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In order to minimize both fifth and seventh harmonics, the coil span in three phase AC machines must be

  1. 144°
  2. 6/7 × pole pitch
  3.  of pole pitch
  4. 120°

Answer (Detailed Solution Below)

Option 3 :  of pole pitch
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Detailed Solution

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For eliminating nth harmonic coil span required is

To eliminate 5th harmonic coil span required is

To eliminate 7th harmonic coil span required is

To eliminate both 5th and 7th harmonic, we should take average of both i.e.

150° is  parts of 180° i.e. pole pitch

Hence, the coil span in three phase AC machines must be  of pole pitch.

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