Over-Volting Electric motors

[SystemParanoia]

Kids have told me that they are going to be building electric race cars from a block of balsa.
they will all get a small DC motor and are allowed to use only a single 9v battery.

Teacher says they will use an elastic band to drive the car's but i feel this will be lossy and inefficient ( ive already ripped the axle out of one of my old scalextric cars to ensure drive efficiency )

my question is this..

What will yield better results from the motor ( that i do not yet have in my possession )


1. keep it simple and just Overvolt the bugger by building a voltage doubler, fit a switch, and hope the motor lasts long enough to get me the kids a trophy.

2. Use the ESC from my old RC car ( brushed ) and hope the increased torque from the PWM operation will earn the win despite much more sensible voltages ( 7.2 ) this method will allow me to use a microbit, a raspberry pi or my old radio gear to drive the motor

3. C02 rocket motor!

4. Shrug my shoulders and let the kids figure it out whilst pointing to my power tools.

[Zad]

If the teacher says they are using elastic bands, then I imagine that any other power source will be disqualified. Not least to stop parents dominating the thing (which they will anyway to be honest).

[SystemParanoia]

Zad said:

If the teacher says they are using elastic bands, then I imagine that any other power source will be disqualified. Not least to stop parents dominating the thing (which they will anyway to be honest).

elastic bands are just for power transfer.

the setup of the contest seems a little strange to be fair, its a flat run along the playground with potential obstacles in the way.
no guidence is allowed and any car the goes off-course is allowed to re-run until it completes.

I can see parents with access to industrial manufacturing equipment being at a distinct advantage over the rest to be fair.

[Goaty Bill 2]

The world is your Oyster; Pendle Slot Racing

I used these guys quite a few years ago. They were very good, lots of interesting power plants

I don't think a rubber band is going to work very well. You want something without much/any stretch in it.
Think in terms of the old overhead shaft driven power distribution systems you would have seen in machine shops 70+ years ago.

[SystemParanoia]

Goaty Bill 2 said:

The world is your Oyster; Pendle Slot Racing

I used these guys quite a few years ago. They were very good, lots of interesting power plants

I don't think a rubber band is going to work very well. You want something without much/any stretch in it.
Think in terms of the old overhead shaft driven power distribution systems you would have seen in machine shops 70+ years ago.

im loving the sidewinder axles



the axle pinion and spur gear i've liberated are a more traditional setup like this

[Murph7355]

(4). Teaching your kids it's not al about the winning but the learning.

Stick to the rules too. I remember my teachers being switched on to people playing the smart arse.

[Penelope Stopit]

Use a 6 Volt Motor and directly power it with the 9 Volt battery, you will save on weight by doing it this way

[Mr2Mike]

SystemParanoia said:

1. keep it simple and just Overvolt the bugger by building a voltage doubler, fit a switch, and hope the motor lasts long enough to get me the kids a trophy.

Depends on the current draw and battery type. If it's an alkaline PP3 type battery, they can't deliver much current since they have quite high internal resistance (which increases with increasing current draw). Doubling the voltage means doubling the current from the battery, at some point the terminal voltage would drop so much that this wouldn't yield much improvement.

You can lower internal resistance though, by heating the battery before use

SystemParanoia said:

2. Use the ESC from my old RC car ( brushed ) and hope the increased torque from the PWM operation will earn the win despite much more sensible voltages ( 7.2 ) this method will allow me to use a microbit, a raspberry pi or my old radio gear to drive the motor

Using PWM would not increase the torque compared a direct connection to a battery, in fact losses through the controller would reduce it. PWM provides an improvement over a simple resistive controller when you want to drive a motor more slowly, but seems like you want to be driving it flat out.

SystemParanoia said:

3. C02 rocket motor!

They wouldn't like you releasing all that CO2. Electric is the future apparently

SystemParanoia said:

4. Shrug my shoulders and let the kids figure it out whilst pointing to my power tools.

Providing hints, and asking them questions that makes them think is the best way IMO.

Rather than outright speed it sounds like you might need to concentrate on obstacle avoidance. Pointless being the fastest car that leaves the course on the first obstacle.

Edited by Mr2Mike on Sunday 16th July 13:47

[SystemParanoia]

thanks for the pointers guys,

Due to lack of information, I have decided to give them:

the drive axles, pinion and spur gears from a pair of scalextrix cars
tape
hammer
nails
mini glue gun
A spare 6-9v motor and random gears torn from an old toy and 9v battery for testing purposes
rubber bands
fist full of mac'd straws
note pad full of rough sketches of different ways to connect the axles and drive gears to the cars that we drew up over the course of the day.

they collect their bits and bobs from school tomorrow, so Ill have to see what we end up with, and hopefully more clarification on the rules and what kind of races there will be!

[buggalugs]

Could you use the rubber band for energy storage - have it wound up before the start to get a boost off the line. Then you could get away with longer gearing to get some serious speed up.

[SystemParanoia]

You mean go 4wd?

