Building an electric racing car

Step 4 The motor and drive system

The rules stipulate a 24 volt motor - the supplied motor is a wheel chair motor style without the attached slow speed gearbox. It operates at 20 amps at 2000 RPM.

With the gearing we have used it gives us a top speed at present of about 25 to 30 Mph - Not super fast but when your bum is only 2 inches from the ground!!

At a later date we will raise the gearing ratio slightly to up the top speed to 30 to 40 MPH provided we can do thig without over stressing the motor.

Cycle gears are also an option but may insert a complex system into something that needs reliability.

Comments

[TheMadScientist]

these motors can by aquired at your local flea market/ resale shop for sure! old and new air compressors alike love these things, and they tend to be pretty massive... also, backup generators use these in reverse to generate electricity, which means that they can be taken out and used as an electric motor aswell. a third source would be the obvious washing machine/drier but they're a bit more difficult to get loose...

[rickharris]

Mmm these are purpose made for the job and are 24 volts.

You can get DC motors second hand but they are surprisingly rare these days. About the best source is Ebay and ex golf cart motors or up to 300 watts (claimed) electric scooters.

[rickharris]

Take care you are getting a DC motor of the required voltage and power rating there are a lot of toys (electric scooters for example) out there that use relativly small motors. Our motors are designed to operate on electric wheel chairs although there they have a step down gear box fitted. You need something in the order of 250 watts to be useful. BUT take care about using a generator to act as a motor. It will not be as effectent as a purpose built motor and could be and AC alternator which is useless. Car / lorry starter motors provide plenty of power BUT eat electricity - there is a drag car contest that uses them but after 200 yards the batteries are cooked. If you can find one perhaps a car DYNAMO will do at a push bit I wouldn't expect the range or power we get.

[The_Tom]

I got a motor from an old treadmill is says its a
2.5 hp at 130 VDC
17.2 AMPS
7090 RPM
25C
Class B
Thermal switch: 120 VAC MAX. 1800 VA MAX

Im kind of unsure as to what most of these mean lol and would it be a good candidate for a small go kart like yours?

[omnibot]

I'd say so. RPM looks a little high but gearing it down should take care of that. 2.5hp is a good amount of power for a small buggy.

[The_Tom]

im guessing it meant 2.5hp at 130 dc volts?

[tim127]

im trying to build an electric bike and the battary i want to use says its 10 ah and 24 volts how many amps is it and how well will it work with a 100 watt motor?

[rickharris]

A 10 Amper hour battery will in principle supply 10 amps for 1 hour - or 20 amps for 1/2 an hour or 5 amps for 2 hours - get the idea?

In actual fact it will fall somewhat short of this if you expect it to recharge and have much of a life.

Assuming the motor is 24 volt you may get 3/4 to 1 hour real running out of it. In principle the 100 watt motor will draw 4.amps to produce 100 watts at 24 volts.

100 watts is a little small for powering much other than a very light weight scooter. 200 - 250 would be better and not a lot more expensive.

in this car a 70 Ahr battery gave us about 2 hours running flat out - 30 Mph.

[tim127]

ok thank you, ill get a 250 watt moter and a better battary

[wee_man]

Around how much did this cost and can you tell me what currency its in and also how much did the gokart weight with motor adn everything in execpt the driver cheers

[rickharris]

IN GBP about £500 - this includes safety items like the 4 point harness, electrical cutouts, tip tilt batteries required by the rules of the contest. Weight around 80 -100 pounds 40 to 50 kg two of us can lift it easily - in fact it is often carried, turned on it's side to exit a narrow door by 14 year old students. I think with a little thought you could build a similar running car for under £100 without the motor cost if you try. - Tubular steel is cheaper than aluminium, Ply wood is even cheaper - look at how simple boats are made - look up stitch and tape boat building. Look at the greenpower site there are many hints there . The motor is the difficult part to get cheaply and there is no easy substitute for the wheelchair type motor.

