easy to install speed control

Your motor is probably pretty beefy, yes? If so maybe you want a beefy controller. By "speed controller for the switch" do you mean that big yellow lever?

It's typical to have a variable throttle which uses a 0-5000 ohm variable resistor, that connects to a controller that varies the speed to the motor.

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The yellow lever is the gear shift for the five speed transaxle.
I know I need a variable resistor or something like that, but I just can't seem to find one. Do you have any ideas on where I can find one?

Hi,

Most folks (99.9%) don't use a variable resistor to directly control the voltage to the motor. Instead they use a controller (Alltrax is most popular, Kelly is newer and cheaper - but unknown). The controller (usually) takes an input from a variable resistor (say 0 to 5 k-ohm - but at a much lower voltage). The controller then varies the power to the motor by controlling the duty cycle of a square wave which oscillates at about 15kHz.

I think you have a 24V system, right? What amp draw do you expect? You could also look for golf cart controllers - which might be cheaper than Alltrax.

It is possible to use a variable resistor directly - but because of the high amp draw it has to be large (and it's also inefficient because it wastes energy as heat). I've seen some home brew continually variable resistors - I've also seen some home brew 2 or 3 position systems. I'll see if I can dig up something if you want to go that route.

Yes it is a 24 volt system, and the motor is supposed to draw around 50-75 amps, though I think it can handle more.
I just need something that will be easy to hook up. I thought of using a golf cart controller, but I was allways afraid of frying it when I first hook it up. I did try to use a controller from an electric scooter with 24 volt system, but the supplier refused to label the connections showing me which wire went where. Instead, he gave me a wireing diagram that could barely be read. Not to mention that it took nearly a month to get the thing.
Anyways, thanks for the help.

A golf cart controller isn't really that hard to install, and is probably your best bet. The connections are labeled: motor(M+, M-), Battery(B+, B-), and throttle (there are three or four: two for the throttle, one to power the controller, and sometimes one to limit the speed in reverse) you will need an appropriate pot, usually 0-5k ohm. most important is to get the battery polarity right, but if you use color coded wire (red for positive, Black for negative), this is a no-brainer. if you have trouble, there are wiring diagrams available in pdf format online for most brands.

Okay, its easy to install, but do they even make a 24 volt golf cart controller? I can get the pot from radio shack but I have to search for the controller. Or would a 36 volt controller also work?

Yes, they make 24 volt golf cart controllers. the curtis 1204 is one. alltrax makes controllers from 12-72 volts.check out this page: http://cloudelectric.com/controllers?b=1 you could also use a 36 volt controller, but you would need to get a third battery, and find out if the motor you have can run at the higher voltage without getting too hot.

Thanks for the link. I think I found just what I was looking for.