Sunder said:
cavallo pazzo said:
As I'm new to FOC controller, I'm impressed by silence and smoothness already in full sensored but don't know what to expect from sensored start+sensorless run mode.
cycborg wrote sensorless run is way to go to take full advantage of FOC, but why ?
I'm keen to know this too. Can anyone please help me understand why sensorless mode takes full advantage of FOC, but sensored mode doesn't?
Three Hall sensors indicate the position of the motor rotor by providing a total of 6 signal transitions for each 360-degree electrical cycle, i.e. 60 degrees apart (The period of an electrical cycle is the time it takes a pair of rotor magnets to pass a fixed point on the stator.) When using Hall signals only, to get a close approximation (within a degree or two) of the rotor position for each PWM cycle interpolation is required, but if the motor speed is changing rapidly a significant error in position can be introduced. By measuring the currents of 2 or 3 phases at a certain point in
each PWM cycle a more timely and accurate estimation of the rotor position can be obtained. However, at low RPMs the back-EMF of the motor is so weak that sensorless mode can be unreliable, so sensored mode is used until the motor reaches a certain speed.
From the current measurements the FOC algorithm can account for phase current lagging phase voltage due to the inductance of the windings and is able to place the peak of the magnetic field of the stator in relation to the rotor magnets in such a way that produces the most torque. This is called the torque angle and is normally 90 degrees. Current lag increases as the electrical frequency increases, so the algorithm needs to adjust the timing of the voltage output of the inverter accordingly. In order for all this to work correctly the FOC firmware needs to know the phase resistance and inductance of the motor which it can discover during an initial auto-tune. Also, as you know, an FOC controller needs to be manually tuned for the specific motor it is running which requires an understanding of how PI torque and speed controllers work.