What is Harmonic Filtering?

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To understand the purpose of harmonic filtering, it helps to understand what harmonics are.The voltage of a standard 50Hz AC (Alternating Current) power supply swings between positive and negative 50 times a second, following a neat sine-wave. Normal (linear) AC electrical loads such as heaters or incandescent lights draw a current that follows this same sine-wave.

The graph below shows typical voltage and current curves – voltage in blue and current in red.

However, with non-linear loads such as VSDs (variable speed drives), the current does not nicely follow the voltage. The graph below shows an example of a non-linear current waveform. Through a set of calculations this non-sinusoidal wave can be broken down into many sine-wave components with frequencies of multiples of 50Hz. These components are called ‘harmonics.’ The amount of harmonics present is defined as a percentage of the 50Hz current.