Hi to all,
I have a three phase alternator which is installed on my motorbike and I would like to use Arduino to read the voltage across each phase.
The problem is that I only have three wires coming from the stator and I do not have any neutral.
I searched on google and there are some examples with transformers but they suppose that there is a phase and a neutral wire while I just have three phases.
Is it possible to read the voltage with Arduino?
Did you check the frame of the alternator? Where are the rectifiers for your alternator?
There is a voltage regulator that convert the AC to DC.
My goal is to have a way to read the voltage from the stator (before the voltage regulator) by using Arduino without having to modify the original system.
I mean, I want to be able to ride my motorbike while using Arduino to log the voltage values generated by the stator coils.
Strange, but it's your project. DO you know the PEAK AC VOLTS for each phase? You have to know that before doing anything else. Use your DVM on the AC scale to measure the AC volts. That will give you the RMS voltage. Multiply that voltage bu 1.414 and you will have the peak voltage.
If you isolate the Arduino's ground (run it from a separate battery or get an isolating DC-DC converter) one of the phases can be connected to the Arduino's ground and you can measure one, or both, of the other phases relative to that "ground" phase.
That's how a multimeter works... Either it's battery powered or the power supply is transformer isolated so it doesn't have a true-ground, just + & - or red and black connections.
Then of course you'll have to protect the Arduino from excess voltage and the negative-half of the AC voltage.
And if you don't rectify and filter you'll be measuring/sampling a waveform (or half of a waveform) and the readings will "look random" and you'll have to find the peak or average.
The neutral is the frame ground ...
You are sure the alternator is "Y" connected and not "delta" connected?
The maximum voltage on each phase is 120VAC.
I would like to just measure the peak value for each phase and I would like to avoid to rectify the voltage.
If I got well what you suggested, I could connect a phase to Arduino GND and then another one to an analog input to read it by using different ground for the Arduino board? If yes, how can I limit the high voltage for the analog input? Should I use resistors?
Thank you!
it is Y connected unfortunately!
Then the PEAK voltage would be about 170 volts.
Is it safe to read this voltage with Arduino using just resistors?
Moreover, will I compromise the motorbike system if I use resistors as additional loads for the stator?
Resistors for voltage dividers or transformers are necessary. The resistors will be not even noticible to your alternator. Do you have an Arduino with the necessary number of AD converter pins?
Same thoughts I had.
Time spent staring at some readout trying to work out what it all means is time spent not watching where you are going, a very dangerous practice.
It the primary reason any warnings on most electrical systems for malfunctions or other are done with various colour small flashing dash lights.
Why you would want to know the ac levels seems more a novelty thing, dangerous as already mentioned.
Why would the voltage be different on different phases? Why would you have any reason to measure more than one phase?
Sounds like a very bad approach! Easiest way to isolate here would be to use a transformer on the AC.
Is there some reason why this is relevant? 
So this clearly is not part of the running equipment of the motorcycle. What is it for and how is it connected to whatever it is powering?
Funny question. 
Whatever you do, you will need resistors, but a transformer for isolation is the most sensible.
The resistors are not "loads", just limiting the current to the Arduino.
As long as all three phases are known good, monitoring the individual phase voltage is a waste of time. Monitor the rectifier output for a cleaner indication of alternator power.
Metric motorcycles use three phase alternator, yel-yel-yel IIRC. These feed into a triple rectifier and then it's easily measurable DC.
Metric electronic ignitions won't make spark under ~11V. The bike will cold crank down to 9V but cannot start because the ignition module shuts down. Wait a while for the temp (and battery voltage) to come, up and all of a sudden the engine starts & runs on the first crank.
The motorcycle alternator will start and run with any two phases, but it won't charge the battery. The symptom is that the motor just seems to cut out and just coast because the battery voltage dropped and the ignition stopped firing reliably.
I had a Honda 1100 that did that exact thing for that exact reason. I glued a little LED car voltage monitor on the bars. If you didn't have a green LED she wasn't going to start.
If you really want to know the actual voltage just buy three AC Input modules and read the voltage sense output. All the isolation and conditioning is built into the module. OPTO-22 comes to mind but there are lots of possible vendors.
That could explain the OP reading such a high voltage to ground on his meter.
Unloaded voltage is quite high but will fall rapidly when loaded. Running voltage will be in the teens.
I can see a carbonised Arduino on the horizon. 
Following your suggestion, I found out this product on OPTO-22:
https://www.opto22.com/products/g4iac5
but I think it only detects if there is any AC voltage and it doesn't provide its value.
Was thinking more of this
