So, I have a project where I have to read 3-phase 380V AC voltage using
Arduino MKR WAN 1310. The board is running on 3.3V. If anyone has any suggestions I would love to hear them. Thanks
Are there any safety constraints such as requiring a transformer to reduce the voltage to something under 50v?
First, determine the PEAK voltages, because that is what you have to deal with. Then determine if your three-phase circuit is a DELTA configuration or a Y configuration. Safety concerns both answers.
If you are not experienced in power at these voltages get a mentor that is or abandon the project. If is simple mistake is mane you can destroy a lot of equipment and more importantly hurt or kill somebody. If it were me and since I like living I would use three separate transformers. Connect them in a delta configuration on the primary side they you will have a much lower voltage to work with. You will need to be cautious of phase relationships if you do more then measure phase to phase voltage. If it is phase to ground then you need to connect them in a Y configuration on the primary side.
Lethal voltages should be electrically isolated from you, the Arduino, and your computer/USB port, etc.
Some people want to use a voltage divider to lower the voltage but it's not guaranteed safe and it's generally illegal to sell something that works that way.
If you build something and put it in an insulated box with no outside connections (like a meter that has a built-in display) or something with optically isolated connections, then it's safe and it's OK to sell something like that. But if you need to connect it to USB during development or testing, that's a problem unless you've built-in optical isolation for the USB port.
There are basically three ways to get isolation and still get some kind of "signal communication".
A transformer is best if you want to measure the voltage because the secondary voltage is proportional to the primary voltage. For 3-phase you need a 3-phase transformer, or 3 transformers.
Transformers are AC and the Arduino can be damaged by negative voltages so it has to be rectified or biased, etc.
Optical isolation is another option but it's non-linear, so not the best for measuring voltage.
Relays are electrically-operated and electrically-isolated switches but they are only for on-off.
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I would suggest safety first. 380VAC from the grid is dangerous. From the technical point of view:
- You need an isolation transformer, that safely isolate you and the microcontroller from the grid. The output (VA) to be kept low. Fused at both sides. Ratio f.eks 380:1, output 0,05VA.
2.You can use different ratio and use a trimable voltage divider for fine tuning, but keep the sec. voltage as low, as it lower the risk of damge of the electronics.
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Now, you have a usable voltate to read. Since most ADC can not read negative voltage, you need a op amp circuit for level shifting of the zero crossing.
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The voltage you read, is the peak to peak value. So you need to calculate the RMS voltage. First, sample the sinus over a time period. Then square the measurements, sum them and find then mean voltage by divide by the number of samplings. Then take the root. You now have the rms voltage, that a good multimeter will give you on the display.
So, it is a challange.
The peak voltage is 380V, each line is 220V, and the configuration is delta I believe but I might be wrong. It's a reefer container, so if this is very important (the configuration) I can get further details. I just need to read the fluctuation of the voltage running through the cable wire that powers the container.
Yeah, kind of important. With a "Y" configuration, the common will be connected to the system ground at the circuit breaker box. So you have a real zero crossing point between one hot lead and common/ground.
With a "delta" configuration. each of the three leads shares two voltage/current circuits. So there will never be a zero crossing of the AC sine wave.
There is a zero cross between two phases. In this case ground is not a referencer point. This will appear on the output of the transformer connected between any two phases, it will be a sine wave unless lots of transients, phase switching, etc on the power feed. The reference point is determined by how the secondaries are interconnected, this is independent and isolated from the primary. The home electrical systems in the US eventually are sourced with 3 phase power. The three phases are 120 degrees out of phase with each other that is why we see different voltages between a delta or Y connected with the same transformers. An important difference between the delta configuration and the wye configuration is the number of nodes: the delta has three (i.e., one node for each phase), and the wye has four. The fourth node in the wye network allows for the connection of a neutral wire.
And there is the problem trying to measure something with an Arduino the reports the voltage value in reference the the Arduino ground.
There is lots of information about measuring electrical power at https://openenergymonitor.org/
Have a look at this section in particular: https://docs.openenergymonitor.org/electricity-monitoring/index.html
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VERY good reading. Especially this part: "The emonTx4 can be used on a 3-wire 220 V system, but it will be necessary to construct a special “emonVs.voltage_sensor_3-wire-220V” version in which the 3 ZMPT101B transformers and their voltage multiplier resistors are connected in delta, rather than in star as in the standard emonVs. Anyone contemplating this should take local professional advice.".
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