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TMS320F28054: PMSM electrical angle at encoder index pulse

Part Number: TMS320F28054


It seems there is a standard way that industrial manufacturer's of PMSM follow when aligning incremental encoders on motors. They align the encoder module to the motor so that the index pulse and U-hall falling edge aligns with the U-W phase back EMF going negative to positive through 0V.   This positions the index pulse (and U-hall falling edge) at an InstaSpin electrical angle that is not zero.

For example, when you align the motor to its U pole as done in the Labs (U+ and V-), and equate this to electrical zero, what I see is you have to rotate the motor 210° electrical to get the index pulse.

After aligning the motor I would like to use the index pulse as a check that the alignment was successful.   So if I know the index pulse should always occur at 210° electrical and it doesn’t then there is a problem.

My question is based on the method I described for positioning increment encoders on PMS motors, what is the theoretical angle at the index and why? 

Iv'e empirically measured 210° electrical and theoretically think that the 210° electrical is because encoder alignment (aligned to UW back emf going negative to positive) is shifted 300° from U-phase alignment, minus the back emf vector shift from d-azis rotor flux alignment of 90°, equals 210°.    


  • Often, "index " of an incremental rotary encoder happens once every mechanical turn, it's used when there is the need of an absolute reference, like position control. Motor used the electrical angle to implement vector control. Each motor has unique electrical angle related to index that should be fixed if the encoder was not changed on the motor. Maybe, the final value of the angle of index has a small error once every running.
  • Hi Yanning,

    This response does not answer my question.

    My question boils down to the following. When InstaSpin aligns a motor (Lab 12b) it is aligning the motor on its U-pole and this is assigned electrical zero degrees. Correct me if I am wrong. Now if an encoder is positioned such that the index occurs at UW back emf going negative to positive, what InstaSpin electrical angle is the index?

    This question relates to the timing (position) of the U-pole vs. BEMF UW.

  • InstaSPIN-FOC aligns the rotor to a predefined position, usually is the zero position which the electric angle is Zero. Often, we assume the U phase is alpha-axis, so the rotor position will be aligned to U-Phase, the rotor flux is aligned to the stator flux after applying the constant current vector to the stator.
    The electrical angle of index is dependent with the installation of the encoder of motor , every motor has its fixed electrical angle for the index of its encoder.

  • Hi Yanming,

    Yes, I agree the electrical angle of the the index is dependent on the installation of the encoder. But I've explained in my posts what is a standard method of installing the encoder and aligning the index to the UW back emf of the motor going negative to positive. Instaspin aligns to U phase, the index is aligned to UW back emf going neg to pos. Therefore there should be a know relationship between the Instaspin alignment and the index. Correct?

    Does anyone have experience with using the index to verify the electrical angle?

  • Usually, we don't use the index to correct the electrical angle for instaSPIN-FOC, only use the index to correct the rotor position for the position control if need. The electrical angle of index should be fixed if you complete the alignment and getting the angle of index. You may use the index to verify the electrical angle if you want to do.