[wee_man]

thanks i was thinking of making something similar to the one you made 24v motor etc etc. but more in the style of a gokart. but weight will be a problem alwell and i was thinking of putting a lawn mower motor in the back with an generater running off it just incase i was a long away from home and ran out of battery

[rickharris]

Mmm - why not just drive from the lawn mower engine? It would take hours to charge the battery once discharged.

Alternative? - and I haven't tried this - you could use a mains generator - there are plenty about and drive a mains motor again plentiful BUT the law will regard this as a petrol powered vehicle.

[wee_man]

but with a lawn mower motor the spinning bit is horizontal whick would be a pain to use. You could use a chainsaw motor but then would it have enough power and speed required?

[rickharris]

A standard type lawnmower puts out 3 to 4 Hp - Not a lot anyway - It would move you about but at no great speed. The drive direction depends on the mower - often vertical but there are ways to overcome that - e.g. Gears - belts. Chainsaw motor - No - Not powerful enough. If you have the mower engine and can get hold of a chainsaw engine and have lots of space to use the vehicle go for a hover craft - Lots of information on the web and great fun. Otherwise have a look at recumbent bikes. -

[wee_man]

i would rather a go-kart feel but i would rather use a eletric motor because petrol costs so much now. but its cool how greenpower is interesting people in the use of eletricty but i reacon i more countrys their needs to be a contest like this

[rickharris]

Of course I agree with you - Organisation costs and take a lot of effort so not many people are prepared to do it. Australia has a similar contest..

Perhaps if low cost - different - transport is what you need have a look at ground hugger An electric assist drive could be added.

You must remember that our car is designed for a race series and wouldn't do very well in normal road / traffic conditions. - if it were that easy every car company in the world would be making them. -

For the 4 hours we run and the 100 miles + we can cover it serves its purpose. In traffic it would be much less distance and would need to be more complex even as a basic go kart.

[DFree]

What about using a car alternator on the wheel in the back that isn't attached to the motor. It would charge the battery while you drive. I don't know if it would put out enough power to keep the battery charged since its always running on electricity but it would certainly increase the amount of time you have.

[wee_man]

(addingto joeofloath's comment)
The reason why no alternator is 100% efficient is because there is wasted energy.  In the motor (for say) 50w of energy is applied.  Some of that will be turned into heat and sound and finally the rest into kinetic energy.  Then theirs also friction with the tires on the ground. heat sound in the alternator too.

[quote]
When energy is transferred,
some of the energy turns into forms we don't want.
This energy is called wasted energy.
Wasted energy takes the form of heat and sometimes sound or light.
[/quote source http://www.gcsescience.com/pen20-energy-efficiency.htm]

[joeofloath]

It won't. No alternator is 100% efficient, so it will take (say) 50w to turn over, but only generate 10w. You can't generate power from no where anyway. You Use the battery power to move the car, slowing it down with an alternator is pointless.

[snipe8]

you can make your go cart be faster by. putting the bigger gear on the motor and a smaller one on the wheel

[rickharris]

true in theory - In reality the car wouldn't go at all because the motor wouldn't have enough power to drive the wheels with such a high gearing. IF it were possible the car would be capable of exceeding 680 miles an hour!

[snipe8]

lol. thats true

[kjjohn]

is there an equation or something for figuring out how fast it will go with a certain gear ratio? I am designing an electric motorcycle and I would like to know.

[rickharris]

Measure the wheel diameter - Work out the gear ration by counting the teeth on the gears - Gear teeth A divided by gear teeth B = gear ratio.

Once you know how fast the wheel is going in Revs per min and how far it is round you can work out how far the wheel travels in a min and so multiply by 60 for MPH.

HOWEVER your motor will have an ideal speed to run at for maximum power. For this you need a data sheet for the motor. e.g. mine was 2000 RPM at this speed it draws 20 amps. and produces maximum torque. So the gear ratios were chosen as a compromise between acceleration and top speed verses battery life.

A spread sheet would help you out.

[al9595]

only one wheel is moterized doesn't that mean it will go in circles?

