Electrical – Brushed DC motor as generator, characterising the motor


I've read through the several topics here concerning using a DC motor as a generator but I've failed to answer my question so, apologies, I'm going to ask again.

I need to use a small DC motor as a wind-powered generator in association with an energy recovery chip (TI BQ25505). This chip will only successfully recover energy if the input voltage to the chip is ~400 mV or greater. I have tried using several DC motors as generators but the only one I have found which produces a terminal voltage greater than 400 mV when the breeze is just sufficient to overcome the back-EMF of the generator took 4 weeks to arrive from China and was provided without a specification.

Hence I would like to characterise the generator in order to purchase an equivalent locally. Here is what I know so far:

  • Winding resistance 75 Ohms.
  • Operating speed at "minimum breeze" 12 to 15 Hz.
  • Voltage produced across 1 kOhm resistor at "minimum breeze" 650 mV DC.

Is there a way that I can convert this knowledge into (or perform more measurements in order to arrive at) the parameters which DC motors seem to be specified in, i.e. terminal voltage, output power, max RPM and max torque?

Best Answer

"100-6000 rounds" probably means 100-6000 revolutions per minute. Used as a generator, the motor will probably produce about 5.5/6000 = 0.0009 volts per RPM. I assume speed in Hz is equivalent to revolutions per second. The voltage produced should be 0.0009 X 60 = 0.055 V or 55mv per RPS. At 12 revolutions per second, you could expect about 55 x 12 = 660 mV.

If the motor/generator produces 5.5 V and 100 mA at 6000 RPM that would be a maximum power of 100 X 5.5 = 550 milliwatts.

You can calculate torque from Power(W) = T(Nm) X RPM X 2Pi/60