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Harmonics

What is harmonics?

When we think about harmonics, we can easily come up with harmony in music, which means the composition of individual sounds. What about electrical power? In an electrical power system, a harmonic is a voltage or current at multiple of the fundamental frequency of the system. Then, What is the fundamental frequency? Before we discuss harmonics, we should know what the fundamental frequency is. Maybe you know the relationship between voltage, current, and resistor, for example, V=IR. Assume that there is a system where a DC voltage supply and a resistor are connected in serial. Then, if the voltage is AC, the current is AC.  And It's totally because of the resistor in this system. That is, the resistor is linear. So in this system, we have one voltage sine wave and one current sine wave. And they can be called a fundamental frequency wave.


Fig.1



















What if the resistor is replaced with other components which is non-linear? The current will not be a complete sine wave such as the complex waveform in Fig.1. Why does it happen?? It's because of harmonics. Harmonics mean unwanted higher frequencies that superimposed on the fundamental waveform creating a distorted wave pattern. Harmonic frequencies in the power grid are one of the causes of power quality problems. Harmonics in power systems result in increased heating in the equipment and conductors, misfiring in variable speed drives and torque pulsations in motors. So we should know and take care of it.
 As I said, harmonics have a higher frequency than the fundamental one. More exactly, their frequency is multiple times as the fundamental one.
Fig.2







As fig 2 is illustrated, the fundamental wave is the first harmonic and there is the second one, the third one, and so on.

Fig.3














Interestingly, in fig 3, frequency response, you can notice that magnitude depends on its frequency. In this graph, the fundamental frequency seems to be 200 Hz. As the frequency is going higher like 400, 600 Hz, ..., its magnitude is getting lower. That makes sense because we can think like as speed goes up, power is lower and its whole power remains.


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