Originally Posted by
howardcano
The current decreases with increasing motor speed because the back EMF increases. (The back EMF of a motor is given by its KV constant times its rotational speed.) This happens even if there is no inductance. Likewise, the efficiency peaks at a higher speed than the power output even with no inductance. (You can see this effect with some simple calculations, but I won't give an example here so as to not clutter up the thread.)
Your calculations didn't include the internal resistance of the motor windings. The resistance in the motor leads must be added to this. The motor in the example dyno results you gave has an internal resistance of about 75 milliohms (8V divided by about 107A at stall, extrapolated from the plot). In this case, decreasing the lead resistance from 1 milliohm to 0.25 milliohm only gives a 1% change in total resistance.
Also, your calculations are for input power, not motor output power.
Thank you for your posts! It's obvious we both enjoy the technical aspect of our hobby. Maybe most guys aren't interested, but for us, its the FUN part!
If the back EMF is not from the inductance of the motor winding, what physical phenomena is it from?
Correct, my calculations did not include any impedance (complex resistance) in the motor windings. I treated it as a constant as there is not any change, unless one opens up the motor and rewinds it like we used to do back in the day. I just extrapolated from the posted dyno data for the examples.
Yes, I calculated the input power to the motor. I presumed that the relatively small changes would have minimal impact on the motor efficiency. More power in would equal more power out.
No problem, it comes pretty naturally. At least it pays the bills.
Cheers