[rickharris]

All of the cars only drive 1 wheel so I guess the answer is NO it makes little difference. Thi prevents one wheel trying to go faster than the other round corners. Look up car differential to see the principle. We avoid the weight of a copmlex gear system by driving 1 wheel.

[al9595]

ok i get it

[carbon12 based]

Hi, Did you do any calculations for the frame or just did a qualitative analysis. By looking at the pictures, you have the motor driving "one wheel" am I right? How does this impact the steering and handling? Would it steer to one side? If so, maybe getting some camber and toe in on the rear probably would fix that. You would loose some power due to the increased friction however that maybe worth it if you are constantly having to steer it back in a straight line. I know for simplicity reasons a Diff is not used and not enough power for a fixed 2 wheel (Drift action!). Great design wish i had this when I was in school, you must be like god to the students.

[rickharris]

The basic chassis came as a kit - It was developed by experimentation and is far stronger than it needs to be although it is very light compared to the weight of the motor and batteries. Driving one wheel makes little difference to handling and removes tyre wear. Driving two wheels is also more complicated - in general more complicated meanes heaver something we try to avoid.

[maxpower49]

how big is the moter not horse power but the dimensions

[rickharris]

About 173 mm long by 100 mm diameter

[skaltura]

Look at brushless systems and LiPo. However the amps you are pushing are very far from even near the limits of NiMh. I haven't worked on this scale of systems, infact 1/10 scale cars, but on these, more power is being used than this on the top end. Older technology is limited on them to under 300watts however. But with modern technology, on sane bucks, you can already achieve 4kW and driving time of hours (as you very very rarely use even nearly the max). Under 1,000USD you can buy an 1/10 RC Electric Car with all you need, which easily outruns a nitro car, top speeds of over 100km/h, or around 65mph. And that's seriously scaled down stuff, weighting under 1kg total for the electrics, imagine 2-2.5kg RC car cruising at 65mph ;) You need scaled up for this, or either seriously short gearing (like 1:13 final drive), and preferrably multiple small motors, if you use what's found for RC cars. Look at Castle Creations, they are releasing new ESC, which can pull through continuously multiple kWs, and motors to go with it :) Also, those speaking of generators to use as motor --> Change the timing, it's all in the timing with brushed motors, however, no matter what kind of brushed motor, it's simply inefficient. Unfortunately, it seems you are restricted on the batteries & motor :( Fortunately, there's A LOT of things you can do besides these to increase overall efficiency and speed :)

[jakesllama]

well i soppose it would be good for turning right... sweet video by the way did you enter the contest? i would love to build an electric car but i only have wood scraps... tah

[rickharris]

everything is possible - the car cost about $100 to build. It could have been cheaper and a LOT more expensive.

[TheMadScientist]

rick, may I ask if you know how to convert a belt drive type electric motor to a chain? the belt mechanism is seemingly friction welded to the shaft, but I have a strong dislike of belt drives...

[rickharris]

Your best bet is to drill the Pulley and a suitable gear and bolt them together. A lot of people do this to make the input gear on a standard 3 speed gear box bigger. At the power ranges we use this works well 4 or 6 1/2 - 3/8 bolts depending on the size of the gear.

[jakesllama]

sorry i ment what contest did you enter?

[rickharris]

The contest is run for British Schools. The cars are built by and raced by 11 to 16 year old students (12 of them) over a 4 hour race. The aim is to go the furthest in 4 hours on the power in 4 car batteries. This is a fair challenge as all the cars use the same batteries and motors. Everything else is up to the designers. Top cars do 40 to 50 MPH and will travel around 110 to 130 miles in 4 hours. It was started as a way to get young people interested in engineering We were number 48 in the video the all aluminium car. We had some problems - a puncture which lost us a lot of time and finished 64th - Still for the first year this was very good. Next year top 10.

[maroadrunner]

There is a similar contest in the United States. It is called Electrathon. People build electric cars and race them on 2x12v batteries over one hour. The goal is like above to go the farthest in that amount of time. As far as I can tell it is very similar as the best cars go 40 to 50 mph too.