Measuring the rectifier output voltage

Nov 03, 2008 5 Replies
K

I my plant we have a rectifier for electrolytic cleaning. 7500A and



30V(Max). We give the reference for current. The problem is when we measure the current through out PLC analog channel we find it fluctuating. The current waveform exactly follows the voltage waveform. The configuration is 2 half wave 3-phase rectifiers working in paralle through the interphase transformer. Should I add something to get an steady feedback. I suspect the fluctuation has to do with the scanning of the AI channel of the PLC. we should send the coltage as DC without harmonics rather than the way it is now.
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snipped-for-privacy@gmail.com schrieb:

Hi,

maybe the flutuations come from beat frequencies between the sampling device in the measuring unit and the mains frequency. To be sure to measure the Current (or voltage ) correctly, you have to integrate the signal over at least one period of the mains voltage, in Europe 20ms (50Hz mains frequency) or 16 2/3 ms in the US etc. To do that, you have to use an integrating measuring principle e.g. a dual slope A/D converter. I did that by multiplying the mains frequency by a factor that kept my A/D chip to measure exactly one period of my mains voltage.

hope i could help

Frank

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L

Filter the measurement or synchronize it to the phase period.

RL

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K

Actually there isn't much that I can do with the PLC analog channels( Alstom 8035 PLC) . Their scanning depends on the PLC internal clock that has nothing to do with AC frequency.

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M

Does the "rectifier" have any filtering?

If not, what you are seeing is correct.

The current waveform will follow the voltage waveform.

But for your application, it probably doesn't matter, what counts is the AVERAGE current. So you can read the average current with a meter or matematically figure the average current value of your waveform.

Mark

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S

What is the output of your current sensor? If it's something like

0-10V you may be able to add a simple low-pass filter to give you average current, and, if necessary, fiddle the value a bit to get it to agree with the instrumentation on your baths. If it's a 5A CT you may have to add signal conditioning ahead of the PLC analog inputs.

Best regards, Spehro Pefhany

"it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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