wrong RMS measurement

wrong RMS measurement

Postby solelunauno » Wed Aug 26, 2015 4:37 pm

Dear Virtins support, I'm an electronic engineer and I valued your product "Multi-Instrument for sound card".
I downoloaded trial version and made some tests, and I can't understand why, according to me, RMS values are not correctly measured if signal waveform isn't a sinewave.
I made two different loads for a sinewave mains source (50Hz transformer): one is a simple resistor and the other is a rectifier, capacitor and resistor sized to make the same "RMS" current absorption from the transformer.
Voltage from the transformer was 29Vrms and direct resistor 25ohm, to obtain about 1,16A rms sinewave.
Capacitor was sized very high, in order to acquire low ripple (about 1%) and to be sure to measure with an hardware multimeter the real voltage and DC current.
The voltage @ rectified load was about 38V, so the resistor was sized higher (41ohm) in order to obtain same power of previous resistive load and therefore same RMS current before the rectifier @ AC source.
I uploaded the two wave measured; the instrument has not been calibrated, as this was only a test.
As you can see, the area of the two half period waves are approximately the same (as expected), but Virtins reported 17,6mV rms for the sinewave and 29mV rms for the peak wave, why?
Can you justify that?
Thanks
Attachments
rectified_load.jpg
This is a current wave from a rectified and capacitor filtered load applied to same 50Hz sine source of previous test and sized tu be about the same RMS load of previus simple resistor.
rectified_load.jpg (253.93 KiB) Viewed 13007 times
sinewave.jpg
This is a sinewave current from a direct resistor load @ mains supply (50Hz)
sinewave.jpg (252.46 KiB) Viewed 13007 times
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Re: wrong RMS measurement

Postby VirtinsTech » Mon Aug 31, 2015 12:47 am

Thank you for your question.

1. There is a little difference in power consumption between the two cases.
Power consumption of case 1: 29x29/25=33.64 (W)
Power consumption of case 2: 38x38/41=35.21 (W)

Of course, the difference is not big enough to explain the difference in the RMS value of current measured at the transformer.

2. Not sure how your rectifier works. Is the second half of the waveform nearly zero or just an inverted shape of the first half. If the former is the case, then the energy conservation equation will have to be established over at least one cycle. Currently only half of the cycle is measured.

3. The areas of the two curves are similar would mean that their mean values are similar, not RMS values.
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