shape of current waveforms measured at shunt

[JRive]

So, I'm expecting the voltage developed across the shunt resistor to be sinusoidal, but I'm having much difficulty measuring these on the STEVAL-ESC001. The shunts on that board are 10mOhms, and perhaps the current through the motors is too low to see anything useful with the scope.

Am I correct in expecting this waveform to be sinusoidal, if the BLDC is a SPMSM? I am running the motor unloaded, and perhaps that is why it is not drawing much current at 10K rpm. I can increase the value of the shunt resistor to see if I can see this waveform.

Does anyone have a current profile for whatever motor they're using they can share so I can see what to expect? from start to constant speed to stop (or ramp down). The motor I'm using at constant speed is drawing less than 250mA, at 8000 RPM.

Thank you

Jorge

[Saurabh SONA]

The voltage across shunt is very low to measure reliably. It is much more effective to measure the current sensing opamp outputs (TP5, TP6 and TP7). The overall gain of the opamp circuit is 4.4 and polarization voltage is around 1.4V DC. The current across the shunt flows only when the corresponding low side switch (MOSFET) is ON. So you will get a PWM signal with sinusoidal envelope centered at 1.4V DC.

One catch is that when the current is very low compared to the maximum measurable current of the signal conditioning circuit, ADC resolution is not optimally utilized and the current envelope shape may not be perfectly sinusoidal due to poor S/N ratio.

Default current sensing configuration for the board enables current sensing up to 37.5A peak (ideally, but real value will be slightly less due to opamp R2R limit). Increasing the shunt resistors value without changing any other component value will proportionally reduce this maximum current sensing range and give much better results when running the motor without load.