Elimination of Harmonics:
In alternators, the primary source, or cause, of harmonics in the e.m.f, waveform is due to the non-sinusoidal field flux waveform. In case the field flux waveform is a sine wave, there would be no harmonics in the alternator generated e.m.f. Thus, first an attempt is made to have almost sinusoidal field flux wave and then the means to reduce the harmonics from the e.m.f waveform should be adopted.
Field flux waveform can be made as much sinusoidal as possible by the following methods:
(i) Small air gap at the pole centre and large air gap towards the pole ends in salient pole synchronous machines tends to make the field flux sinusoidal.
(ii) Skew the pole faces if possible.
(iii) In turbo-alternators, the air gap is uniform. So field winding is distributed in the slots in such a manner as to make the field flux waveform almost a sine wave.
Inspite of all these attempts, field flux waveform along the air-gap periphery is not sinusoidal. As a result, harmonic emfs are always generated in an alternator. These can, however, be eliminated or suppressed as follows:
- Distribution: The distribution of the armature winding along the air-gap periphery tends to make the e.m.f. waveform sinusoidal.
- Chording: With coil-span less than pole pitch, the harmonics can be eliminated.
- Skewing: By skewing the armature slots, only tooth harmonics or slot harmonics can be eliminated
- Fractional slot winding: With fractional slot winding slot harmonics can be eliminated.
- Alternator connections: Star or delta connections of alternators suppress triplen harmonics from appearing across the lines.