Things about PWM modulation

SPWM and SVPWM

The frequency converter using PWM modulates the DC voltage into a variable frequency and variable amplitude voltage (AC or DC). SPWM was first used because it is easy to implement. It only requires comparing the desired output modulated waveform with the triangular carrier wave. In the "ancient" era, analog circuits could easily implement it.

The modulation ratio is defined as the ratio of the output phase voltage to the DC voltage 1/2, and the modulation ratio of SPWM is 1.

SVPWM is now called space vector PWM. It starts from the perspective of the sinusoidal magnetic flux of the motor and aims to obtain a constant circular rotating magnetic field through voltage control, so it is also called sinusoidal flux PWM. The magnetic flux is the integral of the voltage. SVPWM makes the magnetic flux closer to a circle by selecting the appropriate voltage vector and action time. The biggest advantage of SVPWM is that it improves the voltage utilization rate, and the modulation ratio can reach 1.15 (15% higher than SPWM). A more popular understanding of the modulation ratio improvement of SVPWM is that the output line voltage peak can be equal to the DC bus voltage, while the maximum output voltage peak of SPWM is sqrt(3)/2 of the DC bus voltage.

Digital Implementation of SVPWM

Many documents and textbooks will explain how to calculate the action time of 8 voltage vectors and how to act on the voltage vectors in sequence. This process is really too heartbreaking and tests people's patience. However, the people who first engaged in SPWM did not give up and wanted to see what the equivalent carrier looked like under the voltage vector action mode of SVPWM. So many experts came to play a role, and finally found that its modulation wave was based on the sine wave, with the third harmonic injected.

Moreover, the third harmonic is very special. It is not a cubic sine signal, but a waveform similar to a triangle wave. Fortunately, this injected harmonic has an analytical expression, Uz=middle(Ua,Ub,Uc), which is the middle value of the three-phase output voltage ua,ub,uc. That makes it easy. When digitally implementing SVPWM, there is no need to use complex spatial position calculations. It only requires taking the middle value and superimposing it.

Further research, using different values ​​for Uz, in addition to SVPWM, can also be changed to obtain 5-segment PWM, overmodulation PWM, etc. For three-level, five-level or seven-level, the research ideas are similar.

PWM amplification factor

I believe that many people who are engaged in power electronic control will be troubled by the coefficient Kpwm. It appears in many documents and textbooks and is used to deduce the transfer function of the system, but no one tells us what it is. Kpwm is actually used to measure the amplification factor after PWM modulation passes through the power device.

Kpwm=Uout/Uref. As can be seen from the above figure, Kpwm is related to many factors, such as PWM modulation algorithm, chip PWM logic circuit, power device, and everyone has different definitions of these details, so Kpwm does not have an exact value, which I think is why many books are unwilling to explain it clearly. In practical applications, we should assume that if I output a Uref value, after passing various levels, the final output Uout is what, so as to get our own Kpwm.