Sunday, 17 July 2016

How Harmonics Eliminated from Alternator Generated Voltage?

In this post we will discuss about how to eliminate or suppress the harmonics from the emf waveform of an Alternator. In an Alternator, the primary source of harmonics in the emf waveform is due to non sinusoidal field waveform. If the field waveform would have been sinusoidal then there have been no harmonics in generated emf of an Alternator.

Therefore, first attempt is made to make the field waveform sinusoidal as far as possible and them means of reducing or suppressing the harmonics is adopted. Field waveform of an Alternator can be made sinusoidal by the following methods:

  • Small air gap at the pole centre and large air gap at the pole end in an Salient Pole Synchronous machine tens to make the field flux sinusoidal.
  • Skew pole faces if possible.
  • In Turbo-Alternator or cylindrical pole synchronous machine, the air gap is uniform and hence field winding is distributed in slots in such a manner to make the field waveform sinusoidal. Figure below shows the Rotor of an Alternator.

Having adopted all the above mentioned methods, the filed waveform along the air gap periphery is still not purely sinusoidal but it is flat topped. As a result, harmonic emf is always generated in the Alternator. These harmonics can however be eliminated / suppressed by the following methods:

  • The distribution of armature winding along the air gap periphery tends to make the generated emf waveform sinusoidal.
  • With chorded coil, harmonics can be eliminated. If the ξ be the chording angle for fundamental flux wave, then for nth space harmonics the chording angle becomes nξ electrical. Therefore, pitch factor for nth harmonics,

Kp = Cos(nξ/2)
If we want to eliminate 5th harmonics then, ξ = 36° as
Kp = Cos(180/2)
      = 0
Therefore, generated emf E = KpKdπfNphØ

Where Kp = Pitch factor

 Kd = Winding Distribution Factor

f =Frequency

Nph = Number of turns per phase

Ø =Flux

A chording angle of 30° is most useful in an Alternator as it gives the following pitch factors,
0.966 for fundamental, 0.707 for 3rd harmonic, 0.259 for 5th and 7th harmonics and 0.7.7 for 9th harmonics.

Mind that triplen harmonics is eliminated from the generated line emf by Star connection, though 5th and 7th harmonics of reduced magnitude are present in the line emf.

  • By Skewing the Armature Slot tooth / slot harmonics are eliminated.
Thus, using the above mentioned ways, harmonics in the generated emf of a synchronous generator is reduced.

No comments: