Harmonics

Because of a phenomenon called "Half-wave symmetry". Contrary to what 'AVP5' has said, all orders of harmonics are generated. It is not difficult to generate even order harmonics.

Let me explain the Half wave symmetry phenomenon. Take a sinusoidal wave form of fundamental frequency, of say, 50Hz. Now, second harmonics will be 100 Hz. Isn't it? This means that there will be two full cycles of this second harmonic wave form, by the time fundamental frequency wave form completes one full cycle. In the positive half cycle of the fundamental, there will be one positive half cycle and negative half cycle of the second harmonic wave form. Whatever the effects caused by the positive half cycle wave of the second harmonic will be nullified by the negative half cycle wave of the second harmonic (as direction is reverse). And this is true for all even order harmonics.

Now, consider a third order harmonics. If you superimpose a third harmonic wave on a fundamental wave, in the postive half cycle of the fundamental, there will be two positive half cycles and only one negative half cycle of the third harmonic. The one positive and one negative half cycles of the third harmonic would cancel out each other and yet you will still have another positive half cycle of the third harmonic prevailing in the postive half cycle of the fundamental. This has no neutralisation and hence causes problems. This is true for all the odd order harmonics.

That's why we worry more about odd harmonics and not even!

Hope it is clear now!