User Manual

4
Harmonic considerations
for HVAC applications
■ Impact of harmonics on the
line current
A characteristic of the 6-pulse diode
rectifier is that the current drawn from
the AC line is nonlinear, meaning that
the current waveform is not sinusoidal.
This is caused because the rectifier
diodes can only conduct current when
the instantaneous input line voltage is
greater than the DC link voltage. Since
the DC link voltage is held at a high and
nearly constant level by the action of the
DC link capacitor, the diodes only con-
duct current near the peak of the input
voltage waveform. This causes the line
current to be narrow with high ampli-
tude current pulses that charge the DC
link capacitors on a periodic basis. The
resulting AC line current drawn from the
power system has a high amount of har-
monic current. At the input terminal of a
standard VSD, harmonics current can be
120 % to 130 % Total Harmonic Distortion
(THD). The current waveform diagram
demonstrates the affects of harmonics
on the waveform.
■ Common problems that arise from
harmonics
Harmonics can create many problems in
a facility. They can cause additional
motor heating as well as higher RMS
currents through connected transfor-
mers and feeder equipment. Sensitive
equipment such as instrumentation,
computers and communications systems
may fail to function correctly or get
damaged. In severe cases of voltage dis-
tortion, in addition to equipment break-
downs or malfunctions, harmonics can
add costs in oversizing transformers to
accommodate a perceived or false load
requirement that is reflected back onto
the power line.