Harmonics and VSDs


When it comes to the selection of a Variable Speed Drive (VSD), there are often questions about the levels of total harmonic distortion THD on the mains supply created by the installation of the VSD. A six pulse bridge rectifier input on a VSD takes up a non-sinusoidal current from the mains, which increases the input current IRMS. A non-sinusoidal current is transformed by means of a Fourier analysis and split up into sine-wave currents with different frequencies, i.e. different harmonic currents I N with 50 Hz as the basic frequency:

5th harmonic

Example of the fundamental voltage and the fifth harmonic voltage


The voltage distortion on the mains supply depends on the size of the harmonic currents multiplied by the mains impedance for the frequency in question. The total voltage distortion THD is calculated on the basis of the individual voltage harmonics using this formula.

5th harmonic 2

Danfoss offer a free, easy-to-use software package (MCT31) that calculates the amount of harmonic distortion created in the installation of Danfoss VSD’s which can be downloaded here.

For further information contact MJK Automation.