# Optimum resistance for heat

I'm a mechanical engineer getting confused with electrical engineering...
I've made an electrical generator by spinning the shaft of a DC motor by connecting it to my bicycle.

I've tested it with a multimeter, and on 1 kOhm, 2kOhm, 3kOhm, and 4kOhm resistors I'm producing about 70 Volts. Current is varying accordingly with V=IR. Therefore power P=IV is increasing as resistance decreases.

My question is how do I find the optimum resistance to produce the most electric power? By this calculation, power would increase as resistance decreases. However there has to be some limit to this, as I was only putting in about 200 W into the bicycle. According to the equations a 10 ohm resistor would lead to 7 amps and 490 W, but obviously I'm not producing more power than I'm inputting.

When the source resistance is equal to the load resistance, that's when maximum power transfer occurs.

The source is a DC motor, rated at 24 V and 16 amps. Therefore the resistance is 1.5 ohms.
But this is the motor specs, there are no specs for spinning the shaft and turning it into a generator. We are producing 70 V, so a resistance of 1.5 ohms result in 3267 W. This is not possible as I'm inputting roughly 100-250 W.

Thanks.

Averagesupernova
Gold Member
You can't go by the 24 volt/16 amp rating. This means that the motor is designed to run at 24 volts and draw 16 amps when under its maximum rated load. When running idle the motor will draw much less current. Keep reducing the resistance until the calculated power starts to drop. By calculated power I mean calculate it from measurements of voltage and current. What happens is the voltage will drop faster than the current increases. This is when the load impedance is equal to the source impedance as waht has mentioned. You will need some serious high wattage resistors to do this. I would recommend using sealed beam headlights instead. I've used these for testing power supplies before. You can wire them in various combinations to get different loads.

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