How Much Electric Kart Can You Build in a Day?
Last week I rebuilt the kart with fresh fasteners and axle bearings, plus picked up a steering wheel and some #35 sprockets. Then, once the Mars motor looked repaired and good to go onto the kart I picked up a handful of 7/8 keyed #35 sprockets from Surplus Center. The parts were forecast to arrive on Thursday- so I made plans with my friends to bring everything to Laney and try to get the kart running. I came in at two and got started- about eleven hours later the kart was driving, a half an hour later it was broken, and finally at about four in the morning we all went home.
Miles drew up the motor mount plate in CAD and cut it out of 3/16 mild on the CNC plasma table. It’s bolted right to the outer bearing frame support with slotted holes to adjust the chain tension- the stick he’s using is to help support the motor while he tightens the plate in.
Micah made up all the cables out of 6GA welding cable. We didn’t find enough lugs locally at any sort of reasonable price so we made “lugs from the ghett-o” by smashing copper tube around the wire, filling with solder then drilling. A little trick I learned from Home Power Magazine about ten years ago. I mounted the Sevcon Millipak controller, contactor and fuse, and made up the control panel, throttle, and hall sensor wiring harnesses.
See the rest–
Micah set up battery mounting using only the best pallet-wood and ratchet straps
A dead cushman carcass was ransacked for the 0-5k throttle pedal mechanism, which Micah cleverly secured using hose clamps.
Igniton and forward/reverse switches. Classy purple switch cover courtesy of Surplus Center. Of course.
(warning: there’s a little F word in the video)
So, what happened? Let’s just make a list:
- The axle was binding in funny ways at first, we assumed it was the chain but realized the chain was not tight in any particular way while the axle was binding. Turned out it was the axle bearing set-screws interfering with the slot-shaped cut-out in the motor plate. Since it was slot shaped there was non-constant interferance, and because the rubbing pushed the screws counter clockwise it wasn’t possible to simply “wear in” as the screws kept backing out and rubbing worse. I simply pulled the setscrews out (badly mangled, but really loose) and that problem went away.
- Violent motor cogging- somewhere in the electronic commutation system (hall sensors, wiring, controller) it’s getting bad sequence information, causing awful cogging and jerking torque output. Not sure what’s at fault exactly. You can really hear the effects in the wheels-up video above. It seems to be worst at around 1/3 speed and throttle. It usually clears up with higher speed and doesn’t really seem to manifest at very slow speed while starting out. It did this on the bench before we did the motor repair hacking as well.
- Directional confusion- the Mars motor is set up to spin CCW, the fan is only effective while spinning CCW- which I think is correct for most motorcycle applications. In order to put the motor inside the frame not outside we need to spin it CW. I wired up the motor exactly to the recommended diagram which meant we were operating the controller in reverse. Either due to a reduced speed reverse algorithm or a significant timing bias in the motor the kart ran far faster backwards than forwards. Which is somewhat entertaining but mostly stupid.
- Fusing Stupidness- Before even putting the kart on the ground we blew the $20 150A fuse I had just bought. Not sure exactly why, given that it’s a 250A controller and slow blowing fuse- I suspect it had to do somehow with the cogging problem. It’d be real nice to get that solved- good for the batteries, controller, motor, drivetrain… well everything.
- Parking ticket- I got a big effing parking ticket for being parked on the street at about 12:50. Street sweeping. To their credit they did then bring a sweeper through. But it was one of those great ones where you’re walking to the car as they’re writing the ticket. Ok, moving on…
All of these things were non-optimal but did not prevent driving the kart around.
Then the mishap that ended things for the night- Due to a combination of the sprocket carrier axle-clamping screws not being tight and the awful jerking cogging-torque from the screwy commutation the sprocket holder key jiggled out, then the chain derailed. Casualties- the key was smushed up pretty good, the keyway suffered a bit, and the sprocket carrier became a bit pretzel shaped. Turns out that if a chain in a system without any give in the chain-path length derails it’s going to deform the weakest part of the system. In this case the aluminum sprocket holder. At least it wasn’t the motor shaft (though I should probably check that carefully)- that would be significantly more expensive. So that’s probably one of the reasons kart suppliers sell “sprocket guards” – a bit of insurance against chain derailment. Maybe worth it.
