Finding the max speed

[daveevans9]

Posted on February 01, 2005 at 04:48

Finding the max speed

[daveevans9]

Posted on May 17, 2011 at 10:18

Hi,

I have a 2 pole BLDC motor that is to be driven sensorless in voltage mode, closed loop with a target speed of 35000. Working the math gets me: Rpm = 35000 nP = 2 Fm = 1/Rpm = 583 Hz Fe = Fm * nP = 1166.6 Hz Te = 1/Fe = 857 uS Tstep = Te / nP = 428 uS using the a sample rate of 50Khz (20uS) gives me approx 20 samples per step. Is my math correct? Can I assume that there is a point at which the motor will not reach a target speed due to the electrical constants of the windings I.E the time it takes to demag and time taken to saturate, and at this point what happens? How can I tell when I have reached the max speed of a motor? I have no reliable constants for the motor I am driving. Regards DaveE

[gaetano]

Posted on May 17, 2011 at 10:18

Hi DaveE,

your math isn't correct if you mean as 'Tstep' the time of 1/6th of one electrical period (in a six step driving mode). In this case, Tstep is given by: Tstep = Te/6 = 23.8 us and sampling at 50Khz gives you only one sample! (very poor resolution). Of course assuming that np is the number of pair poles, that is, 4 poles in total. The magnetization and demagnetization times limit the operating speed of the motor, accordingly with the applied resistive torque: don't forget that until the D event doesn't occur, it is impossible to detect the Z event; then, the D event depends strongly from the motor constants and the level of current (applied torque) flowing in the motor. Best regards, Tanio