Which are the reasons of these fake high-frequency oscillations?

Hello, I have developed a Simulink model of a speed control system for a three-phase asynchronous engine. I have used the sinusoidal PWM technique, so I implemented the model with a non-controlled three-phase rectifier and an inverter which is controlled by a module that produces trigger pulses comparing three sine waves, which are offset 120 ° each other, with a high-frequency triangle wave.

To built the control loop I used a PI controller configured as a continous-time controller without anti-feature and with a saturation feature in the output. The PID controls the "amplitude modulation index" (m_a) of the "sinusoidal PWM trigger pulses generator". The amplitude modulation index is the ratio of the sinusoidal waves' amplitude and the triangle wave's amplitude, ma = Ar / Ac. And I am using a big motor with a power of 10 HP.

The problem arised when I simulated the model becauses some high-frequency oscillations appeared in the speed plot, which are unreal taking into account the motor's great inertia. I have attached the plots I got after the simulation.

Do you know which could be the reasons of these high-freq oscillations?

• Speed plot:

• Amplitude modulation index (ma) and frequency modulation index (mf):

• Error signal:

The model's block diagram is the next one:

And the attached file is the Simulink model (slx).

I hope you can help me. Best wishes!

[Sofia Gilardini]

Having the same issue...

Answer 1

[Sabin]

Most likely root cause is the LC filter. LC filters can introduce resonant frequencies. If these resonant frequencies are excited by disturbances in the system, they can lead to oscillations. Therefore, designing properly the LC filter should improve the results. Also changing the solver to ode23tb in this instance should speed up the simulation a bit.