the packs, and it’s showing a 5amp discharge setting but is averaging a little over 1.5amps actual discharge.  For a fully charged pack it takes about 2 hours to go from 4.18V to 3.7, or so, and then it hits the time limit.  I’ve hit it for another discharge cycle, and we’ll see where that brings my voltage, but ideally I’d like to get it down to nearly the safe minimum- probably around 3-3.2V.

I’d really like to get this all done by August 14th, for a show I’d like to run it in…  but we’ll see.  This is one process I don’t think it’s safe to rush.

On another note, the power supply I ordered was for a computer peripheral, at around \$10.  Rated at 12V and 6amps, I thought it would do the trick, but no.  After a few minutes it just cuts out.  Right now I’m just running the charger off of my big scooter batteries and it’s going fine, but that’s definitely something that I need to tend to.

## Connecting the Charger (Turnigy LiPo)

You know the old saying about a picture being worth a thousand words and all…  well, I seem to have missed the words, “Connect the Main Output leads to the power leads of the battery”.  No seriously.  I’ve been thinking the battery charged entirely through the balance connectors…  now, of course, I realize that’s kind of silly, considering how small they are…

So here it is, the main power input on the right (just to confuse you, right next to the balance connections) and the main charging output coming out of the left side.  You can get a look at the display here too.

OK.  Now.  Here’s the new wiring plan, based on my newfound Wisdom.

## The LiPo Project… The Connector Strategy

I’ve decided how I’m going to wire these babies up.  Because the Turnigy charger is so cool, and reads out the cell status for each individual cell, I’m not going to be cutting and hard-wiring the packs together because I want to be able to pull individual packs out and let the charger balance them one at a time.  So.

On the balance leads, I’m setting up a circuit board using a simple test board to hold 4 of these suckers.  They are from Digikey, the JST B5B-XH-A.

Then I’m going to make the power feed pigtails using these slick bullet connectors from HobbyKing.  Reports are that they’re a total PIA to assemble, but they’re awesome, so I’m going to give it a shot.  They can be found here, at HobbyKing

Once I have all five modules together, I”ve ordered some 10″ wide shrink tubing to hold each module together.

The catch is, I have to go back to the battery mount on the bike and make room for one more module.  These are only slightly smaller than the 6s packs I was planning for, so 16 fit perfectly.  20, not so much.

The 10″ wide shrink tube I got from AllBattery.com, along with the little JST-XH 5-conductor pigtails that are going to the PC board.

## The lipo 4s1p Pack Wiring Plan

Here we go…  the wiring diagram for putting together 20 4s packs.

One module:

The basic concept?  Everything is paralleled.  All the balance tabs, and all the main + – leads from the individual packs.  (NOTE:  You MUST make sure all the packs are fully charged and balanced before paralleling the balance leads, to avoid any severe current flow between unbalanced packs/cells.)

Then, you series the modules:

DISCLAIMER:  First, mostly all of this is from RemoteContact, AKA Adam Bercu.  He gives credit over to Endless Sphere, but I know for a fact he’s learned all this stuff himself, the hard way – by building such badass robots they won’t let him play in their sandbox anymore…  but that’s a tale for another day.

Second, lipo is dangerous, and you take anything I say here at YOUR OWN RISK.  If your lawyer thinks I have some deep pockets he can go after if you blow your silly ass up doing anything I say here…  well, your lawyer is delusional.  Do your homework.  I may be totally full of crap.

(edit: Please see the updated wiring plan here, due to me realizing that you have to charge through the main power leads.  Duh.)

## How a Controller Works (so you can see)

Controllers work by feeding pulses to the motor- full voltage,  on/off, really fast.  What the geeks call it is “Pulse Width Modulation”.  Now, if you want to see that, you have to set up an oscilliscope, which I don’t know how to spell.  Wait.  Oscilloscope.  There. Thanks to the work of Noah Podolefsky, we can see both the silly scope, and the controller monitor on this video:

So, watch the numbers on the Alltrax readout, especially the throttle setting.  Now, watch the silly scope lines- the lower line is zero voltage, the upper line is 100% voltage, and the length of the lines shows the width of the pulses.  With no throttle, you get a solid line a 0 voltage, right?  The more the throttle feeds the controller, the longer the line at 100% gets, until it’s solid…  You can’t really see the “curve”, or, by that I mean the vertical lines connecting the 0 value to the 100%- don’t let that throw you.

Here are some screen shots of some key events.

First, starting out at dead stop.  The line you see is the “0″ voltage, indicating no pulse.

This is at 12% throttle, and you start to see the little “100%” pulse line.

This is at 67% throttle, the “100%” line gets longer, indicating the pulse’s duration at 100% voltage is increasing.

At 93% throttle the pulse is solid- you’re essentially getting full voltage, all the time.

