Danfoss VLT AHF005 Design Manual

Contents

Danfoss VLT AHF005 Design Manual Manual pdf 9 page image

3.1.2 Non-linear LoadsNon-linear loads (such as diode rectifiers) draw a non-sinusoidal current. The figure below shows the current drawn by a 6-pulserectifier on a three phase supply.A non-sinusoidal waveform can be decomposed in a sum of sinusoidal waveforms with periods equal to integer multiples of thefundamental waveform.f (t) = ∑ah ×sin(h ω1t)See following illustrations.11 2 3 4 5 6 70.00--11 2 3 4 5 6 70.00--130BB539.10The integer multiples of the fundamental frequency ω1 arecalled harmonics. The RMS value of a non-sinusoidalwaveform (current or voltage) is expressed as:IRMS = ∑h =1hmaxI(h )2The amount of harmonics in a waveform gives the distortionfactor, or total harmonic distortion (THD), represented by theratio of RMS of the harmonic content to the RMS value of thefundamental quantity, expressed as a percentage of thefundamental:THD = ∑h =2hmax(IhI1 )2× 100 %Using the THD, the relationship between the RMS currentIRMS and the fundamental current I1 can be expressed as:IRMS =I1 × 1 +THD 2The same applies for voltage.The true power factor PF (λ) is:PF =PSIn a linear system the true power factor is equal to thedisplacement power factor:PF =DPF =cos(ϕ)Introduction to Harmonics a... AHF005/010 Design Guide8 MG.80.C4.02 - VLT® is a registered Danfoss trademark33