I’ve been looking to use a EmonTX-type set-up to monitor AC currents coming out of a VFD. I originally thought I’d be able to use a CT sensor and the same code but a little more research has come up with a number of potential challenges.
The frequency range of a VFD. Our VFD operates at frequencies as low as 30Hz. I understand CT sensors don’t work at low frequencies due to saturation of the iron core. Would this frequency still be ok? And would the EmonLib library still work, or is the method of current calculation dependent on the frequency being 50-60Hz (this is what my research seems to indicate, I just wanted to check). In which case, if I adapted the code, is this approach vaguely feasible?
The VFD outputs a simulated AC signal, from PWM voltage. The current is smoothed by the motor inductance (which the VFD is driving), and I was hoping this would be close enough to a sine wave for us to measure relatively accurately. I know that due to the slightly jagged shape, I need to use True RMS. Does this existing EmonLib code use True RMS, or will I need to write my own code? I’m intending to low pass the CT output with a capacitor, as recommended elsewhere on this site, with a 10uF capacitor to ground. Reckon that’d be enough?
I’ve read that VFD output conductors are unshielded at the point where I’d be measuring them, resulting in extra noise and interference. Some people said meaningful measurements from a CT were impossible to achieve, but one guy said they wrapped the cables with brass foil and soldered a short braid between foil and the cabinet ground rail, which worked. I don’t think my understanding of EMI shielding is good enough that I’d be able to diagnose and fix this in practice. Any tips on how to reduce noise would be amazing. I was just going to have the microcontroller fixed to the wall near the cables, with the CT sensor clamped over the cable.
(For context, the VFD we’re using is a Grundfos RSI. We could measure at the inputs to the VFD instead, as this may be more accurate and less noisy, taking DC voltage and DC current measurements, but I’ve had a whole host of other challenges there, which I’d love to discuss if anyone is knowledgeable. My technique for measuring DC voltage was to use a potential divider, which I’m concerned is not electrically isolated and the voltages could be up to 1000V. I’ve also been having struggles working out how to safely and effectively connect to the ACS hall effect current sensor. See ACS770 Hall Effect Sensors - #16 by Natashaxh )