Electronic – 12A current from 24V power source, are there resistors which can handle the wattage

dc motorheat-protectionresistors

I'm planning on using a 500 W DC motor powered by 2 x 12 V batteries in series. But I want to reduce the power even more by adding 2 Ohm resistance so the power will be 288 W instead.

However, adding a small 2 Ohm resistor will ignite it instantaneously and burn down my house in the process (Thanks to Murphy).

So I was wondering:

  • Is there a resistor which can handle those 288 W?
  • Is there a potentiometer which can aswell?
  • If no, what would be the right way to go about this? Buy a ~300 W motor instead?

Background

The motor will power a bicycle and I'm having troubles finding a suitable DC motor for this purpose. The only one I found was this 500 W DC scooter motor.

Best Answer

Is there a potentiometer which can aswell?

That's called a rheostat (basically a power potentiometer for the purpose you described) -- but I'd recommend against it, since they're pricey.

If no, what would be the right way to go about this? Buy a ~300 W motor instead?

Easy -- use pulse-width modulation by using a MOSFET (and freewheeling diode) to apply 24V to the motor some fraction D of the time. Vary D and you will vary voltage to the motor. (Just watch the motor current.)

The MOSFET should be rated at least 40V and a low-enough on-resistance that you don't have to worry about overheating at your maximum current. Here's a BUK9509-40B which is rated at 40V 75A 9milliohm max at 5V Vgs.

(For some more info about the basics of driving MOSFETs from microcontrollers, read this blog entry of mine.)

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