F28E120SC: Issue with Open loop control Motor Operation V/F Control

Hello,

DMC_LEVEL_2 is an open loop based method to check the ADC scaling. At the end you have to choose right values for:

I would bring the angle and phase voltage to scope to see if they are matching. They should look like:

You can also try starting with DMC_LEVEL_3, where it's a closed current loop.

Best Regards,

Masoud

As the document I have attached above.

In the document, it can be observed that the phase PWM signals are initially different inphase before they reach the commanded frequency (as shown in Figure 3 and Figure 4).

I suspect that this behavior may be the reason for the high current flowing at the initial stage.

Could you please help me understand why this is happening during startup?
Is this expected behavior before the system reaches the commanded frequency, or could it be related to the PWM configuration or control algorithm?

Yes, what you are seeing at startup can be expected. DMC_LEVEL_2 is an open-loop test mode intended mainly to validate the current/voltage sensing path and basic PWM operation. In this build level, the sensed current and voltage are used for observation/verification, not for regulating the motor as in a closed-loop controller. DMC_LEVEL_3 is the step used to validate the hardware with a closed current loop.

Because this is open loop, during startup the controller applies a generated electrical angle/frequency ramp rather than correcting itself from current feedback. As a result, before the rotor locks into the commanded rotating field, the three phase voltages/currents may not look perfectly balanced or 120° aligned, and a larger startup current can appear if the initial electrical angle, V/f ramp, or voltage magnitude is not well matched to the motor/load. That is why I suggested checking the electrical angle together with the phase voltage on the scope.

You can verify level 1 items again. Make sure ADC offset calibration, PWM polarity, deadband, phase shift, and current/voltage scaling are all correct, since Level 2 depends on those being clean. And then try DMC_LEVEL_3 since level 3 closes the current loop, it is usually a better step for confirming whether the issue is simply a normal open-loop startup effect or something wrong in sensing/PWM configuration.

Best Regards,

Masoud

I was able to resolve the initial phase issue that occurred at the start of the PWM generation. However, I am still observing a phase gap between the phases, as shown in Figure 8 of the attached document. I am not able to understand why this phase difference is occurring between the phases. Could you please help me identify the possible reason for this behavior?

Additionally, when running the V/F method, the current drawn is already quite high from the beginning, and after the speed reaches a few Hz, the current further increases and eventually reaches the overcurrent limit.

I also observed that when trying DMC_LEVEL_3, the current is very high  from the beginning and the overcurrent protection is getting triggered. Could you please help me understand why this is happening?

Abhishek,

Overall, this code is a variant of our universal motor control code. We have a user guide explaining the steps of using this SW and ask to follow some steps. For the motor you have, can you tune the parameter based on following user guide and let me know if you face with issue?

https://www.ti.com/lit/ug/spruj26a/spruj26a.pdf

Best Regards,

Masoud