Because the output voltage of the inverter (Variable-frequency Drive, VFD) is generally 380 volts, there should be no problem in direct measurement, but in practice, most multimeters cannot get a correct reading when testing the output voltage of VFD, and in some cases the multimeter may be burned directly.
Assuming that the VFD outputs a sine wave filter or DV/DT filter from the front device, the voltage base at the load end will not rise abnormally, and the safety test with a multimeter is greatly improved. Because of the high price of the filter and the large footprint of the filter, most VFDs do not have a device.
1. Generally speaking, the VFD output is 380V and 50Hz, which means that its fundamental wave (sine wave) is 380V and 50Hz. The output waveform of the VFD practice is a PWM wave, which includes a carrier signal in addition to the fundamental wave. The carrier signal frequency is much higher than the fundamental wave, and it is a square wave signal, including many higher harmonics.
2. Generally, the multimeter can only measure the AC sine wave 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 frequency conversion measurements and tests. Not really, because this kind of table measurement includes both fundamental and carrier waves. For example, in the case of the above-mentioned VFD, when the output is 380V, the measurement result is generally more than 400V.
3. The appearance used for frequency conversion test should have the ability to compose its fundamental wave in various PWM waveforms, and the method of digital signal processing is required for severe measurement, that is, high-speed sampling to obtain the sample sequence, and then stop the discrete Fourier transform of the sample sequence, so as to obtain the amplitude, phase and the amplitude and phase of each harmonic of the fundamental wave.
4. There is also an idea that the calibration uniform value can replace the effective value of the fundamental component in the VFD output PWM signal. Theoretically, the quasi-uniform value (MEAN) is equal to the true RMS value of the sine wave and the fundamental RMS value of the sinusoidal modulated PWM waveform, which is simple to complete. Therefore, MEAN is used to measure the fundamental rms (H01) rather than the RMS of positive harmonics (RMS) or the fundamental rms of PWM. However, in recent years, with the rapid development of frequency conversion speed regulation technology, the application of non-sinusoidal modulated PWM has become more and more. In addition, drive users generally do not know what kind of modulation each drive manufacturer uses.
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