Danfoss VLT FC Series, Руководство по обслуживанию

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5.3.3 Inverter Section
In the inverter section (see Illustration 5.10 and
Illustration 5.11 ), gate signals are delivered from the control
card, through the power card and gate drive card to the
gates of the IGBTs. In parallel drive systems, these signals
from the individual modules are delivered to the multi-
drive control interface card (MDCIC) on the control shelf.
The series connection of each set of IGBTs is delivered to
the output, first passing through the current sensors.
Once a run command and speed reference are present, the
IGBTs begin switching to create the output waveform, as
shown in
Illustration 5.9 . Looking at the phase-to-phase
voltage waveform with an oscilloscope, the pulse width
modulation (PWM) principal creates a series of pulses
which vary in width.
Basically, the pulses narrow as they approach 0 crossing
and grow wider when further away from 0 crossing. The
pulse duration of applied DC voltage controls the width.
Although the voltage waveform is a consistent amplitude,
the inductance within the motor windings averages the
voltage delivered. As the pulse width of the waveform
varies, the average voltage that the motor detects also
varies. The resulting current waveform takes on the sine-
wave shape common to an AC system. The rate at which
the pulses occur determines the frequency of the
waveform. By employing a sophisticated control scheme,
the drive delivers a current waveform that nearly replicates
a true AC sine-wave.
Hall effect current sensors monitor the output current and
deliver proportional signals to the power card where they
are buffered and delivered to the control card. In parallel
drive systems, these signals pass through the MDCIC on
the control shelf. The control card logic uses these current
signals to determine proper waveform compensations
based on load conditions. They further serve to detect
overcurrent conditions, including ground faults and phase-
to-phase shorts on the output.
During normal use, the power card and control card are
monitoring various functions within the drive. The current
sensors provide current feedback information. The DC bus
voltage is monitored along with the voltage delivered to
the motor. A thermal sensor mounted inside each IGBT
module provides heat sink temperature feedback.
130BX136.10
Illustration 5.9 Output Voltage and Current Waveforms
Internal Drive Operation
VLT
®
FC Series, D1h–D8h, Da2/Db2/Da4/Db4, E1h–E4h, J8/J9
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