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Maximum switching frequency

Javier Vara
Expert 1645 points

Other Parts Discussed in Thread: TMS320F28069

Hello,

We want to develop an inverter  (35kW output power) using MOSFET modules for a motor drive application. We have previously successfully used sensorless Instaspin running on a TMS320F28069 at 40kHz (switching frequency) and we were wondering if we could use it again for a much higher switching frequency application (above 100 kHz). If I remember correctly there is a decimation value that allows you to set the frequency of the main ISR with respect to the switching frequency but I don't know if you can increase that value as far a you want to or there is some other physical limit.

If it is possible to run at 100-200 kHz, is there anything else we should take into account? 

Regards,

  • 0
    TI__Guru** 119680 points
    using the HW decimation you are limited to a max of /3
    //! Note: Valid values are 1, 2 or 3 only
    #define USER_NUM_PWM_TICKS_PER_ISR_TICK (3)

    So, practically for MIPS purposes you are limited to somewhere in the 60-90 KHz rate for
    #define USER_PWM_FREQ_kHz (10.0)
    (I've never run faster than 90 KHz PWM)

    however, if you don't need to do motor ID, you can also use the SW decimation during run-time
    #define USER_NUM_ISR_TICKS_PER_CTRL_TICK (1)
    which can keep the CTRL rate lower and allow you to increase the PWM

    EX: PWM = 150 KHz, ISR = 50 KHz, CTRL = 25 KHz

    and you may be able to decimate the CTRL, CURRENT, and EST to a lower rate. You only need these to be ~10x your maximum motor frequency.

    So if your motor frequency is only 800 Hz max you should be able to keep the rates to 8 - 10 KHz instead of 25 KHz.
  • 0 in reply to ChrisClearman
    Expert 1645 points

    Hi Chris,

    Thanks for the quick reply.

    The motor we want to run is a 2kHz one (120 krpm, 1 pole pair). We don't need to ID it so for what you are saying I guess I could do:

    #define USER_PWM_FREQ_kHz (150.0) 

    #define USER_NUM_PWM_TICKS_PER_ISR_TICK (3)

    #define USER_NUM_ISR_TICKS_PER_CTRL_TICK (2) 

    to have: PWM= 150kHz, ISR= 50kHz, CTRL = 25kHz (>10x the motor electrical frequency)

    right?

    Thanks again,

  • 0 in reply to Javier Vara
    TI__Guru** 119680 points
    that's correct. We've never run it like that, but see if it works. You may want to use proj_lab03 to see the CPU loading as I suspect it is going to be quite high

    are you using low side shunt current measurements? what is the current of your motor?
  • 0 in reply to ChrisClearman
    Expert 1645 points
    ok. Thank you.

    we are probably going to use hall effect sensors on the motor phases. The maximum current that we expect to see is about 130 Apeak
  • 0 in reply to Javier Vara
    TI__Guru** 119680 points
    that is a good idea. it will be very challenging to get good low side current measurements with that level of current and such a low inductance motor.