Electronic – What frequency do BLDC motors generate

First of all google the serial number. This may provide a link to the datasheet & this will inform yourself as to whether it is a BLAC or a BLDC machine.

From empirical measurement... It all depends on what you are exactly seeing. You have to remember that the waveform you are measuring is line-line and NOT phase voltage.

So is it a BLAC machine? the Phase voltages would appear like:

And thus the line-line at the terminals would appear:

Thus a BLAC machine would be quite clear. NOTE. In theory the BackEMF is sinus, in practice & depending on build quality... it won't be

So is it a BLDC machine? the Phase voltages would appear like:

And thus the line-line at the terminals would appear:

The number of poles is the number of magnetic cycles in one revolution of the shaft. When setting up for driving a BLDC, it's useful to step thru the drive phases one by one manually. I usually mark a point of the shaft, then add a sticker or something on the stator to record where the shaft is.

I like to use a 12 phase drive. In that case, I go thru each of the 12 phases and mark where the shaft is. This not only tells you what order you need to generate the 12 phases in, but the number of poles falls out by seeing how many times the pattern repeats in a full revolution.

Another way to just get the number of poles is to connect one lead to the supply, one lead to ground, and leave the last lead floating. That drives the motor to a particular phase. By manually rotating the shaft, you can see how many places the rotor is pulled to in a rotation. That is the number of poles.