Harmonic distortion indicators - Crest factor: Difference between revisions
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The crest factor is the ratio between the value of the peak current or voltage(I<sub>M</sub> or U<sub>M</sub>) and its r.m.s. value. | The crest factor is the ratio between the value of the peak current or voltage(I<sub>M</sub> or U<sub>M</sub>) and its r.m.s. value. | ||
* For a sinusoidal signal, the crest factor is therefore equal to <math>\sqrt 2</math>. | * For a sinusoidal signal, the crest factor is therefore equal to <math style="vertical-align:-15%;"> \sqrt 2</math>. | ||
* For a non-sinusoidal signal, the crest factor can be either greater than or less than <math>\sqrt 2</math>. | * For a non-sinusoidal signal, the crest factor can be either greater than or less than <math style="vertical-align:-15%;"> \sqrt 2</math>. | ||
The crest factor for the current drawn by non-linear loads is commonly much higher than <math>\sqrt 2</math>. It is generally between 1.5 and 2 and can even reach 5 in critical cases. | The crest factor for the current drawn by non-linear loads is commonly much higher than <math style="vertical-align:-15%;"> \sqrt 2</math>. It is generally between 1.5 and 2 and can even reach 5 in critical cases. | ||
A high crest factor signals high current peaks which, when detected by protection devices, can cause nuisance tripping. | A high crest factor signals high current peaks which, when detected by protection devices, can cause nuisance tripping. | ||
'''Examples:''' | '''Examples:''' | ||
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{{FigureRef|M7}} represents the current absorbed by a compact fluorescent lamp. | {{FigureRef|M7}} represents the current absorbed by a compact fluorescent lamp. | ||
I<sub>r.m.s.</sub> | {{def | ||
I<sub>M</sub> | |I<sub>r.m.s.</sub>| 0.16A | ||
THD<sub>i </sub> | |I<sub>M</sub> | 0.6A | ||
Crest factor | |THD<sub>i </sub>| 145% | ||
|Crest factor| 3.75}} | |||
{{FigImage|DB422616|svg|M7|Typical current waveform of a compact fluorescent lamp}} | |||
{{FigureRef|M8}} represents the voltage supplying non-linear loads through a high impedance line, with a typical "flat top" distorted waveform. | {{FigureRef|M8}} represents the voltage supplying non-linear loads through a high impedance line, with a typical "flat top" distorted waveform. | ||
V<sub>r.m.s.</sub> | {{def | ||
V<sub>M</sub> | |V<sub>r.m.s.</sub> | 500V | ||
THD<sub>u</sub> | |V<sub>M</sub> | 670V | ||
Crest factor | |THD<sub>u</sub> | 6.2% | ||
|Crest factor| 1.34}} | |||
{{FigImage|DB422617|svg|M8|Typical voltage waveform in case of high impedance line supplying non-linear loads}} | |||
Latest revision as of 09:48, 22 June 2022
The crest factor is the ratio between the value of the peak current or voltage(IM or UM) and its r.m.s. value.
- For a sinusoidal signal, the crest factor is therefore equal to [math]\displaystyle{ \sqrt 2 }[/math].
- For a non-sinusoidal signal, the crest factor can be either greater than or less than [math]\displaystyle{ \sqrt 2 }[/math].
The crest factor for the current drawn by non-linear loads is commonly much higher than [math]\displaystyle{ \sqrt 2 }[/math]. It is generally between 1.5 and 2 and can even reach 5 in critical cases.
A high crest factor signals high current peaks which, when detected by protection devices, can cause nuisance tripping.
Examples:
Figure M7 represents the current absorbed by a compact fluorescent lamp.
Ir.m.s. = 0.16A
IM = 0.6A
THDi = 145%
Crest factor = 3.75
Fig. M7 – Typical current waveform of a compact fluorescent lamp
Figure M8 represents the voltage supplying non-linear loads through a high impedance line, with a typical "flat top" distorted waveform.
Vr.m.s. = 500V
VM = 670V
THDu = 6.2%
Crest factor = 1.34
Fig. M8 – Typical voltage waveform in case of high impedance line supplying non-linear loads
