BLDC motor BEMF problem

[ashugtiwari]

Hi All,
Im making BLDC motor speed and torque control application and to control that im using sensorless algorithm. when i take BEMF input from one of motor phase and finds its zero cross corrects the speed, it works but if i suddenly by force stops the motor, the motor stalls, my algorithm checks if BEMF zero cross is exactly as wanted if not it updates the speed. to my surpise even when stalled the BEMF still comes as feedback with the same frequency as desired. My question is what am i doing wrong that makes this behaviour? te motor stall should not gives any back emf but i can see BEMF waveform on motor phase even if motor stalled.

Thanks for answers
Ashutosh

 

[FvM]

Very generally, the phase voltage of a synchronous motor (e.g. BLDC) is Iphase*Zmotor + EMF. So there will be a voltage with non-zero motor current for n=0 (stalled motor). A sensorless algorithm has to compensate for Iphase*Zmotor, apparently you're not doing it correctly.

 

[ashugtiwari]

Thanks for reply FvM
alright! so, is Zmotor = R + XL(motor inductive impedance)?
Also, in my case, im measuring the period of back emf waveform and adjusting the commutation speed. So, as u said, i also have to measure the emf voltage value and subtract it from Vdd supply to find the back emf? And if i measure the Iphase and calculate the drop in motor and subtract it from Vdd, will the result be back emf?
im not much of a guru in motor control, so that may sound little novice. sorry for that.

 

[FvM]

Vdd should be eliminated in back emf measuremt because emf is an AC quantity.

I must confess that I don't fully understand your concept of "adjusting commutation speed". I'm presuming that BLDC is a synchronous machine, thus the motor will either run synchronous or be unlocked/stalled.