This application detects the phase sequence of a three phase 380V 50Hz system and signals also
a faulty phase.
The circuit for each phase detection input is the same as the one depicted here. So we will use three inputs on the MCU for mains detection.
The fault output can be attached to an optocoupled relay and then to the contactor M switch.
The LEDs can be directly connected to the outputs of the MCU.
In each of the three IO lines, R, S and T, there will be a square wave shifted 120 degs each. For sequence detection, we use the same principle used in quadrature encoders. There will be two possible sequences: RST or RTS. So we start looking for R to be low a couple of us, then look for the R phase to transition LOW->HIGH. Then check which of the remaining phases is HIGH. If S is HIGH while R rises, then the sequence is RTS. But, if S is LOW while R rises, the sequence is RST.
Once this is detected, the corresponding LED is lit for over a second, and then the cycle repeats. There are timers in each state that control the timing of the transitions, and if some transition does not occur within the correct period of time, then an alarm is displayed for a second, and the motor contactor is opened. After that time, the system enters in the initial state trying to detect phase sequence again. But the contactor remains open until Start is pressed.
The FSM is written in standard C++ code and it is abstracted from the I/O by means of a Platform class. For testing and demo purposes I have implemented an x86 and an Arduino platforms respectively.