The difference between ac and DC Fan fan is an alternating current communication, it will be positive and negative power supply voltage, and, with the power supply voltage of the dc fan fixed in so it must rely on circuit control, make the two sets of large working coil produces different fan have a fixed frequency, magnetic field communication so silicon steel sheet produced by the magnetic pole change speed depends on the frequency on the power supply frequency is higher,  The faster the magnetic field switches, the faster it will theoretically be, just as the more dc fans there are, the faster it will be.  But the need to pay attention to is that the frequency should not be too fast, otherwise it will form a difficult activation.