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Measuring AC current from a phase angle controller

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Hi all, I am using a Watlow SCR power controller in phase angle mode to control five small heaters hooked up in parallel at 240VAC drawing no more than 1000W total. The heaters run at low power so the sinewave out will be 25% or lower in the phase angle output. I have cDAQ and want to use two current input modules NI 9246 or NI 9247  with the current from each heater running directly through each channel or through a current transformer for each of the heaters. My software is a Labview executable that i cannot change and will read Amps or A-rms only and I have no way to process the phase angle power signal. My question is will I get an accurate amperage measurement Vrms from this setup? If not what other method would work ie a 240V Variac to manually power the heaters - it should have a clean sinewave output. Please advise.


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Ooops I meant "My question is will I get an accurate amperage measurement Amp-rms or Amp from this setup?" 

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Accepted by topic author Kennerd

This is basically a question of "How sophisticated is the software I cannot change?". I suggest you purchase a True_RMS multimeter, and compare it's readings with your existing set-up at various power settings.


Be cautious around 240 volts. Make sure someone certified & trained checks your wiring. If you do use current-transformers be aware of the high voltages that can be produced if their secondaries are open-circuit.

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Thanks for the good advice. Unfortunately I cannot build this system to find out that our executable cannot process the waveform correctly as we need high accuracy to calculate power on the fly. I'm sure the Variac auto transformer would have been the ideal solution to get a good true RMS reading but engineering department decided to go with DC power supplies instead for some reason so I cannot follow up with any findings of errors.


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