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It is generally believed that the inverter output voltage is 380 volts, and direct measurement should not be a problem. However, in reality, most multimeters cannot obtain correct readings when testing the inverter output voltage, and in some cases, the multimeter may even burn out.
For example, in 2014, I encountered an issue while testing an ABB inverter that drives a 30 kW water pump. This inverter did not have input and output filters and was installed in a control room, linked to the water pump by a 20-meter long cable. Using a domestic 9025 general multimeter in the 700-volt range to test the three-phase to ground voltage, the result was an overload reading on the multimeter, displaying 1. After a while, the multimeter’s range dial broke down. To test the cause of the failure, I used a high-voltage probe rated for 2500 volts and tested the waveform with an oscilloscope, finding that the peak to ground voltage at the three terminals of the water pump exceeded 2000 volts, making the breakdown of the multimeter understandable.
Later, I tested with an American brand Omega multimeter that has true RMS functionality, and obtained normal effective voltage readings at the water pump terminals. Currently, many brands of multimeters have low-pass filter functions and claim to be able to measure true RMS values. However, the maximum AC range of these multimeters is only 700 volts, and safety during testing cannot be guaranteed. Some imported brands of multimeters have passed over-voltage tests of 8000 volts, ensuring testing safety, but these multimeters are priced above several thousand yuan, making them expensive.
If the inverter is already installed with an output sine wave filter or DV/DT filter, the voltage at the load end will generally not rise abnormally, greatly improving the safety of testing with a multimeter. However, due to the high cost and large footprint of filters, most inverters do not have them installed.
Final Summary
1. Typically, when we say the inverter output is 380V, 50Hz, we refer to its fundamental wave (sine wave) being 380V, 50Hz. The actual output waveform of the inverter is PWM, which, in addition to the fundamental wave, also contains carrier signals. The frequency of the carrier signal is much higher than that of the fundamental wave and is a square wave signal containing a large number of higher harmonics.
2. Ordinary multimeters can generally only measure AC sine waves in the range of 45~66Hz or 45~440Hz. Some true RMS multimeters have a much wider measurement frequency range, leading many to believe they can be used for frequency conversion measurements. However, this is not the case, as these meters include both the fundamental wave and the carrier in their measurement results. For instance, in the mentioned inverter, when the output is 380V, the measurement result is generally above 400V.
3. Instruments used for frequency conversion testing should have the ability to decompose the fundamental wave from various PWM waveforms. Strict measurements require the use of digital signal processing methods, which means high-speed sampling to obtain a sample sequence, followed by discrete Fourier transformation to obtain the amplitude and phase of the fundamental wave and the amplitudes and phases of each harmonic.
4. There is also a perspective that calibrating the average value can replace the effective value of the fundamental component in the PWM signal output by the inverter. The mean value (MEAN) theoretically equals the true RMS of a sine wave and the effective value of the fundamental wave of a sine-modulated PWM waveform, and it is simple to implement; hence, MEAN is used in many instruments to replace the effective value of positive harmonics (RMS) or the effective value of the fundamental wave of PWM (H01) measurement. However, in recent years, frequency conversion speed regulation technology has advanced rapidly, and the application of non-sine-modulated PWM has become increasingly common. Moreover, inverter users often do not understand the modulation mode used by their inverters, leading to greater limitations of the MEAN value in PWM measurements.
Therefore, from a safety perspective, never use a multimeter to test the inverter output voltage.
Source: Internet, copyright belongs to the original author, infringement will be deleted
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