Current limiting a starter motor

Question

I'm planning to try to build an electric go-kart. My idea was to use a 12V starter motor from a car to power it, since used ones are fairly cheap and easy to get. (I already have 2 of them to experiment with).

I've read that starter motors aren't a good choice because they aren't meant to run continuously and will get too hot. But I was wondering if I limited the current, maybe with external resistance connected in series with the motor, will that help to prevent the motor from overheating?

Starter motors normally draw a huge amount of current in order to have enough power to crank an engine, which needs a lot of force because of the engine compression. But I figure I wouldn't need to run the motor at full power just to drive a go-kart.

Could limiting the current help with this?

I thought of several different ways, including using heater elements or even just a long coil of wire for enough resistance. Since starter motors draw about 100 amps when cranking an engine, I figured if I limited the current down to maybe 40 amps, that might work to allow the motor to run longer.

How can I limit the current to a starter motor to 40Amps?

I need a cheap, simple way to do this if possible. A PWM motor controller isn't really option because it's hard to find ones that can handle enough current, and they seem to be expensive.

Answer 1

This should be a comment, but I can't create comments yet. A starter motor can afford to run at 100 A because it does so for a very brief time. I don't think limiting it to 40 A is going to allow you to run it continuously. As an estimation, what you've done is decrease heating power by 4 roughly and this allows you to run it 4 times longer. It is not as simple as that but it is a ballpark.

What you should do is look for an engine with a nominal current at least as high as the one you need.

Other than that, a resistor is a very inefficient way to limit current and only affordable for a brief time (i.e. starting).