Posted on May 17, 2011 at 10:19Hi Oren,
the concepts that explain what motor pole pair means and the differences between the mechanical frequency and the electrical one is written in whatever textbook treating motor aspects. It is not so straightforward to explain it in few rows :-?
By the way, it is enough to know that, given Np as the number of motor poles (magnetic pole present on the stator), the relation between the electrical frequency (the freq of sinusoidal voltages applied to the motor phases or, equivalently, of the current in the motor) and the mechanical one (the frequency seen at the motor shaft or at the rotor) is:
Freq_mech = Freq_elec / (Np/2).
Then, in a BLDC motor, driven in six-step mode, the motor currents are periodic waveforms costituted by 6 levels (see fig.51). So, as we are referring to the motor current, one can say that a electrical period is made up of six more little intervals or steps.
The second formula you wrote, descends directly from the previous considerations.
Let's suppose you turn the motor rotor (with your hand) by one round (1 mechanical period, hence). The electrical periods are equal to (Np * 1) = Np. Since each electrical period contains 6 steps, you will get a total of Np * 6 steps for one rotor revolution. Viceversa, if you get, let's say, M steps with one mechanical rev of the motor rotor, the second formula allows you to know about the motor poles.
N.B. Often, the motor poles can be referred as motor pole PAIRS. The relation between the two values is banally:
Motor poles = Motor pair pole * 2.
Our Np refers to motor poles.
I hope all is more clear.
With my best regards,
Tanio