This is interesting.  The motor is now at maximum RPM (unloaded) and the throttle is wide-open.  However, the controller has backed off on the pulse duration.  Makes sense…  it’s not working as hard since the motor is not accelerating.

This curve stayed like this for quite a while, as the motor was spinning down.  It’s interesting, too, to take a look at the various performance numbers- voltage, temperature, draw…

Take a look at the dialog about this work here, on ElMoto: “What happens at 100% throttle?”.

Great stuff Noah, and thanks!  If you want some fun, stroll over to Noah’s YouTube page and browse through the work he’s done.  Definitely one of the more badass looking builds, and he’s a man who got his wife to ride his bike.  He said the secret of that has something to do with starting the build in his kitchen.  I’m not really sure how that works, and I can assure you, I’m not going to be exploring that method any time soon…

Got any good reference or support links for builders?  Gimmee!

Big shoutout to Noah Podolefsky, and this re-post is for the folks who got to see his great presentation today! Wish I could have been there…

Reference:

Suppliers (with Reference Info):

Classifieds and Used Parts:

Groups and Forums:

Blogs and Magazines:

Books:

## The Second Build- A Different Story

As soon as I built my first boat, I immediately started planning my next. It’s a natural thing, I think, with anyone who’d build something like this, and the bike was no different. You immediately see things you’d do differently, changes you’d make, and usually they’re changes that go right to the basic design.

The Honda VF500F was a great chassis for a build, in fact it’s one of the most typical for a conversion.  I’m not much into the frame design though, and it’s more a product of my breeding in motorcycling.  I’m stuck in the ’70s.  I like tube steel frames.  I love me a good double downtube, and I immediately started thinking about a frame I’d like to mess with more than the ’84 VF500F square-tube monoshock.  I came right back to my first build, a bored, ported, blueprinted 1975 Yamaha RD350.  So, in my random travels, I kept an eye out for a chassis.

What I found is a frame, for \$40, with no title and precious few parts.

The process of building this bike is completely different than what I went through with the Honda.  First, it is a restoration of sorts.  I’ve pulled together enough to get the thing rolling, actually safely, too.  I know all about the handling modifications on this frame so I could pick and choose what I felt was most effective.  Bronze swingarm bushings, roller bearings in the steering head, like that…  all well known and time-proven.

The electric part of this was a no-brainer.  I don’t have a ton of money, and I love this frame.  The Honda is, frankly, something I got my mileage out of and didn’t have much interest in finishing up.  The Yamaha, on the other hand, is so light and nimble, it seems a natural for the motor and controller I had.  For that matter, it was all wired up.  It’s painful to say it, but I scavenged most of the parts from the VF500F, stripping it pretty much bare, electrically.  The motor mount just needed a little tweaking, and even on that score I’d learned a lot from the first bike.

It was really a totally different process.  The first build started when I got the bike, which immersed me in the project.  From there I developed the concept, researched the components and designed as I built.  For the second bike, I started with the final concept fully formed in my mind.  I have a pretty clear idea of what it will be, and what it will do.  I also know what I want to do differently, and mostly, that’s about the batteries.  I’m still determined to develop a battery system that’s somewhat universal, or, at least, versatile.

There’s another side of this, and that’s the fact that I have no title.  The truth is, I could probably get it registered by some sleight of hand, but building a bike that’s not intended to be street legal is completely liberating.  There are so many things on a bike that you tolerate, or build on, or try to get around, that are about making the thing meet the requirements of the DOT.  When you let that all go, you can do simply whatever you want.  You can cut off anything at will, you can configure the bike just as you want, with no constraints.  It’s a joy.

Granted…  it’s going to be a challenge actually finding a place to ride this thing, especially at it’s hoped-for top speed, but it’s well worth it.

This time, I’m not so much converting a gas bike.  It’s more like I’m building a motorcycle, and it happens to be electric.

One of the cool things about it, additionally, is that the motor and associated components are pretty much interchangeable.  I literally pulled them off the Honda and put them on the Yamaha.  It’s a point worth remembering when you’re looking at a system for your first build: look at the purchase as something that could have a life beyond that first project.

I don’t want to give the impression that I have it all sorted out though…  there are a few things that seem like they’re never solved particularly well.  Yes.  I’m talking the batteries.  But more on that later.

Here’s how it sits right now…  within a couple of months, a functional machine:

Interesting, too has been the aspect of the tools.  I don’t have to run out and pick up all the sundry tools that I didn’t have, or couldn’t find, although buying tools is one of the fun things about projects like this.  The only thing I needed to add to my tool collection was the sandblaster, something I now wonder how I lived without.  I’ve been there, sorted out the fabrication processes, got the tools together and now don’t have to spend all that time and effort trying to figure out how to get something done.  Partly it’s being equipped, and partly it’s having a little experience.

Now.  If there only wasn’t a four foot snowbank in front of the shop door…