The role of multi-function oscilloscope in the analysis and measurement of frequency converter

　　Overview of the role of multi-function oscilloscope in frequency converter analysis and measurement

　　As a speed regulating device, intelligent inverter is widely used in various motor controls, such as metallurgy, papermaking, electronic product assembly and other production lines, due to its versatility, high reliability and high energy saving. Once the inverter fails during production, especially when a board-level failure occurs, it is sometimes difficult to make a correct judgment based on experience alone. At this time, it is necessary to use high-performance test instruments for fault analysis. The Ulead portable multi-purpose oscilloscope integrates an oscilloscope, a multimeter, and a paperless recorder, and has a bandwidth of 60MHz. It is an ideal test and maintenance tool for modern power electronic devices.

　　Working principle of general frequency converter

　　The universal inverter uses industrial frequency AC power to rectify into DC power, and then inverts the DC power into three-phase AC power with continuously adjustable frequency and voltage, that is, AC-DC-AC mode. The so-called "universal" contains two meanings: one is that it can be used with universal cage asynchronous motors; the other is that it has a variety of optional functions and is suitable for various loads of different natures.

　　The control circuits of modern pulse width modulation (PWM) inverters are mostly digital circuits with microprocessors as the core. Their main function is to accept various setting information and instructions, and then form PWM signals to drive the inverter according to their requirements.

　　Waveform analysis and measurement of frequency converter

　　The multi-function oscilloscope can be used to detect and analyze the gate PWM drive signal of the inverter (UI) part of the inverter, as well as the output voltage waveform. The following uses the inverter series of Beijing Jintai Keyi Testing Instrument Co., Ltd. as a reference.

　　1. First, connect the dedicated grid adapter to the input channel of the oscilloscope. Then connect the three grid connectors (X9, X11, X13) to the corresponding grid slots on the power board.

　　2. Use a DC power supply to power the power board and control board to be tested, connect the oscilloscope to the test point, and use the monitoring software to observe the SPWM waves of the three phases U, V, and W. When the voltage of the DC power supply line reaches the rated value, the gate voltage of the three-phase upper and lower half bridges of the IGBT thyristor should be around -12V to cut off the IGBT.

　　3. Then run at 0Hz and observe whether the modulation frequency of the SPWM wave of each phase upper and lower half bridge is normal (this frequency is different for different power boards, and this board shows 733.7Hz).

　　4. Adjust the 400V AC power supply to a given frequency of 50Hz and observe the PWM output voltage waveform. From the waveform, it can be seen that the ripple is relatively small and the output is relatively stable.

　　5. Finally, use the provided test report macro template to automatically generate a valuable test analysis report.