40A motor speed control

[fizz126]

I've got a little class project where we're trying to control the speed of a small electric car. A kiddie car or such like
I've measured the total current that the motors use to be 40A+ under extreme load, but normal running the current is around 10A - 15A.

I guess the easiest way to do this will be with a large variable resistor, do variable resistors capable of doing this exsist and is it practical?

I've been looking online for Speed controllers that can handle this current but i'm looking at a cost of nearly £100 for some of them which we can't possibly justify.

Can anybody point us in a cheap/ efficient direction to acheving speed control either by a project that we could build, or a unit we could purchase in the UK?

I have a basic knowlegde of electronics and understand the principles of PWM speed control though actually designing a circuit with suitable power transistors and timing is well beyond me at the moment - i have forgotten every thingfrom college.

I was eventually hoping to expand the proect by "plugging in" an old scalextric control or something ( i could push the boat out to radio control) to adjust the speed. I assume this will basically be a case of removing the pot of the motor speed control and wiring in the the new controller in.

Thanks
Robert

[garfild]

40A Electronic Speed Controller

http://norbique.rchomepage.com/esc/schematic.htm


PWM controller

http://www.uoguelph.ca/~antoon/circ/pwm555.html

use 2 X BUZ11 in parallel for 40A motor.

[fizz126]

Thanks very much.

I'll have a go at building the circuits

[Meir]

Hi, I've joined recently; I'm an electronics enthusiast and have built many circuits mostly from EPE. However my theory isn't that great!
I've seen the Elector circuit of a 12V DC motor speed controller using a 555 timer ic. I would like to ask whether I could use IRF830 or IRF840b MOSFETS instead.
My need is for a 24V DC motor used in an orthopaedic armchair to provide back massage, so the load would not be anywhere near 40A. I would say 5A max at 24V.
The existing (obsolete) control circuit uses a PIC. I have replaced all peripheral transistors and the circuit still doesn't work. I'm now convinced there's a problem with the pic itself. It has a handheld controller with buttons for increase/decrease speed.
Anyway I could try the 555 circuit (maybe it would do for this application) and leave the pot set at a reasonable setting for back massage!
Any advice would be much appreciated.

[piratepaul]

You don't say the voltage of the car... Off the top of my head I am thinking the controls on an electric cooker or something like that.

[Meir]

It's not a car. The working voltage of the orthopaedic armchair is 24VDC derived from the mains. As I say, the load of the back massage motor is 5A max, probably around 3 or 4 Amps.

[piratepaul]

The thread is about an electric car. Google 50 amp potentiometer there are loads of them and cheap.

[Meir]

OK sorry.

[piratepaul]

I spect you could use MOSFETs, I spect it would be more efficient than a pot or variable resistor, is efficacy a problem? ( chair sub thread).
You could switch between different voltages using transistors and some sort of digi control.