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

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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. $\frac{5}{6}th$ of pole pitch
4. 120°

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Correct Answer - Option 3 : $\frac{5}{6}th$ of pole pitch

For eliminating nth harmonic coil span required is

$= 180 \times \left( {\frac{{n - 1}}{n}} \right)$

To eliminate 5th harmonic coil span required is

$= 180 \times \left( {\frac{4}{5}} \right) = 144^\circ$

To eliminate 7th harmonic coil span required is

$= 180 \times \left( {\frac{6}{7}} \right) = 154.285^\circ$

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

$= \frac{{144^\circ\; + \;154.28^\circ }}{2} = 149.14^\circ \approx 150^\circ$

150° is $\frac{5}{6}$ parts of 180° i.e. pole pitch

Hence, the coil span in three phase AC machines must be $\frac{5}{6}th$ of pole pitch.