User Manual

3
■ What causes harmonic current
problems?
When harmonic currents flow through
the impedances of the power system,
they cause corresponding voltage drops
and introduce harmonics onto the vol-
tage waveform. This causes the system
voltage waveform to become distorted
and since this voltage is distributed to
other users on the power system, it
causes harmonic currents to flow
through otherwise linear loads. For
example, if the system voltage has a 5th
harmonic component and it is applied to
an induction motor, then some 5th har-
monic current will flow into the motor.
■ How conventional VSDs work
To understand harmonics, one must first
understand how a conventional VSD
works and how its power flows. Conven-
tional VSDs consist of a 6-pulse diode
rectifier, DC link capacitor, IGBT inverter
and a processor-based controller. The
diode rectifier is used to convert AC line
voltage to a constant fixed level DC vol-
tage. The DC link capacitor acts as a filter
to smooth the DC link voltage and help
keep it constant.
The inverter is used to convert the DC
link voltage to a variable voltage, vari-
able frequency 3-phase output for con-
trolling the speed and torque of an
induction motor and providing overload
capabilities necessary for high dynamic
motor performance. The controller is
used to supervise the operation of the
inverter as well as implement powerful
vector control algorithms to obtain opti-
mum dynamic performance from the
induction motor. This type of drive is
well suited for constant torque loads that
require high dynamic performance, such
as fast-acting speed- or position-control-
led applications. This type of VSD is often
used for HVAC applications even though
the high performance and overload
capabilities are not required.
Rectifier DC link Inverter Motor
Three
phase
power
C
+
0
2
4
6
8
10
12
14
16
5791113
Current (%)
Harmonic order
Power flow in a VSD single line diagram
Current harmonic spectrum up to the 13th fundamental
Harmonic results of a typical 6-pulse 3-phase rectifier