Stuff I’ve learned, Part 2

Honestly, I didn’t really realize I was approaching this milestone until after I’d passed it, but getting a few good runs on the bike this week really means one major thing. I got it running. It ain’t too pretty right now, but it works, and in the words of the old windsurfing advice- sheet in and sail ugly!

I have, especially in the last few months, learned some important things.

I think everyone kind of obsesses about what motor to get, and really, it’s not too difficult a decision. My vote was go for tried and true, and simple. The Mars motor is pretty much the standard- a legend in EV circles, really, and ultimately the simplest to implement. I’m not at all sure it’s the ultimate ultimate, but in comparing all the specs it’s only percentage points away from the top end. The motor/controller package is really the most important part of the equation, and there is just a ton of information out there on running this motor with any number of controllers.

Maybe on my second or third build I’ll go with some hugely high-tech option, but this thing is a simple, permanent magnet, tried and true solution. It’s sincerely badass too.

Motor Primer 101.
I’m going to do a little video on this later, but understanding the motor/battery combo is actually very simple. Here’s how it works. First, forget about everything you know about gas motors, and trying to apply that to electric. Deep breath. Ready?

1. Voltage = RPM. Simple as that. You give a motor more voltage, it spins faster.
2. Torque = power. And torque, in electric motors, basically comes from size.

You can’t forget all that other stuff, watts, amps, you know, but that’s the basic equation. Some discussion.

You can feed a little tiny motor a ton of volts, as long as it can take it, and it will spin really really fast. It won’t, however, pull you at any kind of speed, because it doesn’t have the torque. If you feed a nice big motor a ton of volts, it will spin really really fast and drive you really really fast, too, because it has gobs of torque. If you have a really big motor but it can’t take a lot of voltage, it may pull a moose, but it won’t pull it very fast- volts = RPM.

Of course, this begs many questions, some interesting, some not so much, but here are a few.

Take a few motors of roughly the same size and output. You’ll get different RPMs per volt out of them. The differences amount to the efficiency of the motor- but that said, my feeling is they aren’t so different as to say one is hugely better than another. Likewise torque- some are better than others, but overall the performance within a size/weight class are pretty much the same. Honestly, you have a few standard solutions- but if you take a good hard look at the specs and the tradeoffs, you could basically close your eyes and pick.

Ome more thing. Amps. Electric motors draw as much power as they need to turn- to apply torque- so if you have a big load they’re going to draw as many amps as you give them. What you “give” them depends on how much your batteries can provide (and for how long), the wiring that feeds the motor, basically the whole energy “train”. The motor has a maximum it can draw, but again, the bigger the motor the more it will draw (to apply the torque it is capable of… starting to see how this all fits?). You have a little bitty motor like on Sparky, it will only draw a small amount of amperage, you got a big nasty old Mars, it may pull as much as 400amps.

How to blow motors up.
Overload them. Then they draw more amps than they can handle, if you feed them enough. Give them too many volts. Then they spin too fast, and the electrical parts, like the brushes and stuff, burn up. Motors run most efficiently at about 80-90% of their rated load. Motor ratings, by the way, are not particularly nebulous. They are derived from very specific build criteria, basically what you can do to the thing without it blowing up, and the engineers know what they’re doing. Although you can boost the voltage, for example, about 20% and the motor will run, it won’t run for long.

The Contactor.
I didn’t really understand what the contactor did, but it’s basically a bigass relay that allows the controller to shut everything down if things go horribly wrong. Yes, you can make the thing run without it, but you’re risking the health of the controller at the very least.

Next installment will be batteries- but I can tell you the bottom line right now. Weight = power. There’s more energy density in various battery technology, like lithium you get more power/weight than lead, but within a technology “group”, you want to go fast (or go long) you need weight. Maybe this will help. Batteries make electricity as a result of chemical reactions. More chemicals = more electricity.

Finally, a personal observation on projects like this. I find it most rewarding to take things a bite at a time. There are a few major steps in the process, like actually buying the motor, but dive in and plunk away. Building Sparky, for example, was incredibly rewarding and instructional. I was at kind of a standstill on the big bike, but I got this little minibike running, and blew up a few cheap controllers, batteries and brushes in the process- what’s more educational than blowing stuff up?

Dive in, and good luck!



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