ET521A multi-function oscilloscope detection effect in VFD (Variable-frequency Drive) maintenance

Core Tip:

ET521A is an "automatic oscilloscope on-site comprehensive tester" developed and produced by Zhuhai Yiwan Electronic Technology Co., Ltd. It integrates the test functions often used in electrical maintenance, with 25M digital oscilloscope, 6600 code automatic range multimeter, LCR digital bridge and 60M frequency/duty cycle measurement, remote control/crystal oscillator detection, 10Hz~156kHz function signal output function. It also has automatic measurement and paperless recording functions, that is, it has a storage capacity of 40 DSO measurement waveforms, automatic measurement (voltage) records and stores 200 sets of data, and manual measurement data saves 200 sets.

First of all, its oscilloscope function is very digital and intelligent, changing the cumbersome adjustment link of traditional oscilloscopes during measurement. Just press the automatic measurement key, and the measured waveform will be automatically displayed on the screen through the calculation of the adaptive software, and the true effective value, frequency, period, positive and negative peak value and peak-to-peak value and other measurement results will be digitally read out. In order to observe and analyze the waveform of the measured signal from different angles during measurement, the amplitude and time base of the waveform can also be changed through the combined control of the function key and the coding switch. As the only rotary switch in the whole machine, the coding switch can not only quickly and conveniently adjust the relevant test parameters of the instrument, but also experience the retro rotation pleasure for friends who are used to using analog oscilloscopes.

VFD (Variable-frequency Drive) has many measurement points, and most of them are not grounded, so the shielded ground wire crocodile clip on the original probe is not very convenient during measurement. The author made a small modification. As shown in the figure below, the crocodile clip was replaced with a long plastic handle needle with a hook for shoes. It can be operated with one or both hands. The modification effect is excellent through actual use.

A large number of drive waveforms need to be measured during VFD (Variable-frequency Drive) maintenance, and the drive waveform of VFD (Variable-frequency Drive) is a PWM wave. Therefore, in order to make ET521A read the carrier frequency more accurately, the number of waveforms taken per screen should be 4-10, that is, a more reasonable scanning time base should be selected, and the drive PWM wave changes continuously with the change of time. At the same time, we must carefully observe the refresh on the screen to carefully identify the waveform that may have problems. As shown in the figure below, the drive waveform on Yaskawa E7 400V22KW measured using the automatic measurement function.

Using the automatic measurement function, you can observe the details of the constantly refreshed signal under test. If you need to observe a part of the waveform carefully, you can change the scanning time base to determine the number of waveforms displayed per screen (because the amplitude during automatic measurement is very reasonable, generally no need to adjust again). You can also use the pause function of the oscilloscope to freeze the waveform collected at the moment of pressing to observe. When the frozen waveform is not ideal, you can freeze it repeatedly to select the waveform you want to measure. If you want to analyze some details, especially some special fault signal waveforms, when you have mastered the various parameters of the measured signal through automatic or manual measurement, it is most suitable to use the single scan function. The captured fault waveform signal is located in the center of the screen with the trigger level as the starting point. You can also press and rotate the encoding switch to observe the waveform displayed in the buffer (2K can be collected each time), so that you can observe and analyze the fault waveform more clearly. The following is an example of the abnormal drive waveform measurement method on Mitsubishi F740 30KW. As shown in the figure below, it is when the amplitude of the measured fault signal exceeds +30V. In order to see its details clearly, the parameters of the single scan such as amplitude are set to 10V/grid, rising edge trigger, trigger level is set to +30V, and time base is set to 2.5μS.

The Mitsubishi F740 30KW drive circuit has such an abnormal drive waveform, and it was finally found that the performance of the drive optocoupler was aging. The waveform shown in the figure below is an abnormal drive waveform measured on a Delta VFD series VFD (Variable-frequency Drive) 18.5KW machine. In the end, it was found that the performance of the drive optocoupler was aged.

As shown in the two typical drive fault waveforms above, the drive optocoupler is also aged, and similar "overcurrent" fault codes are shown in each machine, but they show completely different fault waveforms. Of course, these details are also impossible to distinguish with an ordinary multimeter.

ET521A is known as a multi-function meter, and it does have many measurement functions. I will not introduce them one by one here, but only give a brief experience of the three most commonly used functions.

1. Automatic/manual measurement function of AC and DC voltage. The AC voltage of ordinary multimeters is only up to 750V. ET521A can measure AC and DC voltage up to 2000V, and automatically convert the full range. This provides convenience for measuring high voltages of about 1000V. In addition to the standard "true RMS" reading, it also has a peak detection (P-D) mode. At the same time, its voltage measurement function automatically records 200 sets of data, which is very convenient when measuring multiple voltage readings such as ICs online.

2. Resistance/on/off/diode measurement function. The author believes that its biggest feature is that the open circuit voltage of the diode range is as high as 3.2V, which is not much higher than the 1.5V or 2.8V open circuit voltage of ordinary multimeters, but it is enough to expand its measurement range. In addition to being able to measure the voltage drop of light-emitting tubes with higher forward voltage drops, it can also make the light-emitting tubes emit light, making it more intuitive to judge whether they are good or bad. More importantly, it will provide more valuable test data for measuring ICs. All ICs of the same model use exactly the same combination of PN junctions and resistors. Some pins that were originally measured as open circuits by ordinary multimeters provide higher open circuit voltage tests in ET521A.