Danfoss AHF005 Design Manual

Also see for VLT AHF 005: Instruction manual Operating instructions Operating instructions Design guide

Contents

Danfoss AHF005 Design Manual Manual pdf 7 page image

3 Introduction to Harmonics andMitigation3.1 What are Harmonics?3.1.1 Linear LoadsOn a sinusoidal AC supply a purely resistive loads (for examplean incandescent light bulb) will draw a sinusoidal current, inphase with the supply voltage.The power dissipated by the load is:P =U ×IFor reactive loads (such as an induction motor) the current willno longer be in phase with the voltage, but will lag thevoltage creating a lagging true power factor with a value lessthan 1. In the case of capacitive loads the current is in advanceof the voltage, creating a leading true power factor with avalue less than 1.In this case, the AC power has three components: real power(P), reactive power (Q) and apparent power (S). The apparentpower is:S =U ×I(where S=[kVA], P=[kW] and Q=[kVAR])In the case of a perfectly sinusoidal waveform P, Q and S canbe expressed as vectors that form a triangle:S 2 =P 2 +Q 2The displacement angle between current and voltage is φ. Thedisplacement power factor is the ratio between the activepower (P) and apparent power (S):DPF =PS =cos(ϕ)3.1.2 Non-linear LoadsNon-linear loads (such as diode rectifiers) draw a non-sinusoi-dal current. The figure below shows the current drawn by a6-pulse rectifier on a three phase supply.A non-sinusoidal waveform can be decomposed in a sum ofsinusoidal waveforms with periods equal to integer multiplesof the fundamental waveform.f (t) = ∑ah ×sin(h ω1t)See following illustrations.Introduction to Harmonics a... AHF005/010 Design GuideMG.80.C3.02 - VLT® is a registered Danfoss trademark 73 3