According to the general view, the output voltage of the inverter is 380 volts, and there should be no problem in directly implementing the measurement. However, in fact, most multimeters cannot get correct readings when testing the output voltage of the frequency converter, and in some cases the multimeter may burn out directly.
For example, I had a problem testing an ABB drive with a 30 kW water pump in 2014. The drive has no input and output filters, and the drive is installed in the control room and is linked to the pump via a 20-meter-long cable. Using the domestic 9025 ordinary multimeter in 700 volts to test the three relative ground voltages, the result multimeter reading overloaded, displayed as 1.
After a while, the multimeter's measuring wheel broke down. In order to test the cause of the failure, the use of a high voltage probe with a withstand voltage of 2500 volts, the waveform was tested with an oscilloscope, and the peak voltage of the three terminals of the pump was found to exceed 2000 volts, so the multimeter breakdown was easy to understand.
After testing with an American brand of Omega multimeter with true rms function, the voltage rms value of the pump terminals is normal. At present, many brands of multimeters already have low-pass filter functions, and also declare that they can test true RMS values. However, the maximum AC range of these multimeters is only 700 volts, and the safety of the test cannot be guaranteed. Some imported brands of multimeters have passed the 8,000-volt overpressure test, which can ensure the safety of the test, but the price of these multimeters is more than a few thousand yuan, and the price is expensive.
If the inverter has been installed with an output sine wave filter or DV/DT filter, the voltage at the load end will basically not rise abnormally, and the safety of testing with a multimeter is greatly improved. Because the filter price is high and the footprint is large, most of the frequency converters are not installed.
Final conclusion
1, usually we say that the inverter output of 380V, 50Hz, refers to its fundamental wave (sine wave) is 380V, 50Hz. The actual output waveform of the inverter is a PWM wave, which contains a carrier signal in addition to the fundamental wave. The carrier signal is much more frequent than the fundamental wave and is a square wave signal that contains a large number of higher harmonics.
2. Ordinary multimeters can generally only measure AC sine waves of 45~66Hz or 45~440Hz. Some true RMS multimeters have a much wider range of measurement frequencies, and many people think that they can be used for variable frequency measurements and tests. This is not the case, because this kind of table measurement results include both the fundamental wave and the carrier wave. For example, the above-mentioned inverter, when the output is 380V, the measurement result is generally above 400V.
3. The instrument for frequency conversion test should have the ability to decompose its fundamental wave in various PWM waveforms, and strict measurement needs to use digital signal processing, that is, high-speed sampling to obtain the sample sequence, and then discrete Fourier transform the sample sequence to obtain the amplitude of the fundamental wave, the phase and the amplitude and phase of each harmonic.
4. There is also a way of thinking that the calibration average can replace the rms of the fundamental component in the PWM signal output by the frequency converter. The quasi-mean (MEAN) is theoretically equal to the true rms of the sine wave and the fundamental rms of the sinusoidal modulation PWM waveform, and is simple to implement.
Therefore, MEAN is used in many instrumentation to replace the rms value of positive harmonics (RMS) or the rms value of the fundamental wave of PWM (H01). However, in recent years, the frequency conversion speed regulation technology has been changing with each passing day, and the application of non-sinusoidal modulation PWM has become more and more. Moreover, drive users often do not know what modulation mode their drives use, and MEAN values are increasingly limited in PWM measurements.
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