Chuck Yung
EASA Senior Technical Support Specialist

I used to joke that if you mention harmonics, engineers get excited while the eyes of non-engineers glaze over. The truth is that harmonics can be easily understood when explained in layman’s terms. Harmonics are simply multiples of the fundamental frequency, with positive, zero or negative sequence. The fundamental frequency is line frequency – also called the first order harmonic -- that being 60 Hz in North America or 50 Hz in most of the rest of the world.

Other harmonic numbers (5th, 7th, etc.) can be viewed as that order times the fundamental frequency, or visualized as having that number of waveforms in the same distance as a single waveform of the fundamental. So in a 60 Hz system, the 5th harmonic is 5x60 or 300 Hz. There will be 5 complete waveforms in the span of a single 60 Hz waveform. When the positive and negative portions of the sine wave are symmetrical, even number harmonics are non-existent.

Any harmonic that is a multiple of three, in the three-phase world, is a zero-sequence harmonic; and, when we are considering a sinusoidal power system, cancels out (except for synchronous alternators, which are outside the scope of this discussion).

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Categories: Technical topics, AC motors, Rotors, Squirrel cage rotors, Repair procedures & tips

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