2-23
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
The waveform of the commercial power supply will be distorted if the commercial power supply contains
excessive harmonics. Machines with such a commercial power supply will malfunction or generate ex-
cessive 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 current, however, contains harmon-
ics.
Design Chapter 2
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