3.3.3 Inverter Section
In the inverter section (see Figure 3-7), gate signals are delivered from the control card, through
the power card and gate adjustable frequency 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 sen-
sors.
Once a run command and speed reference are present, the IGBTs begin switching to create the
output waveform, as shown in Figure 3-6. 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 approached
and wider the farther from zero crossing. The width is controlled by the pulse duration of applied
DC voltage. Although the voltage waveform is a consistent 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 adjustable frequency drive is capable of deliv-
ering a current waveform that nearly replicates a true AC sine wave.
This waveform, as generated by the Danfoss VVCplus 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.
During normal operation, the power card and control card are monitoring various functions within
the adjustable frequency 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 inside one of IGBT modules provides heatsink temperature feedback.
130BX136.10
Figure 3.6: Output Voltage and Current Waveforms
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
3 Internal Adjustable Frequency Drive Opera-
tion
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
3-9
3
Summary of Contents for VLT HVAC FC 100
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