Detection of 3 phase system

Question

I need some help with the below circuit.

Circuit:

3-phase AC circuit (3 phases and neutral). 120deg phase shift between the 3.

I need to provide a signal to the Microcontroller.

If one of the phases is missing, or most importantly, if phase difference is greater than 120deg between any of the two phases, then I need to convey this information to the MCU.

Initially, I thought of using an OR gate IC and giving the output of the gate to the MCU. But using the OR gate, it is not possible to convey the phase difference information to the MCU.

Answer 1

One single two-state signal to detect phase problems of a three-phase system:

Use one comparator each to compare each phase to zero and generate the exclusive or of the comparator output signals.
Each comparator output changes state whenever the potential difference at its inputs changes sign, one input being tied to the reference potential ("ground"), whenever the input signal "crosses zero".
An ExOr's output (equivalently, an ExNOr's output) changes state whenever an odd number of inputs changes state; in particular, when one input changes state.
An MCU can monitor the time between state changes, there should be six for every mains period.
Evenly spaced when the phase difference is 120° or 2π/3.

Here's a sketch at falstad.com - try changing one of the phase angles.

Electrical safety may mandate to isolate signal processing from "the three phases".

Answer 2

If the voltages on each phase are equal, this will give zero volts output if the phases are 0, 120, and 240 degrees.

^{simulate this circuit – Schematic created using CircuitLab}

Balanced voltages and phase angles:

Voltages with unbalanced phase p1=0, p2=120, p3=250

Voltages with unbalanced amplitude p1=0, p2=120, p3=240, V(p3)=160