Siemens DTU3005-B Specifikace stáhnout pdf (Strana 23)

When a harmonic waveform is superimposed on the

fundamental sine wave a distinctive waveform is produced. In

this example, the third harmonic is seen superimposed on the

fundamental frequency. The problem of waveform distortion

becomes more complex when additional harmonics are

present.

Total Harmonic Distortion Harmonic distortion is a destructive force in power distribution

systems. It creates safety problems, shortens the life span of

transformers, and interferes with the operation of electronic

devices. Total harmonic distortion (THD) is a ratio of harmonic

distortion to the fundamental frequency. The greater the THD

the more distortion there is of the 60 Hz sine wave. Harmonic

distortion occurs in voltage and current waveforms. Typically,

voltage THD should not exceed 5% and current THD should not

exceed 20%. Some of the power meters offered by Siemens

are capable of reading THD.

Phasors Phase rotation describes the order in which waveforms from

each phase cross zero. Waveforms can be used to illustrate this

relationship. Phasors consist of lines and arrows and are often

used in place of waveforms for simplification.

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 87 88

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Siemens DTU3005-B Specifikace Strana 23