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MCT8316A-Q1: MCT8316A-Q1

yenwu lo
Prodigy 100 points
Part Number: MCT8316A-Q1
Other Parts Discussed in Thread: MCT8316Z

Tool/software:

Dear TI technical support, I have a question regarding Precision labs series: Brushless-DC motors

specifically on Trapezoidal commutation video, 

the question can be seen in the attached word document file, which the phase voltage A, B, C timing and backemf ea, eb, ec seems not lined up? 

& also the time dead time T between PHASE A voltage & the dead time between backemf ea seems not matching each other?

also the backemf changes polarity in one electrical cycle, but the PHASE A voltage always keep the same polarity,

please see circled area in the attached word file, 

not sure if they should be lined up between PHASE A voltage & back emf ea waveform?

Or PHASE A voltage should change polarity like the backemf waveform does ?

Or my understanding & concept is wrong or not? 

If possible & convenient? can you help to point out where is the mistake in my question or my concept? 

Thanks and look forward to hearing from you when convenient. 

yenwu loTI_trapzoade commutation.docx

  • 0
    Prodigy 100 points

    FYI, The attached picture file is copied from the video-playing screen shot at around halfway of playing the Precision labs series: Brushless-DC motors series, 

    "Trapezoidal commutation" video.  If you play that video, then you will see that picture which is attached in the post.

    Hope this may help to explain where the picture is coming from.

    thanks

    yenwu lo. 

  • 0 in reply to yenwu lo
    Prodigy 100 points

    FYI, this question is more a general type BLDC MOTOR DRIVE related question, not directly related to IC: MCT8316A-Q1, 

    I do not know how to post my question originally, so I randomly pick a part number = MCT8316A-Q1, so that I can create my question thread and post it. 

    Thanks

    yenwu lo

  • 0
    TI__Genius 13460 points

    Hi yenwu lo,

    yenwu lo said:
    the time dead time T between PHASE A voltage & the dead time between backemf ea seems not matching each other?

    The time period labeled T' is the same time period as T. For T' the half-bridge for Phase A is in the Hi-Z state. T on the other hand includes 3 commutations steps one Hi-Z and two where the low side MOSFET is on. For T to match T', T must be shortened to only include the time period where the voltage is ramping down.

    yenwu lo said:

    not sure if they should be lined up between PHASE A voltage & back emf ea waveform?

    Or PHASE A voltage should change polarity like the backemf waveform does ?

    The Phase voltage is input to the motor and is referenced to GND. When the phase voltage is low the low-side MOSFET of the half bridge is on, pulling the voltage to GND (0V). We expect the backemf to match closer to the phase current polarity since it is generated by the motor.

    Regards,

    Joshua

  • 0 in reply to Joshua Horlander Cruz
    Prodigy 100 points

    Thanks. Joshua, 

    1st question, regarding the T and T' not matching issues in the attached picture. 

    I think the right-side PHASE A voltage on the right side of the attached picture is referenced to DC bus ground, while the left side T' time interval is related with the backemf  ea generated in PHASE A  and it is referencing 3 phase neutral point, and this 3 phase neutral point  & DC BUS GND difference may have caused the T & T' time interval not matching as we see now in the attached picture? 

    not sure if this above reasoning is reasonable? can you confirm. 

    P.S. Basically, I think this may be the main reason which may have caused the questions about this 2 sets of waveform not matching , 

    if we plot the right side and left side waveforms using the same reference point, then the left side waveform will match the right side waveform. 

    Thanks

    yenwu lo 

     

  • 0 in reply to yenwu lo
    TI__Genius 13460 points

    Hi yenwu lo,

    If Phase A was measured with the 3 phase neutral, then it would match the BEMF waveform. An example of a phase voltage referenced to the 3 phase neutral can be seen in figure 8-5 of the MCT8316Z datasheet. 

    A capture of the phase current and phase voltage is shown in Figure 9-11 in the MCT8316Z datasheet.

    Regards,

    Joshua

  • 0
    Prodigy 100 points

    Thanks. Josha, I see, I will check Fig. 9-11 in more detail, if I see further questions, I will check with you again. Thanks again for your great help. 

    Best Regards, 

    yenwu lo