Danfoss VLT series, Service Manual

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2-8
VLT is a registered Danfoss trademark
Inverter Section
In the inverter section (see Figure 2-7), gate signals are delivered
from the control card, through the power card and gate drive
card to the gates of the IGBTs. 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 Figure 2-6. Looking at the phase-to-phase voltage
waveform with an oscilloscope, it can be seen that the Pulse
Width Modulation (PWM) principal creates a series of pulses
which vary in width. Basically, the pulses are narrower as
zero crossing is neared and wider the farther from zero
crossing. The width is controlled by the pulse duration of applied
DC voltage. Though the voltage waveform is a consistant
amplitude, the inductance within the motor windings will serve
to average the voltage delivered and so, as the pulse width of
the waveform varies, the average voltage seen by the motor
varies as well. This then equates to the resultant current
waveform which takes on the sine wave shape that we expect
to see in an AC system. The frequency of the waveform is
then determined by the rate at which the pulses occur. By
employing a sophisticated control scheme, the drive is capable
of delivering a current waveform that nearly replicates a true
AC sine wave.
This waveform, as generated by the Danfoss VVC
plus
PWM
principle at the control card, provides optimal performance
and minimal losses in the motor.
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. These current
signals are used by the control card logic 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.
Figure 2-6. Output Voltage and Current Waveforms
During normal operation, the power card and control card
are monitoring various functions within the drive. The current
sensors provide current feedback information. The DC bus
voltage and AC line voltage are monitored as well as the
voltage delivered to the motor. A thermal sensor mounted on
the heatsink provides temperature feedback.