GE Industrial Solutions AF-600 FP High Power (460V_575V 150HP and above) - Manual (Page 40)

3.3.3 Inverter Section
In the inverter section (see Illustration 3-7), gate signals are delivered from the control card, through the power card and gate frequency converter 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 3-6. 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 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 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 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 one
of IGBT modules provides heatsink temperature feedback.
130BX136.10
Illustration 3.6: Output Voltage and Current Waveforms
AF-650 GP and AF-600 FP High Power Service Manual
39
3