Electric motor issues... any techies?

BMbound · Post by **BMbound** » Sun Feb 17, 2008 1:33 pm

So I'm in the process of planning my mutant vehicle for 2008. in the better interest of reducing BM carbon emissions as well as the amount of petrol I have to haul out to BRC I'm trying to determine how powerful of an electric motor I will need to propel a 1700lb (fully loaded) art car from 0-7.5 or so MPH. I have a basic understanding of math but have no Idea how to determine my power requirements. once I have this down I can figure out my battery/peripheral needs. also which do I need to focus on torque or horsepower?
well thanks for any help

Teo del Fuego · Post by **Teo del Fuego** » Sun Feb 17, 2008 2:08 pm

will you recharge your batteries using a generator or some solar array?

wedeliver · Post by **wedeliver** » Sun Feb 17, 2008 2:19 pm

If someone doesn't give you the answer, you might start looking here for help

http://en.wikipedia.org/wiki/Kinetic_energy

MikeVDS · Post by **MikeVDS** » Sun Feb 17, 2008 2:40 pm

You need to focus on both torque and horsepower. One key part that is needed for any of to help you is to know the size of your wheels. You can always gear things up or down but it's easier if you don't.

BMbound · Post by **BMbound** » Sun Feb 17, 2008 2:52 pm

I plan on using a solar setup primarily with a supplemental generator charging at night/when not in use if neccesary.

since this is a budget/recycled material where possible type project I was planning on using a donut style spare tire from a car. measuring my roomates donut I found the OD to be approx 23"

the vehicle will have two freewheeling fixed front wheels with a rear steering rear drive wheel

EspressoDude · Post by **EspressoDude** » Sun Feb 17, 2008 3:31 pm

Seat of the Pants guesstimate will be between two and five horsepower. Since you will be on Playa-time, the horsepower required to accelerate the load is almost negligible, because you can take minutes to get up to speed. The biggest factor will be rolling friction, that is the energy needed to deform the tires as they roll, in addition to the energy required to deform the playa surface, continually climbing out of a hole. Radial tires on smooth concrete might require a rolling force of a few percent of the vehicle weight. ( one person can push a car on flat asphalt, so say 100 pounds force) On the playa it is likely to be 10 -15% or more if there is wind load. In dry beach sand or mud it can be in the order of 100% of vehicle weight.

one horsepower is 550 ft -lb / sec

10 mph is about 15 ft / sec

10% of 1900 pounds is 190 pounds

15 x 190 = 2850 ft lbs/sec

divide 2850 by 550 = 5.2 hp

Note that this is an approximation.

One hp = 770 watts; so 5.2 hp is about 4000 watts

If you are using 12volt battery power, that works out to 330 amps.

A typical car battery is about 60 - 80 amp hour rating, so 5 batteries would last about an hour or so.

If you are using solar panel to recharge, and you will operate your art car the same number of hours that the sun is directly on the panels, you would need 4000watts of solar panels.

same is true of the generator.

5 car batteries weigh 250 pounds, so add that to the car. Want to operate longer, add more batteries...

Methinks you want a lighter artcar

BMbound · Post by **BMbound** » Sun Feb 17, 2008 3:43 pm

thanks expresso dude. sounds to me like it would be much wiser and cheaper to just use the 8hp small engine that I already have and maybe contribute a donation to cooling man in the order of however much carbon emissions I plan on producing

Captain Goddammit · Post by **Captain Goddammit** » Sun Feb 17, 2008 3:45 pm

This is a friend of mine's (Reverend Gadget) mutant vehicle.

It was electric powered, and the roof was entirely covered in solar panels. I drove this thing and at moderate speed it actually still showed positive charging! It had a bunch of batteries, I forget how many, but it had no problem getting all over the playa after dark.

The catch is that Gadget had access to a lot of really cool surplus parts; he said the solar panels this thing had would have cost several thousand dollars.

MikeVDS · Post by **MikeVDS** » Mon Feb 18, 2008 7:17 am

thanks expresso dude. sounds to me like it would be much wiser and cheaper to just use the 8hp small engine that I already have and maybe contribute a donation to cooling man in the order of however much carbon emissions I plan on producing

You do realize that your contribution would probably have been bigger with electric, don't you? Using a generator to make electricity is going to be less efficient than using it for direct drive. I know you mentioned solar, but unless your borrowing panels from your house that you use all year, the energy it takes to make the panels will be greater than the power you get out of them. Not to mention the problems with disposing lead acid batteries.

