# Max motor speed using FOC

http://www.microchip.com/forums/m619210.aspx#619979

I'm building a driver board to work with Field Oriented Control on the dsPIC33FJ12MC202.
Starting from AN1078, I'm a little unclear from reading the App Note and Tuning Guide- what is actual the limit on how fast the motor can be driven?

consider the following timings: -
FOC calculation takes 22uSec. This time also includes the execution time of "estimator" for back emf. The back emf is reflection of the speed of the motor. For a good sinusoidal back emf to be estimated, the estimator should run some 80 samples. Thats an assumption and might vary a little bit.
So, for a good back emf waveform, the sine wave should have 80 samples. So, 80 * 22usec = 1760uSec = 568Hz.
Hence, you can consider that 568Hz (aprox) is the highest electrical speed you can achieve.
If your motor is 4 pole, max mechanical speed = (120 * 568)/4 = 17,000RPM
If your motor is 8 pole, max mechanical speed = (120 * 568)/8 = 8,500RPM

Oznog
I'm looking at different motors too, for different apps. Some would need to go into the range of 20,000 RPM.

For high speed drives, FOC might not be the best option. Try sensorles back emf based drives. Those needs lesser execution and hence can run motor at higher speeds. Try AN1160.
For extremely high speeds, we might want to do away with any executions. That will need external comparator to detect the zero crossing of back emf.