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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.