Introduction To Harmonics And Mitigation; What Are Harmonics; Linear Loads - Danfoss VLT AHF005 Design Manual

Introduction to Harmonics a...

3 Introduction to Harmonics and Mitigation

3.1 What are Harmonics?

3.1.1 Linear Loads

On a sinusoidal AC supply a purely resistive loads (for
example an incandescent light bulb) will draw a sinusoidal
current, in phase with the supply voltage.
The power dissipated by the load is:
P = U × I
For reactive loads (such as an induction motor) the current
will no longer be in phase with the voltage, but will lag the
voltage creating a lagging true power factor with a value less
than 1. In the case of capacitive loads the current is in
advance of the voltage, creating a leading true power factor
with a value less than 1.
AHF005/010 Design Guide
In this case, the AC power has three components: real power
(P), reactive power (Q) and apparent power (S). The apparent
power is:
S = U × I
(where S=[kVA], P=[kW] and Q=[kVAR])
In the case of a perfectly sinusoidal waveform P, Q and S can
be expressed as vectors that form a triangle:
S 2 = P 2 + Q 2
The displacement angle between current and voltage is φ.
The displacement power factor is the ratio between the
active power (P) and apparent power (S):
DPF =
®
MG.80.C4.02 - VLT is a registered Danfoss trademark
S
φ
P
P
= cos (ϕ)
S
3
Q
7
3