Does a multimeter measure the output signal of a frequency converter
To determine the measurement purpose and the measurement principle of the multimeter:
1. When we usually refer to a frequency converter outputting 380V, 50Hz, it means that its fundamental wave (sine wave) is 380V, 50Hz. The actual output waveform of the frequency converter is PWM wave, which includes not only the fundamental wave but also the carrier signal. The frequency of the carrier signal is much higher than that of the fundamental wave, and it is a square wave signal containing a large number of high-order harmonics.
2. A regular multimeter can generally only measure AC sine waves at 45-66Hz or 45-440Hz. The measurement frequency range of some true effective value multimeters is much wider, and many people believe that they can be used for frequency conversion measurement and testing. Actually, it's not because this type of meter includes both the fundamental wave and the carrier wave in its measurement results. For example, when the frequency converter outputs 380V, the measurement result is generally above 400V.
3. The instrument used for frequency conversion testing should have the ability to decompose its fundamental wave from various PWM waveforms. Strict measurement requires the use of digital signal processing, which involves high-speed sampling to obtain a sample sequence, and then performing discrete Fourier transform on the sample sequence to obtain the amplitude, phase, and amplitude and phase of each harmonic of the fundamental wave.
4. There is also a way of thinking that calibrating the average value can replace the effective value of the fundamental component in the PWM signal output by the inverter.
Mean is theoretically equal to the true effective value of a sine wave and the fundamental effective value of a sine modulated PWM waveform, and is easy to implement; Therefore, MEAN is used in many instruments to replace the measurement of the effective value (RMS) of positive harmonics or the fundamental effective value (H01) of PWM.
However, in recent years, the technology of variable frequency speed regulation has been advancing rapidly, and the application of non sinusoidal modulation PWM has been increasing. Moreover, frequency converter users usually do not know what modulation mode their frequency converter adopts, and the local limitation of MEAN value in PWM measurement is becoming larger and larger.
5. The AnyWay broadband power testing system provides users with a comprehensive solution for frequency conversion measurement and testing, including sensors and instruments.
AnyWay performs spectral analysis on sampled signals based on high-speed sampling, and calculates the fundamental effective value (H01) of the measured signal in real time. This method is suitable for testing the effective value of PWM signals with any modulation method and any other sine or non sine signals.
AnyWay uses fundamental effective value testing as the basic testing mode, and provides measurement modes such as true effective value (RMS), calibrated average value (MEAN), rectified average value (RMEAN), arithmetic average value (DC, mainly used for DC measurement) for users' reference.
