msebold said:
Hall sensors: I understand (from reading smart-guy articles) that in brushless motors they detect the position of the rotating magnets and vary the voltage in response.
For us dummies what does this mean? Why are Hall sensors necessary? Why are there two Hall sensors? What happens with no hall sensors?
Not necessary, just an improvement.
Picture a triangle. Each corner represents a coil that is magnetized by the brush contacting the commutator and it can be attracted to that nearest nonmoving magnet. The only corner that can receive power is the one that will be approaching the magnet, as it reaches the magnet it is disengaged from the brush, then on the opposite side another corner repeats the process. That's fine for a brushed motor, but on a brushless those corners are magnets that don't have to be activated, the nonmoving coils do but have no positioning to tell them when to come on.
So you understand the hall sensor literally tells the coil to come on? If the hall sensor does not send a message that the magnet is near the coil then the coil has no way to know to come on. Doing it this way spares having those pesky brushes that made slot cars so difficult when I was a kid. Basically the sensor times the rpm of the motor and the controller knows if it's running too fast or too slow.
Or even if you want to run a servo that way, it knows where to move to and stop. The controller will actually know right where the armature turned to.
Ah, what about those sensorless motors and controllers. It's just a guess. If you think about it, brushless motors without sensors are like politics, just a huge assumption it'll work fine without any feedback. Not always the case. Flying on faith. Maybe when you start the motor will run in reverse. Dang.
But in fact there are ways of getting out of flying on faith. Such as an encoder. The more closely you can time the on/off of the coils the better your motor will work.Hall sensors are the simplest way to be more efficient, but have their own drawbacks.
