Measuring 3 Phase Voltage With Arduino - Page 1
Forum section: electronics / projects

author: ZeTeX
author_rank: Frequent Contributor
timestamp: March 26, 2021, 11:35:57 am
content: Hello,I have a 3 phase line voltage I need to monitor using arduino, for a data logger I am designing.The goal is to measure line to neutral rms voltage (220v) without utilizing expensive components or transformers.Neutral line might not be available.The first challenge is making virtual neutral, this can be done via resistors, however they will dissipate a lot of power, and if you would use high value resistors, you wont be able to use voltage dividers as they would cause a drop.Does anybody know a solution?Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: March 26, 2021, 12:35:17 pm
content: You've been around a while so I'll *ASS*U*ME* you are competent to design divider chains for mains measurement with adequate creepage and clearance, respecting individual resistor voltage ratings, and verifying that their frequency responce is sufficient.Why not use three phase-to-ground divider chains each greater than 2 Meg, with sufficient resistors in the upper arm to keep the worst case current under 0.2mA even if a couple of resistors short out, buffered by OPAMPs then derive the Neutral reference for your differential ADC on the ground referenced low voltage side, by a resistor star between the three phase buffer OPAMP outputs, then put it through another buffer OPAMP.Another way if you need to read all phase voltages relative to the true Neutral if its present, would be to add a fourth identical divider chain, feeding the +In of the three buffer OPAMPs and configure them as inverting differential amplifiers. (i.e. divider tap to -In, divider low end to Out). Because their +In draws negligible current they can share the Neutral divider. If Neutral isn't present, it drops back to the inverting version of my first proposal.The resulting ground leakage current should be acceptable even if its inadvertently connected to a 480V corner grounded delta supply running 10% high.

author: jonpaul
author_rank: Super Contributor
timestamp: March 26, 2021, 12:50:28 pm
content: Hello: A Dangerous practice to mix instruments esp with digital/CPU direct on 240V 3 ph line.Possible damage to equipment, ground currents or exposure of users to shock.A three phase line is never perfectly balanced in practice due to neutral harmonics and load imbalance. Thus each phase may have different currents and slightly differet voltages.An accurate measurement eg voltage, current VARS, VA or power required that each phase be measured indicvidulayy and not by a phantom neutral with a resistive divider etc.Use a wye to delta conversion if the source is wye config.A simple 3 ph transformer provides stepdown, safety isolation and WYE><Delta conversion if needed.Three separate single phase transformers with 3-20 VA ratings, the common LV power transformers can be calibrated with a specific resistive load.Signal Transformer and many other vendors make 240V (or dual pri) to 5, 10 or any desired LV stepdown.For precise measurements use a "potential" transformer whose ratio calibrated for the rated voltage and frequency eg 240V : 5VAny electronic /electrical measurements book has details on three phase measurements and conncetions.Kind Regards,Jon