AEMC 721 User Manual page 16

to the supply of the harmonic-producing device. Harmonics can interact
within the system through direct system connections or even through
capacitive or inductive coupling.
A harmonic may be defined as an integer multiple of a fundamental fre-
quency. Harmonics are designated by their harmonic number. For our
discussion, we will focus on the 60 Hz power frequency. The second har-
monic would be two times the fundamental or 120 Hz. The third would be
three times the fundamental or 180 Hz, and so on.
Nonlinear equipment generates harmonic frequencies. The nonlinear
nature of a device draws current waveforms that do not follow the voltage
waveform. Electronic equipment is a good example. While this is a broad
category that encompasses many different types of equipment, most of
these devices have one characteristic in common: They rely on an internal
DC power source for their operation.
LOADS WHICH PRODUCE HARMONIC CURRENTS:
• Electronic lighting ballast
• Electric arc furnaces
• Electric welding equipment
• Industrial process controls
• Saturated transformers
• Medical equipment
This is by no means an exhaustive list of equipment which generate har-
monics. Any electronic-based equipment should be suspected of produc-
ing harmonics.
Due to the ever increasing use of electronics, the percentage of equip-
ment which generates harmonic current has increased significantly. The
harmonic problem manifests itself with proliferation of equipment using
diode-capacitor input power supplies. This type of equipment draws cur-
rent in a short pulse only during the Peak of the sine wave. The result of
this action, aside from improved efficiency, is that high frequency harmon-
ics are superimposed onto the fundamental 60 Hz frequency.
The harmonics are produced by the diode-capacitor input section which
rectifies the AC signal into DC. The circuit draws current from the line only
during the peaks of the voltage waveform, thereby charging a capacitor to
the Peak of line voltage. The equipment DC requirements are fed from this
capacitor and as a result the current waveform becomes distorted.
Harmonic Meter Model 721
• Adjustable speed drives
• Personal computers
• Solid state rectifiers
• UPS systems
• Solid state elevator controls
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