I like that thought, especially combining that with having the gear ratio set in such a way that will stall the motor whilst stationary

[Goaty Bill 2]

SystemParanoia said:

Goaty Bill 2 said:

The world is your Oyster; Pendle Slot Racing

I used these guys quite a few years ago. They were very good, lots of interesting power plants

I don't think a rubber band is going to work very well. You want something without much/any stretch in it.
Think in terms of the old overhead shaft driven power distribution systems you would have seen in machine shops 70+ years ago.

im loving the sidewinder axles



the axle pinion and spur gear i've liberated are a more traditional setup like this

Do not be fooled!

The traditional pinion and spur always proved superior in my experience.
Owner of 20+ FLY slot cars, including some heavily modified monsters - long since retired.

[buggalugs]

SystemParanoia said:

You mean go 4wd?

I like that thought, especially combining that with having the gear ratio set in such a way that will stall the motor whilst stationary

I actually hadn't thought of 4wd but yeah that's a good shout you could have a pre-would elastic band on the rears and the motor on the fronts? Might help with stability!

[98elise]

buggalugs said:

Could you use the rubber band for energy storage - have it wound up before the start to get a boost off the line. Then you could get away with longer gearing to get some serious speed up.

Agreed. I would be looking at the elastic bands as an energy store.

[Wait Here Until Green Light Shows]

If the drive has to be a rubber band, then if it was me I'd recommend cutting a slot/groove all the way round the tyre and have the rubber band drive the tyre directly from the motor. If they aren't allowed to cut the tyres, then just drive it directly on the tyre. If anything, this would probably give more grip and hopefully the v-pulley on the motor will keep the band from slipping off. If you do drive directly onto one tyre, try and find another smaller elastic band for the other tyre to keep the circumferences the same (otherwise it may steer to one side)
The most important thing is to keep the design simple...simple = reliable.
Definitely overdrive the motor. A 3v motor will run for long enough to win the race on a 9v battery as long as you don't stall it.

[SystemParanoia]

the drive doest have to be rubber band... the teachers intention is to use the rubber band like a bike chain to get power from the motor to the axle.

looking at the comments above it seems having the motor with a silly gear ratio drive the front axle

i.e

and have a twisted rubber band provide the initial oomph needed to let the motor tun said silly gear

i.e https://www.youtube.com/watch?v=qrmJqEmlsaM

the jury is out on whether it is a good idea to drive the motor with more than 9v as the provided battery may not be up-to the challenge

also, the fact that there will be obstacles on the track is i variable i have no idea how to manage in the time i have available.
with more time, i'd have built a raspberry pi robot that guided itself using the opencv library

Edited by SystemParanoia on Monday 17th July 15:18

[boyse7en]

SystemParanoia said:

the drive doest have to be rubber band... the teachers intention is to use the rubber band like a bike chain to get power from the motor to the axle.

looking at the comments above it seems having the motor with a silly gear ratio drive the front axle

i.e

and have a twisted rubber band provide the initial oomph needed to let the motor tun said silly gear

i.e https://www.youtube.com/watch?v=qrmJqEmlsaM

the jury is out on whether it is a good idea to drive the motor with more than 9v as the provided battery may not be up-to the challenge

also, the fact that there will be obstacles on the track is i variable i have no idea how to manage in the time i have available.
with more time, i'd have built a raspberry pi robot that guided itself using the opencv library

Edited by SystemParanoia on Monday 17th July 15:18

As there will be no external method of control to avoid obstacles, the only obstacles I can think of is some sort of ramp or maybe a change in surface (water, sand?). Either of those are easier to navigate with more power or speed

[Buzz84]

One problem I can see with a wound up rubber band boost feature is that it would have to be wound up with exactly the right amount of turns to last at least the distance of the track,

because as soon as it has completely unwound, it would start to wind back up again in the opposite direction, if it got to this point before the finish line it would slow the car down or even stop it completely.

[Mr2Mike]

Buzz84 said:

One problem I can see with a wound up rubber band boost feature is that it would have to be wound up with exactly the right amount of turns to last at least the distance of the track,

because as soon as it has completely unwound, it would start to wind back up again in the opposite direction, if it got to this point before the finish line it would slow the car down or even stop it completely.

That can easily be prevented by not permanently fixing the band to the axle, just wrap it around the axle for the required number of turns and when the axle reaches the last turn of rubber band it disconnects itself.

[SystemParanoia]

update.

Kids arent allowed to bring them home so im reduced to having a glorified Consultancy role.
also the teacher has moved the goalposts and they now only have x2 Double AA battery packs giving 6 wonderful volts.

Ive given them a lesson a series vs parallel wiring telling them volts are all that matter here. more volts = more rpm etc
Ive also demonstrated to them what i mean about elastic band drive by using some old lego and the same construction tools that they have at school... i.e nothing! + pva glue





This had major traction issues that may have discouraged them about the idea entirely
The obstacles will be disposable water cups, and gentle humps.

Im absolutely gutted that i wont be able to directly help on this one