Building an electric racing car

Step 5 The motor control/electrical system

This is VERY simple - just on and off. A 70 amp 24 volt automotive relay is switched by the red button on the steering lever and this in turn switches current through to the motor.

Although this system is very simple it is enough and reliable. Pulsing the switch on and off provided a crude speed control although for most of the race the motor is on if going in a straight line and off is the wheels are turned.

In line with the motor is a 70 amp thermal cut out to protect the motor from over current - I strongly recommend you fit this - the motor costs £100 new so protection seems advisable.

The motor runs at about 80 deg C in normal use but if stressed, e.g. pulling away from a slow spped too often, driving the motor when the wheels are turned, gearing up the drive too highly in the hope of attaining supersonic top speeds the motor temperature can ramp up alarmingly to the point of breakdown of the epoxy glue holding the windings in at about 200 deg C.

Some teams use water cooling as the motor is water proof, some like us use fins to provide air cooling.

Comments

[bmlbytes]

For a different type of speed control, you could add a potentiometer in series with the motor's positive line. This would give the motor a little less power when turned down, and will be better on the motor than "pulsing" the power switch.

[rickharris]

Sorry to disagree - The Potentiometer will absorb energy and release it as heat - this means that at the 70Amps our motor may draw the pot would need to be HUGE. to get rid of the heat. The push switch is simple and easy to use. We may like to a pulse width modulated system in the future to give proportional control.

[annodomini2]

Yup, the switching is necessary, the pot would have to be huge and expensive to handle the current. Also the system would permanently be drawing some power so the battery goes flat faster A Higher frequency electronic PWM system would be an improvement, but obviously more expensive.

[rickharris]

Some, in fact quite a few cars do use Pulse Width Modulation (PWM) to control speed, Mainly because this allows them to have a boost and coast system that is computer controlled so as the speed drops off only enough power is feed in to keep the speed up - thus using less battery overall. It certainly works - BUT so does an intelligent driver and an on off butten is the watch the speed and use it correctly.

[annodomini2]

A driver can't operate at 5-30Khz though

[maxpower49]

can you get a picture of the fins on your motor or if possible a picture of the watter cooling system

[rickharris]

Of all the pictures I have none show the motor heat sink clearly - I will take some tomorrow. We do not use water cooling although many cars do. In general a mist of water frequently is better than soaking it once in a while. The cooling effect is by evaporation not by sitting in a pool of water. A regular squeeze spray gun can easily be modified to put the spray head at the end of a pipe to spray onto the motor. IF you choose to wrap some absorbant material round the motor to hold the water remember IF you run out of water this becomes a good insulation blanket.

[keebmn82]

I might have missed it but what size gears did you use and where did you get them? I have a Jazzy power wheel chair that I'm going to strip down for the motor, speed controls and batteries. The way gas prices are going I'd drive this to town to pick up grain and what nots, I only live a mile out. I have a elec scooter but they don't come w/ a lot of room to carry very much...Appreicate any help. Here's a photo from the Rhoades website

[Yerboogieman]

looks like a truck from a LONG time ago

[rickharris]

The motor runs at 2000 RPM at the shaft. the motor gear has 11 teeth and the driven gear on the wheel 44 teeth. The wheels however are only 12 inches in diameter.

The choice was made from calculations to acheive a top speed at 2000 of about 30 MPH - Actually we seem to ba about 28 MPH peak speed from road tests so the gearing needs to be raised slighthly - 1 or 2 teeth.

Many engineering suppliers will provide a range of gears which they sell to fit to electric motors. have a search in your area because any addresses I give you will be UK based and not a lot of use.

The greenpower site has a number of links to UK suppliers where you can get a feel for the type of product you need to be looking for.

Remember we are running very light! and get about 40 miles from our 2 batteries - If you load it up the range will drop dramatically (that's why ther e are, as yet, few dedicated electric cars on the road)

[keebmn82]

I plan on adding a couple more batteries and solar panels, but all that will have to wait til the snow melts. The bike is actually buried under a snow bank. Thanks so much for your help. Keith