Explanation on Whether a Multimeter Can be Used to Measure the Output Signal of an Inverter
1. Usually, when we say that the output of an inverter is 380V and 50Hz, it means that its fundamental wave (sine wave) has a voltage of 380V and a frequency of 50Hz. The actual output waveform of the inverter is a Pulse Width Modulation (PWM) wave. In addition to the fundamental wave, it also contains a carrier signal. The frequency of the carrier signal is much higher than that of the fundamental wave, and it is a square wave signal, which contains a large number of higher harmonics.
2. An ordinary multimeter can generally only measure AC sine waves with a frequency range of 45~66Hz or 45~440Hz. The measurement frequency range of some true RMS multimeters is much wider. Many people think that they can be used for frequency conversion measurement and testing. In fact, this is not the case because the measurement results of this kind of meter include both the fundamental wave and the carrier wave. For example, for the above-mentioned inverter with a 380V output, the measurement result is generally above 400V.
3. An instrument used for frequency conversion testing should have the ability to decompose the fundamental wave from various PWM waveforms. For strict measurement, a digital signal processing method should be adopted, that is, obtain a sample sequence through high-speed sampling, and then perform a discrete Fourier transform on the sample sequence to obtain the amplitude and phase of the fundamental wave as well as the amplitudes and phases of various harmonics.
There is also an idea that the calibrated average value can replace the RMS value of the fundamental wave component in the PWM signal output by the inverter.
4. The quasi-average value (MEAN) is theoretically equal to the true RMS value of a sine wave and the RMS value of the fundamental wave of a sinusoidally modulated PWM waveform, and it is simple to implement. Therefore, MEAN is used in many instruments and meters to replace the measurement of the RMS value of a sine harmonic (RMS) or the RMS value of the fundamental wave of a PWM (H01).
However, in recent years, the frequency conversion speed regulation technology has been advancing rapidly, and the application of non-sinusoidally modulated PWM is increasing. Moreover, inverter users usually do not know what modulation mode their inverter uses, and the limitations of the MEAN value in PWM measurement are becoming more and more significant.
The AnyWay broadband power testing system provides users with a professional overall solution for frequency conversion measurement and testing, including sensors and instruments.
5. Based on high-speed sampling, AnyWay conducts a spectrum analysis of the sampled signal and calculates the RMS value of the fundamental wave (H01) of the measured signal in real time. This method is applicable to the RMS value testing of PWM signals with any modulation mode and other sine and non-sine signals.
