This article briefly talks about a problem that is easier to ignore in the motor control process, but has a greater impact on the motor performance-harmonic performance.
1. What are harmonics? What are the hazards of harmonics?
In order to write this place clearly, I checked many related papers. First, let me talk about this from the physical meaning: Harmonic refers to the electric quantity whose frequency contained in the current is an integer multiple of the fundamental wave, generally refers to the periodic non-sinusoidal The electric quantity is decomposed by Fourier series. Except for the electric quantity of the fundamental frequency, the electric quantity generated by the current greater than the fundamental frequency is called harmonics . The harmonic order is the ratio of the harmonic frequency to the fundamental frequency (n=fn/f1). The harmonic waveform diagram is shown in Figure 1 below, and the harmonic decomposition diagram is shown in Figure 2 below. (This is from https://zhuanlan.zhihu.com/p/22481177 ). There is a detailed explanation about the harm of harmonics ( https://zhuanlan.zhihu.com/p/27674405 ), and the same is true for the harm of motors.
So back to the signal level, you can analyze this in depth, if it is only a sine, as shown in green in the figure. At this point, it can be considered that there are no harmonics. Everyone knows the principle of signal superposition. If a triple-frequency waveform is superimposed on this basis, the sine wave will be distorted and become a saddle wave, which is a red waveform. By analogy, if a sine wave is superimposed on more than one kind of harmonics, but 3, 5, 7, 9, 11, etc. harmonics are superimposed, will its waveform no longer be sine at all? What is even more amazing is The sine wave is superimposed by the Nth harmonic, and it turns out to be the most familiar rectangular wave . (Detailed explanation of harmonic superposition in signal analysis URL https://www.bilibili.com/video/BV1Sb411a7aq?from=search&seid=17118871288027897792 )
2. Where do the harmonics in the motor control system come from?
What is the rectangular wave? Isn't this our PWM? Wow, thinking of this, I suddenly realized what kind of fairy theory this is, and Fourier is really a god and man. Let’s get back to the subject, now that the process of generating harmonics is linked to PWM, where the harmonics in motor control come from can be explained.
I personally think that the root cause of the harmonics in motor control is the nonlinear characteristics of the inverter . Why do you say that? The inverter is composed of switching elements IGBT\MOS, whether it is a three-phase inverter circuit or a single-phase. For a device, there are actually only two states of on and off, and digitization means that there are only two states of 1 and 0. The waveform produced by continuously switching between 1 and 0 within a period of time is a rectangular wave . The rectangular wave generated by the switching characteristics of the inverter is the source of the harmonics generated by the entire motor control system. This is the source of the harmonics from the perspective of device characteristics.
From another perspective, I believe that the following picture is not unfamiliar to students majoring in electrical information. We generate PWM through unipolar PWM mode. The method of generation is to compare the triangular carrier wave and the sinusoidal modulation wave, and take the switch At the moment, the drive inverter generates the rectangular wave of (b). At this point, we can see that, in fact, our variable frequency control wants to reproduce the sine wave through the pulse waveform, but our state only has the switch only 0 and 1, so we need multiple pulses to fight this sine wave, because the switching frequency is limited , The number of pulses in a fundamental wave cycle is also limited, it is bound to be unable to completely reproduce the sine wave, this distortion is also the source of harmonic generation.
3. How to optimize the harmonic performance
At its root, if the interference caused by the control is excluded, the harmonics of the motor control mainly come from the nonlinear characteristics of the switching tube, so it can be understood that if there are enough pulses in a fundamental wave cycle to reconstruct the sine wave, then The harmonic content can be very very small. So based on this conjecture, let's do a simulation to verify this conjecture.
It can be seen from the following waveforms that as the switching frequency becomes higher, the current waveform burrs are significantly reduced, and the harmonic performance is significantly improved. This is just a way to improve the harmonic performance. Sometimes when the switching frequency cannot be changed, a special modulation strategy is needed to improve it. This is what needs to be discussed later.
Current waveform when the switching frequency is 2k:
Current waveform when the switching frequency is 5k:
Three-phase current waveform when the switching frequency is 10k :
4 summary
1. Harmonics come from the non-linear characteristics of the inverter, which are more harmful to the motor and inverter, causing larger current burrs and torque ripples
2,. For a device, there are actually only two states of on and off, and digitization means that there are only two states of 1 and 0. The waveform produced by continuously switching between 1 and 0 within a period of time is a rectangular wave . The rectangular wave generated by the switching characteristics of the inverter is the source of harmonics generated by the entire motor control system. This is the source of the harmonics from the perspective of device characteristics.
3. In fact, our variable frequency control wants to reproduce the sine wave through the pulse waveform, but our state only has the switch only 0 and 1, then we need multiple pulses to fight this sine wave, because the switching frequency is limited, a fundamental wave The number of pulses in the cycle is also limited, so it is impossible to completely reproduce the sine wave. This distortion is the source of harmonic generation.