BLDC Motor speed Control

Hi,

We were trying to control the speed of a BLDC motor using (additional) sensorless back emf monitoring. We were trying to control the motor with full sinewave signal control. We found that though the motor responds good in open loop, once the closed loop system is brought in and the control is done by either change of Amplitude or by realigning of the input frequency with the rotor position we lost control.

We then tried implementing the sensor (using Hall Sensors) based Speed control. But still the motor is not responding. That brings us to a conclusion that either we have understood wrong or that the motor may be faulty (We think the former maybe the case).

We would really appreciate if you could just guide us towards the understanding the nuances of BLDC Motor speed control.

What we think is: The rotor may lag (most often) or lead (maybe) the rotating electrical field of the stator, this leads to speed variation of the rotor. To compensate this (instead of boosting the motor frequency directly) at each hall data crossing or at comp,etion of one rotation, the wave pointers of the sinewave generation table are realligned to the current rotor position to aid the rotor in catching up with the stator frequency.

If this is right the I guess the motor is faulty, else please guide

regards,

Vishal...

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