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TMS320F28027F: Lab02b: Induction motor identification accuracy

Jason Tribe
Prodigy 220 points

Part Number: TMS320F28027F
Other Parts Discussed in Thread: MOTORWARE

Hi,

I am using a custom three phase inverter to control an Induction motor. The induction motor is connected in delta, and the average resistance measured between any two phases is 1.4 ohm's.

After repeating the motor identification Lab02b multiple times I get a stator resistance measurement of 0.2 ohm's. I understand that InstaSPIN assumes the motor is connected in star and from a control system point of view in doesn't matter, but I would have thought that the software would identify the stator resistance as 0.7 ohm's. Meaning it would measure 1.4 ohm's between two stator terminals and divide that by 2 to get the individual phase resistance.

Can anyone tell me if my understanding is correct? In which case there is an error in my feedback circuit.

Kind regards

Jason Tribe

  • 0
    TI__Genius 17175 points

    There may be an issue with the feedback network. Have you verified correct operation using lab01b and lab01c in Motorware 18?

    Sean

  • 0 in reply to Sean Bigley
    Prodigy 220 points
    Hi Sean,

    Yes I have run lab's 1b and 1c with Motorware 18:

    Lab01b results:
    When running the lab with the motor connected, the motor starts spinning at a set point of 2000rpm and increases with speed as the set point is increased. I am running the inverter on a 30V current limited supply just for testing as I don't have a larger voltage supply with current limiting, which I believe is the reason for the motor only spinning at higher set points of speed . I also can't do any data logging in CCS or record outputs on DAC pins as I am using the F28027F controller. That being said the motor spins very smoothly during this lab.

    Lab01c results:
    When running the motor with this lab, the motor spins at much higher speeds even with the 30V bench supply. The motor speed changes with set point speed, but I don't see a change in the speed when I adjust IdSet_A and IqSet_A. Again I cannot perform data logging in CCS or output to DAC pins, but the motor spins very smoothly as before. I do see changes in the magnitude of the phase currents when viewing the feedback from the current sense resistors, but I can't correlate the changes in magnitude on the scope the the changes in the current set point values.

    Do you have any advice on how I can confirm the current and voltage feedback without being able to data log or ouput signals on DAC pins?

    Kind regards
    Jason
  • 0 in reply to Jason Tribe
    TI__Genius 17175 points
    Jason

    If you're using the F28027F without a board with built in LPF for DAC, it may be easier to try and diagnose the issues using lab2b/2c. Is the motor able to run well during the identification process?

    Sean
  • 0 in reply to Sean Bigley
    Prodigy 220 points

    Hi Sean,

    During the identification process I don't see any issues with the motor. I've tried to follow the documentation process as well as I can in order to improve the accuracy of the identification but I still get around 0.2 ohm's every time for the stator resistance .

    The first time I see instability when running the motor is directly after the identification process. The motor battles to accelerate and sounds like it's grinding, which I am guessing is due to an unstable control system (incorrect tuning, incorrect parameter or noisy feedback).

    Jason

  • 0 in reply to Jason Tribe
    TI__Genius 17175 points
    "Directly after the identification process" refers to you still running lab2b or moving on to lab3?

    Sean
  • 0 in reply to Sean Bigley
    Prodigy 220 points

    Directly after the locked rotor test has completed, I set gMotorVars.Flag_Run_Identify again to test out the speed control and PowerWarp and that's when the motor becomes unstable. That's page 57 of the instaspin_labs.pdf.

    I have not tried Lab2c before as I thought is was specifically for PMSM.

  • 0 in reply to Jason Tribe
    TI__Genius 17175 points
    What type of meter are you using to determine the resistance between the two phases? Are you using a high resolution LCR meter? Even though 0.2ohms doesn't seem too low, it could be that a multimeter has trouble measuring such a low value

    Sean
  • 0 in reply to Sean Bigley
    TI__Genius 17175 points
    Is there any update here? If not, I'll close this thread for now

    Sean
  • 0 in reply to Sean Bigley
    Prodigy 220 points

    Hi Sean. I apologise for the delay.

    You were 100% correct my multimeter was way off. I used a voltage source with current limiting to calculate the resistance across each terminal and I get 0.4 ohms which is exactly what I expected as InstaSPIN report 0.2 ohms per phase.

    Unfortunately that doesn't help explain why our motor is so unstable after parameter estimation. One thing I have noticed is that the DC link voltage reading in CCS bounces between 31 - 34 V even when the motor is not running. I've confirmed with a scope that there is no ripple on the DC link which makes me thing it's either noise or a resolution problem.

    Do you have any advice?