Sensorless FOC (STO-PLL) – noise, speed oscillation, not reaching max speed

[Mani2]

Hello,

I am using STM32 Motor Control SDK / MC Workbench with sensorless FOC (STO + PLL) to drive a low-speed, high-inertia 24V 40W BLDC fan motor.

Power Board: X-NUCLEO-IHM07M1

Issues observed:

• Motor rotates stably but produces intermittent (not continuous) rubbing noise, mainly after settling at certain RPM.
• Large speed oscillations are visible in Motor Pilot (speed dial deviation).
• Motor does not reach max speed even when increasing current limit in MC Workbench.
• Reducing Iq PI gains(cut off frequency) does not significantly reduce the noise.
• Very low PLL gains are required for stable rotation; higher PLL gains cause oscillation or loss of speed feedback.

pilot_register_observation

Questions:

1. For low-speed, high-inertia fan motors, what is the recommended observer / speed LPF cutoff frequency parameters?

2. Can I reduce this intermittent noise and speed oscillation and run smooth motor control without noise?

3. Which parameters should be prioritized to:

   • Reduce speed oscillations

   • Eliminate intermittent torque/current stepping noise

   • Reach rated max speed?

4. Are there any techniques recommended for this heavy load BLDC motor applications?

 Thanks in advance for any guidance.

[GMA]

Hello @Mani2,

Rated max speed could be limited by the Maximum Modulation Index available in 

Reduce the PWM frequency to obtain an MMI value as close as possible to 100%.

[Prof_McGonagall]

Hello @Mani2,
Did you find any technique to tune out these issues??
I am facing similar problems and would like to know if you found any solution for these. 
Thanks.

[Mani2]

 Hi @Prof_McGonagall,

I haven’t found a complete solution yet. The issue persists with this low-speed, high-inertia fan load, and I’m currently waiting for feedback from ST support. I’ll update here if I receive any concrete tuning recommendations.

Hi @GMA,

I have not yet received a concrete solution or tuning guideline from ST that fully addresses:

• Speed oscillations at low RPM

• Intermittent torque/current noise

I am currently waiting for further feedback or recommendations. 

If anyone else has practical experience or tuning strategies for similar issue with high-inertia BLDC fan applications, it would be great to hear your inputs.

Thanks.

[GMA]

Hello @Mani2 and @Prof_McGonagall,

With the Motor Pilot Advanced Configuration, tune the speed PI regulator parameters to optimize RPM stability. Tune the torque/flux PI regulator parameters to optimize torque response.