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H Harmonics
D Definition
Harmonics consist of electric power produced from AC power and alternating at fre-
quencies that are integral multiples of the frequency of the AC power.
The following frequencies are harmonics of a 60- or 50-Hz commercial power supply.
Second harmonic: 120 (100) Hz
Third harmonic: 180 (150) Hz
Second harmonic (120 Hz)
Basic frequency (60 Hz)
Third harmonic (180 Hz)
Problems Caused by Harmonics Generation
Thewaveformofthecommercialpowersupplywillbedistortedifthe commercialpower
supply contains excessive harmonics. Machineswith such acommercial powersupply
will malfunction or generate excessive heat.
Basic frequency (60 Hz) Third harmonic (180 Hz)
Distorted current wave
form
D Causes of Harmonics Generation
·Usually, electric machines have built-in circuitry that converts commercial AC power
supply into DC power.
Such AC power, however, contains harmonics due to the difference in current flow
between DC and AC.
Obtaining DC from AC Using Rectifiers and Capacitors
DC voltage is obtained by converting AC voltage into a pulsating one-side voltage with
rectifiers and smoothing the pulsating one-side voltage with capacitors. Such AC cur-
rent, however, contains harmonics.
Design Chapter 2
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