So I will pick up a new sprocket carrier and a new key, this time I’ll get one that is the entire length of the keyway- so that there’s nowhere for it to escape to, and to fortify around the mooshed part of the keyway. Then we should be back in business, for some faster gear ratios and higher voltage…
on July 8, 2010 at 10:13 am
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[…] Herndon] is working on building an electric kart, mostly with surplus parts. He’s got some experience with electric vehicles but that […]
on July 8, 2010 at 3:04 pm
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you need some 90’s in that motor mount plate to keep it from flexing
on July 8, 2010 at 3:30 pm
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I considered bending the edges of the plate forward or back for rigidity- it would be almost trivial to throw it on the brake and do so- however the plate is made of 3/16 steel and is pretty stiff as it is. We didn’t see much flex when testing it. It’s definitely something I’ll consider doing down the road. Right now there are more pressing things to fix! Thanks for the constructive comment.
on July 8, 2010 at 3:45 pm
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where do you get all these high amperage motors? i have some kd221k75 transistors and i’m looking for something to test them on.
on July 8, 2010 at 3:58 pm
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some of that groaning in the turns is probly from lack of a diferential, one of the wheels always has to slip.
on July 9, 2010 at 5:51 am
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do you know that swapping two of the motor wires results in the motor spinnig the opposite direciton? this means that the controller can operate in the right direction. i think rc brushless controllers have a less aggressive/power throttle curve in reverse.
on July 9, 2010 at 10:40 am
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Indeed switching the phase and sensor wires around can reverse the rotation of the motor. In this case I’m not sure if the reduced speed was only due to the controller of if the motor has a timing bias as well. This controller is designed for all sorts of sweeper machines and warehouse machines and has a bunch of options about reduced speed forward/reverse- but we have no idea how it was setup. On the night we put this together we were so fried just getting it together we definitely didn’t feel like experimenting with motor hookup variations- and now the Sevcon controller is completely dead and unresponsive… Back to square one!
on August 4, 2010 at 8:25 am
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[I didn’t make a mark on the case before pulling it apart so I may have slightly altered the timing (the hall sensors are on the inside of the front case).]
An incorrect position of your hall sensors after your repair could be the cause of part of the problem. I had a current sensor reversed (on a induction motor system), giving reversed feedback to the controller. This caused the controller to produce maximum negative current instead of a small positive current. Weird thing is, the motor still ran, but very noisy and with crazy currents.
My guess is you need to change the position of the hall sensors relative to another, instead of just swapping them; so if #1 is at 0 degrees and #2 is at 60 degrees, move #2 to -60 (or 300) degrees.
Good luck!
on August 4, 2010 at 7:45 pm
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If you guys want me to check/document the configuration of your Sevcon controller, I’d be happy to. There is normally no fwd/rev bias in the setup for the Mars BLDC but there are some phase timing differences that Sevcon recommends when using that motor.
BTW, great job on fixing the magnets. I sent the link of that page on to John F. who designed the motor! 🙂
Dan Danknick
Team Delta
on August 22, 2010 at 10:48 pm
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[…] last time’s disappointment with the performance of the “Alt-Kart” set up with Mars BLDC motor and malfunctioning (now dead) Sevcon PMAC controller it’s been on […]
on February 26, 2012 at 6:22 am
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Dear Builder!
I read your post: “malfunctioning (now dead) Sevcon PMAC controller”
I’m a student and soulful EV (small motorbike) builder.
Do you have this failed Sevcon controller now, or can you sell it possibly?
Cheers
on February 28, 2012 at 10:14 pm
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I’m afraid the millipak controller is long gone, sorry. Good luck in your endeavors!
on October 13, 2010 at 6:18 am
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[…] experience, I am seriously considering making an electric racing go-kart. Mythbusters built one . This is another attempt and this is an MIT project. So it can be […]
on December 4, 2011 at 1:03 am
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I recently purchased the Sevcon Millipak controller and a Mars PMAC motor for my electric car. I’m having quite the spot of trouble trying to get the thing wired for one direction only. Any ideas, the diagrams are a little complex for me
on December 4, 2011 at 5:09 pm
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Hi Aaron. Diagrams are the best way to express how to wire these systems up- the diagram in the Sevcon manual is of some help, but it is generalized and doesn’t specifiy the Mars motor- here’s a diagram the does, although small and a bit hard to read, it’s useful. IIRC there was a pretty good diagram for Mars/Sevcon Millipak interconnection from Electric motorsport, however since they no longer sell the milipak I wasn’t able to find the diagram. Be aware that the Sevcon can be a little picky about having absolutely every element of the diagram present- switches, throttle, etc. They are a bit picky and complex. For example, you will likely need to attach the forward/reverse switch (or an equivalent wire jumper) even if you only plan to spin the motor one way. Good luck. Since it’s been a while since I set the Millipak up and it ended up burning up anyways (probably already defective or broken) I don’t have the information on hand exactly how to do it.