Finding the max speed

- 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

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