Harmonics

Harmonics are the multiples of the fundamental waveform frequency being produced by the generator and the load. Normally, since generators are magnetically symmetrical, only odd harmonics are significant. For example, a 60 Hz generated waveform will contain the 60 Hz fundamental, and may contain a 180 Hz 3rd harmonic, a 300 Hz 5th harmonic, a 420 Hz 7th harmonic, etc. Generally, the magnitude of the harmonic decreases with increasing harmonic order. Harmonics of lower order (i.e. the 3rd, 5 th, and 7th, are the most significant).

As with motors, harmonics produce undesirable effects in the generator. Harmonic currents cause heat to be generated in the winding, core, and rotor and essentially act as derating factors since generator ratings are limited by allowable temperature rise. Harmonics also cause sine wave distortion. The distortion increases with increased harmonic content in the generated wave and, when severe, can cause voltage regulator sensing difficulties and inaccurate instrument readings.

The choice of coil pitch significantly impacts harmonic generation. As noted previously, in a generator, the magnetic field induces voltage in coils placed in slots in the armature. Maximum voltage is produced when the span of each of these coils is exactly equal to the span of the north and south field poles. This positioning is referred to as full pitch. Most generators are fractional pitch windings. For example, a 2/3 pitch eliminates most 3rd harmonics, while a 4/5 pitch eliminates most 5th harmonics. An example of a design that minimizes harmonics of lower order is a 5/6 pitch and a three-wire Delta connection. With a 5/6 pitch, 5th and 7th harmonics are low and, with the three-wire Delta connection, 3rd harmonic currents have no path.

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