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I am measuring the phase voltage of a BLDC motor as shown in the figure below. As the motor doesn't have neutral point, a virtual neutral point(VN) is created by the resistor network and three-phase voltage are measured with oscilloscope Probe A, Probe B and Probe C. First let me know if my measurement setup is correct!!!

schematic

simulate this circuit – Schematic created using CircuitLab

I want to understand why there is a spike in the back EMF as shown in the figure below. As described in theory the Back EMF of floating phase ant any instant should ramp up or down but why there is a huge spike? How to reduce the spike? Phase voltage at 4200 rpm

My second question is at lower rpm why there are so much noise in back EMF? Is it actually happening to the back EMF or it is just an oscilloscope trigger visualisation.? enter image description here

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  • \$\begingroup\$ Do you have flyback diodes installed? \$\endgroup\$ – testname123 May 17 '18 at 15:47
  • \$\begingroup\$ yes, all 6 mosfets have 6 antiparallel diodes. \$\endgroup\$ – litun bls May 17 '18 at 17:03
  • \$\begingroup\$ Well, if I had to guess, then, I would say that the noise you're seeing is real and that the issue may be that the noise is coupling into your ADC readings. If this happens, it's feasible that the ADC thinks that the motor is in a position that it really isn't after a few bogus readings. \$\endgroup\$ – testname123 May 17 '18 at 19:19
  • \$\begingroup\$ This would manifest in the motor being commutated before it wants to, and causing that kickback. I say that because the unusual pulses are at least synced up in such a way that two of the 3 phases has an anomoly at the same time...This makes me think that the FW is commutating, but something is driving it to commutate at the wrong time, which sounds exactly like something a banged up ADC frontend circuit could do. \$\endgroup\$ – testname123 May 17 '18 at 19:19
  • \$\begingroup\$ Also, is there a reason your unipolar drive forms are modulating GND with a common VBUS connection? Why are you not holding down GND and modulating VBUS? And, are you using as a timing parameter? zero crossing and then a hold off, flux integration, etc. ? \$\endgroup\$ – testname123 May 17 '18 at 19:22

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