For eliminating the 7th harmonic of the emf wave of an alternator the fractional pitch must be
For eliminating the 7th harmonic of the emf wave of an alternator the fractional pitch must be 6/7.
The emf waveform of an alternator contains harmonic components in addition to the fundamental frequency component.
The magnitude of the harmonic components is proportional to the winding pitch of the alternator.
The winding pitch is defined as the distance between two consecutive coils in terms of the number of slots, expressed as a fraction of the total number of slots per pole per phase.
For example, in a 2-pole, 3-phase alternator, the total number of slots per pole per phase is 3, and a full-pitch winding would have a pitch of 3/3 = 1. This means that consecutive coils are placed in every third slot.
Now, to eliminate a particular harmonic component, the winding pitch must be chosen such that the harmonic component of the emf waveform is canceled out.
The winding pitch that achieves this is called the fractional pitch, and its value depends on the order of the harmonic component to be eliminated.
In the case of the 7th harmonic component, the fractional pitch must be 6/7, which means that the distance between two consecutive coils must be 6/7 of the total number of slots per pole per phase.
This will result in a cancellation of the 7th harmonic component of the emf waveform, leaving behind only the fundamental and other lower-order harmonic components.