How does lock protection work on HVAC fan

The mpu detect the rpm and if no match it will stop the motor
how to differentiate if it is from a bad motor or a bad mpu circuitry  

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Find out how its detecting speed - there is probably a sensor somewhere near the motor, or the edge of the fan.
Looks like it - else what ELSE can electronically set the speed - there isn't a relay anywhere else I noticed
WHICH speed ? If its fast only, change the SSR (SSR101)

Looking at it, they are making a single phase motor turn at different speeds just by switching the AC on and off.

Its possible to make your own three phase supply, if you throw enough electronics at it. Electronics are cheaper than motors with multiple windings now.

I'd need a copy of the full circuit to explain it in detail - I must say, this seems a very clever piece of equipment you have there !

Well the motor could be a three phase motor, and the system makes its own AC.

I think, given the history, I'd go with the new MPU board, especially since the pathway you think is broken would be pretty close to the way a lightning strike would find a weakspot.

New board, if the motor is spinning freely.

The right side is the wiring for the internal sensor. What I am trying to do for you is to see if the sensor is doing something - I suspect it should generate a little pulse every revolution, and you should be able to see that. The computer measures the time between pulses - too long and it sees the motor running too slow. Does it LOOK too slow ?

OK, the measurement is inside the motor. Have you got access to an oscilloscope by any chance ?

Failing that, put a DVM between pins 3 and 2, and observe the reading when the fan is "spinning".

Show me where the signal goes to the RIGHT of the picture./
So this IS a mains powered fan isn't it ?