Land-based AC power systems typically operate on a 50Hz or 60Hz sine wave, depending on what country you are in.

Electronic loads can draw current which is not a sine wave, and this can be represented as a

Fourier series of other frequencies which are multiples of the mains frequency (eg 50Hz turns into 100Hz, 150Hz, etc). This has the following problems:

- These harmonics cannot be used to power electric motors

- Transformers and motors waste energy due to hysteresis currents. This waste is much higher for these higher frequencies.

- In three-phase power systems, the third harmonic can cause current in the neutral conductor, which normally should carry almost no current

Electronic systems which are designed to minimise interference will filter out these higher-frequency harmonics.

See:

http://en.wikipedia.org/wiki/Harmonics_%28electrical_power%29