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Internal Frequency Converte...
3.3.3 Inverter Section
In the inverter section, gate signals are received from the
control card (through the MDCIC), and sent to each
inverter module's power card and the gate drive card to
3
3
the IGBT gates. (See illustration titledIntermediate and
Inverter Sections).The output of each IGBT, connected in
series, first passes 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 the illustration below. 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 frequency converter is capable of delivering a current
waveform that nearly replicates a true AC sine wave.
This waveform, as generated by the Danfoss VVC
principle at the control card, provides optimal performance
and minimal losses in the motor.
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High Power Service Manual for Modular F Frame Drives
Hall effect current sensors monitor the output current of
each inverter module and deliver proportional signals via
the power cards to the MDCIC where they are summed
and 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 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 middle IGBT module
provides heatsink temperature feedback for each inverter
module.
Illustration 3.9 Output Voltage and Current Waveforms
plus
PWM
®
MG90K202 - VLT
is a registered Danfoss trademark
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