GE Industrial Solutions AF-600 FP High Power Unit Sizes 6x - Manual (Page 40)

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 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 GE Advanced Vector Control PWM principle at the control card, provides optimal performance and minimal losses in the
motor.
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.
130BX136.10
Illustration 3.6: Output Voltage and Current Waveforms
High Power Service Manual for Unit Sizes 6x
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