I have a dc motor with the following specifications:
Operating Temperature: -10°C ~ +60°C
Rated Voltage: 6.0VDC
Rated Load: 10 g*cm
No-load Current: 70 mA max
No-load Speed: 9100 ±1800 rpm
Loaded Current: 250 mA max
Loaded Speed: 4500 ±1500 rpm
Starting Torque: 20 g*cm
Starting Voltage: 2.0
Stall Current: 500mA max
Body Size: 27.5mm x 20mm x 15mm
Shaft Size: 8mm x 2mm diameter
Weight: 17.5 grams
Now, suppose I wanted to use it as a generator. How fast (in RPM's) would this motor need to rotate in order to obtain the rated voltage? What current can I expect at this speed?
Best Answer
A typical motor will behave as an ideal motor in series with a certain amount of inductance and resistance. At all times, the rotational speed of a motor will be a certain multiple of the applied voltage, and the torque will be a multiple of the current flowing through it. These relationships are bidirectional, so whether something acts like a motor or a generator will depend upon whether relative directions of the applied voltage and the current flow.
Based on the above description, I would estimate that your the voltage on the ideal motor will be roughly 1 volt per 1500rpm (look at no-load speed divided by rated voltage). I'm not sure whether the stall current is measured at 2 volts or 6 volts, but the short-circuit current from a motor spinning at a given speed will be roughly equal to the stall current of a motor driven at the open-circuit voltage that would correspond with that speed. To get maximum power output, you should draw less than half of the short-circuit current,.