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What is a Harmonic?

Waveform Distortions

The purest repetative waveform is the sine wave. This has a single frequency and amplitude and can be described
by the equation v = V x sine(Ø).
Any changes to the waveform are referred to as distortion and the level of distortion is usually quoted as THD or Total Harmonic Distortion.

Distortion to a voltage waverform is called voltage distortion and is quoted as THDv. Similarly, distortion to a current waveform is called current distortion and is quoted as THDi.
A distorted waveform is made up of the original undistorted waveform plus a collection of additional waveforms at multiples of the original waveform frequency. These additional waveforms are called harmonics and the amplitudes and phase angles of these determine she shape of the resultant distorted waveform.
The frequency of the original waveform is called the fundemental frequency and has a harmonic number of 1. The second harmonic has a frequency of twice the fundemental frequency and the third harmonic has a frequency of three times the fundemental frequency.

In New Zealand, the power line frequency is 50Hz, so the second harmonic is at 100Hz and the third at 150Hz etc.

Even harmonics, 2nd, 4th, 6th, 8th.... cause the sinewave to exhibit assymetrical distortion, whereas odd order harmonics cause symmetrical distortion.

The THD measured is the ratio between the RMS of the total waveform minus the fundemental waveform and the RMS of the total waveform.