speed limit for BDLC control ?

[lionel]

Posted on October 29, 2004 at 12:47

speed limit for BDLC control ?

[lionel]

Posted on May 17, 2011 at 10:17

Hello,

I would like to use the ST7MC for BDLC up to 13500 rpm (this is a low torque and low voltage BDLC motor with 12 poles);

Was is the limitation on the ST7MC to control the speed at this rpm (graphic show up to 10000 rpm)...

In advance many thanks for your kinldy cooperation

Lio

[gaetano]

Posted on May 17, 2011 at 10:17

Hello,

13500 rpm means a mechanical frequency equals to 225 Hz.

Then, with 6 pair poles (or 12 poles) you have an electrical frequency of:

Fe = Fm * 6 = 1350 Hz,

and an electrical period of

Te = 1/ Fe = 740 us.

So the step time, in six steps driving mode, is equal to:

Tstep = Te / 6 = 123.33 us.

Using a PWM frequency of 50KHz (20 us as period) you have more or less 6 sampling points to detect the BEMF zero crossing when using ST method (or 3 resistance method).

Otherwise, if you prefer to use GE method (sampling at High Frequency during PWM ON time), you may use better resolution for Z.C. detection but you have to use additional circuitry (voltage divider) to condition the signals coming from the motor phases.

So, for me, from a theoretical point of view there are no limitations to reach that speed.

With my best regards,

Tanio