Electronic – 3 phase Bldc hall sensor problem

Your question seems to center around the use of the Hall Effect sensor in a 2-wire fan. I believe what you are asking is, "If the Hall Effect sensor isn't feeding a tachometer signal back to the motherboard, what is it doing?"

In a two wire system, the only signals fed in a voltage and ground. There is no control. So what is it that is alternating the phases and causing the rotation? If it were just a DC signal, the permanent magnet on the rotor would spin until it's magnetic field aligned with that of the coils in the stator and stay in that position.

Instead what happens is, a DC is fed in and it finds the path of least resistance to ground. Assuming the transistor on the left is open, it would flow through the coils and transistor on the right. The rotor spins and the magnetic field passes the Hall Effect sensor. The Hall Effect sensor sees this rotation and turns on the transistor on the left. This side now has a lower resistance so the current flows through it, reversing the field in the stator which pushes the rotor around again. The Hall Effect sensor senses a change in the magnetic field from the continued rotation of the rotor and then turns off the left transistor. Current then flows through the right.

This alternation continues as long as power is supplied and is the source of the alternating magnetic field that causes the rotation of the fan.

You could implement back-emf as a backup control system to aid detecting fault conditions matching sensor data and rotor position calculated by back-emf, thus being able to detect even those fault conditions producing a "good looking" set of values, and eventually bring the motor to a soft stop using BEMF for rotor position detection.

Another option, if you want to go the fault detection way, is to implement redundancy in your sensors and communication line. This can be implemented in various ways and probably is the best way to add a functional fault detection that not only detect faults but also enables you to not have to immediately halt the motor.

If both such solutions seems too complicate you probably are in a case were a serious fault detection algorithm is not needed and stopping the motor in case of incoherent data is "just the right solution" so you can fix that cold soldered hall sensor before finals :-)

As far as fault detection goes is probably more important to focus on current detection.

Have fun! :-)