BMbound · Post by **BMbound** » Mon Feb 18, 2008 3:32 pm

Mike you pose a valid point that I hadn't fully considered.

Although I was also trying to get away from the heat and exhaust noise generated by a gasoline engine it seems like these would be small disadvantages compared to the trouble it would cause me to fully engineer a solar powered setup to be infrequently used.

Captain Goddammit · Post by **Captain Goddammit** » Mon Feb 18, 2008 9:51 pm

BMbound wrote:I was also trying to get away from the heat and exhaust noise generated by a gasoline engine

Gasoline engines don't make "noise". They make "sound". Really cool sound.

unjonharley · Post by **unjonharley** » Tue Feb 19, 2008 8:51 am

One thing that is surplus in this country is ride on lawn mowers/garden tractors..

I see them as having the perfect drive train for the playa..

Captain Goddammit · Post by **Captain Goddammit** » Tue Feb 19, 2008 6:23 pm

unjonharley wrote:One thing that is surplus in this country is ride on lawn mowers/garden tractors..

I see them as having the perfect drive train for the playa..

Absolutely, I've done that before! In '01 I had the Playa Flya, a little bi-plane made from a riding mower, obtained free by mixing parts from two broken ones.
You need to do two things to a mower to make it a decent playa ride. First, you gotta gear it up; they're made to go about 1 or 2 miles an hour, way too painfully slow even for the playa. If it has belt drive like mine did, there's a small pulley under the motor and a large pulley at the rear. I just replaced both with same-size old car water pump pulleys, and it worked great.
Then, you'll probably need to tighten up the steering by doing whatever your particular rig requires.

Substituting a full-size car battery is also a good thing, and if you're crafty, a real alternator mounted below, belt-driven from a second pulley that you attach to the motor, is great.

Bring spare wheels and tires and filter/tune-up stuff, as you should with any machinery bound for the playa.

Caine · Post by **Caine** » Wed Apr 02, 2008 10:37 am

Go here:

http://evparts.com

and also check out the EVDL:

http://www.evdl.org/

1700 lbs is not too heavy, and at 7.5 MPH you won't need a lot of battery. BUT, you should get real deep cycle batteries. Don't use car batteries.. they are designed for lots of output for a small period of time. You won't get good performance in an EV application.

The cheapest way is to get Golf Cart batteries (good ones if possible.. Trojan T-105s are the standard), string them in series, and use a high voltage DC motor, like this one: http://www.evparts.com/prod-MT2112.htm

If you want speed control, you need a controller as well. Or, you could experiment with a contact controller. That is a mechanical device that puts more or fewer of your batteries "on line" with your motor. Basically, it is a big multi switch. You do not want to use a resistor (like a big dimmer) to control current in this kind of thing...

AND you'll need a charger for the batteries. Using those cheap 12V chargers from the auto supply stores is possible, but its pretty hit or miss, and they are not really designed for deep cycle batteries.

Another thing here.. Horsepower in DC is not the same as Horsepower in ICE. ICE engines rate their Horsepower at PEAK output.. DC is rated at continuous. So in other words, a 10HP DC motor is a hell of a lot more powerful than a 10HP gas engine.

Alternatively to all of this, if you are good with electrics you can put together everything from salvaged parts, old forklifts, and etc. The batteries are the only thing you probably would need to get new.

Caine · Post by **Caine** » Wed Apr 02, 2008 10:48 am

MikeVDS wrote: You do realize that your contribution would probably have been bigger with electric, don't you? Using a generator to make electricity is going to be less efficient than using it for direct drive. I know you mentioned solar, but unless your borrowing panels from your house that you use all year, the energy it takes to make the panels will be greater than the power you get out of them. Not to mention the problems with disposing lead acid batteries.

Actually not true, depending.

If you are using an old garden tractor motor or lawnmower engine, they are incredibly inefficient, and are even MORE inefficient being used as motive force for travel.

In contrast, a genset is always working inside of it's peak efficiency curve, and even with conversion losses you could easily end up equal or over a straight gas engine setup. This is one of the reasons ships and trains use Diesel Electric power trains.

If you are using a well tuned gas engine/power train, designed for motive force, point made. You'd be right.

As far as lead batteries go.. the lead recycling industry is becoming very efficient and greening up lately. To be honest, I'd rather have to deal with those problems than smog and engine exhaust!