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Needed DC motor strength? Answered

So me and a pal are looking at building an electric racer, and wonderd, if two 36V 850W were enough?

its gonna carry one person, the motors, 3 12V lead-acid and the frame of the car (made out of less than 12meters of 30/30/3mm steel)

thanks :)


How fast do you expect it to go?

over flat tarmac?

Do you expect it to start from a dead stop with no extra help

How long do you need to battery to last (range)

You may - will find this information here useful

1. With 250 watts you might expect to get to 30 MPH on the flat
2. Acceleration is brisk without being smash your eyeballs out.
3. A 250 watt continuously rated 24 volt electric motor will need a 75 Amp circuit breaker.
4. "2 x30 A/hr batteries will last approx 2 hours at full speed.
5. the body needs to be as light as you can get it.
6. Don't forget you need to stop and bike brakes at 30MPH + are not very effective. Disc brakes work.

thanks alot :D we were beginning to think we would need 2*kw motors :O

i now see that the 2*850 will be more than enough :D

its a racer, so we want it to go FAST. Breaking will be done by the motors themself (if thats sufficient?) through the H-bridge driver.

so all in all thanks alot :D

the batteries will be 3 12v 90Ah lead-acid, and will supply the two 36v 850W motors.

It should be able to start from a dead stop (problem?)

Check out the PDF link above It;s all about making electric racing cars.

You will need brakes. Even efficient regenerative braking isn't effective as you slow down - and actually in my experience not all that good anyway. All too often you need to brake hard and the go to acceleration the motor braking doesn't do this very well.

If you buy your motors on Ebay they may not be as powerful as you think the 250 watt continuous rated motor above weighs in at about 20 pounds and is about the size of an old car dynamo. Its peak power is 500 watts drawing 20 amps.

Although they often blow the 70 amp breaker if they try to move off from a standing start and don't use gears. or some kind of soft start.

thanks, im buyng the motors from a scooter replacement place, and they are pretty big. continues 850W.

its not a racing car per say, just a have fun driving fast car. would you recon (with 30 amp per channel) that a H-bridge designed to take 50A per channel (EASILY (50 °c Tjunction)) would handle startup? would the pulse current exeed 169A for more than 4 seconds??

My 250 watt motor draws in excess of 70Amps if you try to move from a standing start - I know this because it blows the 70 Amp trip. Nominally at optimum RPM (3000) it draws 20 Amps - this is because of the efficiency curve. Although rated for 250- watts continuous running. It gets HOT and needs cooling. So be aware of this.

To get the Greenpower cars to run from a standing start you need to use a gear box of some kind. we tend to use bike equipment because it is easily available and fairly light.

How fast you go depends MUCH on your gearing. The motors should be able to run at 3000 RPM (if you can get a data sheet so much the better) so you can work out your available speed BUT increasing top speed as in a standard car WILL mean reduced torque. Its a balancing act.

If pure speed is the aim Greenpower did have a contest using standard car starter motors to power a drag racer. These go fast and accelerate well BUT it totally ruins the battery because of the high current drawn.

The contest was dropped a couple of years ago Although several schools had a go at it. 30MPH with your bum 1 inch from the ground seems very fast to the driver believe me. Stopping is a real issue so fit good operating brakes even if you don;t think you will need them.

hmm, okay, guess ill buy mosfets to make a spare motorcontroller, then in the software, limmit the startingspeed.

2.5 HP, should be alright.