Why does a fan make noise while running on an inverter and not on normal electric supply?

Inverter is an electronic device for converting the direct current (DC) from storage batteries or solar cells into alternating current (AC) to match the domestic supply which is basically 220 volt and 50 Hz (cycles/sec).

It also forms the basic part of a UPS device that supports the computers when domestic current fails. The current variation of the good domestic AC supply is represented by a smooth sine wave, similar to a smooth wave on a still water surface.

The sine-wave form AC alone generates uniformly rotating magnetic field in the induction motor of a fan for noiseless rotation. But inverters can generate AC of any shape current variation. A well designed inverter alone supplies sine-wave AC.

The cheap inverters can generate, for example, square wave current variation. The square wave is composed of superposition of many harmonic sine waves (Fourier’s theorem — multiples of basic frequency).

This square wave AC, when connected to a fan, creates jerkily revolving magnetic field inside, making the rotor to make jerky motion and hence noise. Also these unfiltered harmonics in the AC create vibrations in the coil, generating humming noise.

These types of inverters are used for thermal and lighting applications only where the shape of AC change does not matter. Sensitive equipment like computers and motors need sine-wave producing inverters.

Source: The Hindu