I got this small drone motor yesterday, so I like to show you a few test results with its use as a tachogenerator. I think the right term for such a devise in this context is:
Three-phase 12-pole synchronous generator with permanent magnets.
I placed it in my test-set up like you see here:
Then I made the DC motor turn this drone motor with a near constant speed and measured the voltages from this drone motor. One phase is connected to a center voltage between the supplies of the Arduino Nano and then the two other voltages are measured on two analog inputs. The ADC clock is 1 MHz and the voltages are read every 0.2 ms. I got some software in the Arduino to record 5x140 integer values and afterwards send it to the PC with Excel Datastreamer.
This is a plot when the motor runs 1830 rpm forward:
This is a plot, when the motor runs 1120 rpm in reverse:
Disregard the yellow and dark blue plots. They are from the driving DC motor.
The interesting value is the gray plot that is the calculated value for speed. It is derived from this code:
BLDCV1 = analogRead(AnalogBLDCV1Pin) - zeroBLDCV1;
BLDCV2 = analogRead(AnalogBLDCV2Pin) - zeroBLDCV2;
V1V2square = (long) BLDCV1*(BLDCV1-BLDCV2) + (long) BLDCV2*BLDCV2;
BLDCRes = broendegaard4_isqrt(V1V2square); // Fast integer square root function
As you see, this gray line is not that constant, as I had hoped for. The peak to peak voltage of the ripple is about 10 % of the DC value. It is a bit better than the brushed DC motor with five rotor poles. What is better is the ripple frequency, that is 36 times the rotational frequency. It was 10 times with the brushed DC motor used as tachogenerator.
I think the reason for the ripple is, that the voltage curve generated by this drone motor is no perfect sine wave curve. When I study the curve, the top values are more flat than a sine wave and near zero crossing, the slope of the curve is less than should be expected from a sine wave. The time difference between measuring the two voltages are about 15 us, and it may also influence result.