Project: The Electric Booger (budget EV conversion - '92 Mazda MX-3)

[TheSGC]

The tachometer is one of the few things I just haven't gotten around too in my Civic EV, and all I need to do it wire the countershaft wire harness sensor to my Zilla!

DC motors have sweet spots where speed/current/cooling fan operations are the best. My Warp9 has a recommended top speed of 5000 RPM, but likes to be around 3000 RPMs for best performance. I think the best RPMs really depends on the car and gear ratios.

[mechman600]

Originally Posted by thingstodo

For a DC motor, I don't think that the speed has a lot to do with the efficiency.

My understanding is that staying in a higher RPM range for a given load means that the voltage at the motor will be higher and the current lower (since Watts = Volts*Amps), and since higher current (amp) levels = generated heat/inefficiency, this is better.

But I have been wrong many times before.
This week I will do 5 days of "short shift commuting". We'll see what happens.

This motor's fan really blows. In a good way. I can see there a bit of loss at higher revs through the fan.

[thingstodo]

Originally Posted by mechman600

My understanding is that staying in a higher RPM range for a given load means that the voltage at the motor will be higher and the current lower (since Watts = Volts*Amps), and since higher current (amp) levels = generated heat/inefficiency, this is better.

Hmmm...

For a given motor resistance, the power lost in the windings is I*I*R. So less current should be WAY LESS losses. That makes sense.

Reducing the motor current by increasing the applied voltage will drop the winding losses. Check.

What about the brush-to-commutator interface? Does higher voltage increase the losses there, even without arcing? I don't see why it should ...

So is there a down side to running the motor at (relatively) higher rpm?

If you confirm the sweet spot, it would interesting to find out WHY it's a sweet spot.

[mechman600]

The sweet spot for power is 3400 RPM. That's the point where voltage at the motor = pack voltage and current has not yet dropped because of back EMF.

A rough diagram:

66 Volts is usually what the pack sags to under full power.

Amps is proportional to torque.
Volts is the "pressure" required to push the amps in, and this value is proportional to RPM thanks to back EMF.

What does this mean for efficiency? I have no idea.

[mechman600]

The Electric Booger has been running great. Now that the weather turned cold...well...for around here, around freezing, I definitely notice the lack of performance (AKA voltage sag). But my range hasn't been affected much, or so it seems. Not that I ever exploit the full range of the car anyway.

A few weeks ago I started to get a squeaking noise coming from the right rear strut area. At first I thought it was a problem with the ex-rear seat, battery rack, hatch or even the body cracking because of the extra weight. Then the noise progressed to a clicking noise. Then on Thursday, as I rounded a corner and hit a bump, there was a large BANG, followed by a constant RATTLE CLUCK KNOCKIDY KNOCK KNOCK. I stopped for a quick visual and finding nothing obvious, I kept driving.

Up on the jack with some interior panels removed, I was startled to find that the right rear strut mount had rusted right through and broke. Here's a video:

http://www.youtube.com/watch?v=vFWgV5S1kgo

It turns out that a guy at work used to have a Mazda Protege and an extra brand new strut mount, which also broke on him and also fits MX-3s. I gave him a few bucks for it, borrowed his spring compressor and made short work of making the repair.

Check out the rustiness:


And the knackered strut mount:


I should probably get another strut mount for the left side and replace it as well, just to be a little bit proactive, hey?

[MetroMPG]

But, but... cars aren't supposed to rust where you live!

[mechman600]

Originally Posted by MetroMPG

But, but... cars aren't supposed to rust where you live!

I KNOW!

Actually, I think this was a Kamloops car. I think cars actually rust there. But it's funny that there is absolutely no other rust on this thing. Just the strut mounts. Totally weird.

[Astro]

Originally Posted by bennelson

When I've moved motors on pallets like that, I've usually used a ratcheting tie-down strap, like what you use to strap down items in a pickup truck.

If you want something inexpensive/disposable, YES zip-ties work fine, as long as you use the giant industrial ones. There are some VERY large zip-ties out there. In some places, police use them as leg restraints, and then slam the tail of the zip-tie in the back door so the "perp" can't kick the back of the squad-car seats!

Ouch, wouldn't like to see the mess that causes if the squad-car has an accident. The sudden movement of the perp's legs would pull that zip tie ridiculously tight. Then again i don't suppose the cops would have much sympathy if they think the perp is a trouble maker enough to restrain their legs like that.

BTW, very entertaining and informative thread.

Not that anyone could replicate this in Australia, rules and regulations are so tight here.
My old insurance company would not pay any claim if your car had more than 2 modifications. They considered changing the radio, adding a tow bar, changing to alloy rims, tinting the widows, etc. all as modifications and so you weren't covered if you had done more than two of them. They would have a heart attack if they saw the Electric Booger. This is why i call them my "old" insurance company. The new one is better but not by much.

I am always getting ready to do a conversion, hopefully this year will be the one.
The conversion doesn't scare me anywhere near as much as making sure it follows all the regulations and the engineers sign off on it. It will be heart breaking after doing a conversion only to not be allowed to drive it on the roads.

[mechman600]

So yeah, the Electric Booger is up for sale, but I think it will be a hard sell. I am already dreaming up a contingency plan if I cannot get rid of it.

First off, I will ditch the MX-3. It's in rough condition (dents and scratches), old in the tooth (285,000 km/177,000 miles), and a bit porky (2332 lbs original curb weight). 30 HP moving over 1.5 tons of car is getting a bit old. It works fine, but it's not ideal, and less weight would mean being able to do the freeway for short stints. So if nobody wants it before my insurance runs out in May, I am stripping the EV goodies out of it and the scrap guy will haul the glider away.

Possible replacements:
Festiva: 1797 lbs
Swift/Firefly/Metro: 1895 lbs
Tercel: 2090 lbs

My first choice is a Ford Festiva. Maybe the coolest nerdy eco car ever.
"Here, Cheyenne [my 4-1/2 year old daughter], draw me a car. Wow, perfect, you just drew a perfect looking Festiva."
Curb weight of 1797 lbs. That's 535 lbs lighter. Also, I will likely buy a lithium battery pack, which will save an additional 574 lbs and actually fit in the car.

With over 1100 lbs lost, I think I will be happy with my current motor and controller. My Alltrax 7245 controller is meant for 72V nominal but it has a max voltage of 90V, so with lithium I will be able to "cheat" a bit by going with an 80V system voltage (25 cells), where 90V will be 3.6VPC.

[thingstodo]

I bought a 1990 Chev sprint sight unseen a couple of years ago and discovered that, at least in Saskatchewan, 'batteries' are understood to be lead-acid and are treated that way, even if you are using Lithiums. The frame was pretty rusty anyway so I never did get it back on the road.

So in order to put ANY batteries inside the cabin (it's a hatchback) I needed to build a battery box that is sealed well enough to avoid hydrogen and oxygen from the lead-acid charging cycle entering the cabin. So the vehicle inspector wanted a top-vent that sloped upward to where it exited the car, or a power-vented box that had a fan that would not ignite the hydrogen/oxygen mixture, to push the gases outside the cabin.

It may be worth your while to check into any battery location restrictions in BC. A lot of the Ford Festivas were hatchbacks.