Service manual

3.3.3 Inverter Section
In the inverter section (see Illustration 3.8), gate signals are
delivered from the control card, through the power 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 Illustration 3.8. Looking at the phase-to-phase
voltage waveform with an oscilloscope shows 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 pulse duration of applied
DC voltage controls the width. Although the voltage
waveform is a consistent amplitude, the inductance within
the motor windings 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 expected in an AC system. The pulse
rate determines the waveform frequency waveform. By
employing a sophisticated control scheme, the frequency
converter can deliver 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. The control
card logic uses these current signals to determine proper
waveform compensations based on load conditions. They
further serve to detect over current conditions, including
earth faults and phase-to-phase shorts on the output.
During normal operation, the power card and control card
are monitoring various functions within the frequency
converter. The current sensors provide current feedback
information. The DC bus voltage and mains voltage are
monitored as well as the voltage delivered to the motor. A
thermal sensor mounted inside the sixpack IGBT module
provides heatsink temp feedback for the inverter. This
applies to unit up to and including H6 and I6 frame sizes.
For H7-H8 and I7-I8 frame sizes, the inverter consists of 3
dual IGBT modules and the thermal sensor is mounted
separately on the heatsink.
130BX136.10
Illustration 3.7 Output Voltage and Current Waveforms
Internal Frequency Converte...
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HVAC Basic Drive Service Manual
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