Alan B said:
Phase currents are AC so compensating for DC offset should be possible.
I thought about that one. It is possible to run a low pass filter and get the DC offset. Another possibility
would be to run a calibration routine where you make sure all FET's are off (so no phase current)
and measure the DC offset.But I realised going the 3 current sensor route actually has the additional
advantage of also eliminating high frequency noise common in all 3 sensors.
Imagine, the current sensors are in the motor phase lines. all the PWM signals are generated
by the same source meaning that there will be instances where all high side FETs and at other times
when all low side FETs are on. The voltage on the current carrying path of the sensors goes up and
down by about 100 V (I'm planning to run 100V battery). The isolation from current input to voltage
output is not specified but is not going to be an infinite amount of dB. Another sources of noise
which 3 sensors can compensate for is ripple on the 5V supply.
Something else which speaks against using just 2 sensors it that in this case the 3rd motor phase current
is compute as the negative of both other currents combined. So this computed value is going to carry
the combined error of both sensors. While with 3 sensors part of the error is common to all 3 and will
be cancelled out in the 30F.
If you're not versed in analog design it might all sound complicated and stuff but differential signal
processing is very common in integrated analog circuits...
I just ran the motor with 3 current sensors, results are excellent. The 30F will have a menu option
where you can chose 2 or 3 current sensors. For the case of 2, there'll be a small potentiometer
in the schematic that will need to be calibrated. The main difference between 2 and 3 sensors ?
2 sensors will result in a bit less accurate control when motor rpm and currents are low (so low
motor speed and low throttle). Any other case there won't be much of a noticable difference (maybe
some small amount of torque ripple in the 2 sensor setup, but there are coefficients that you can adjust
to reduce this)
Engineering pride though requires me to strongly recommend 3 current sensors