"AC Fan" and the difference between DC fans. The former AC power supply, the power supply voltage of alternating positive and negative, unlike the fixed voltage DC power supply fan, must rely on the control circuit so that two sets of coils in turn work to produce different magnetic fields. AC type fan supply frequency is fixed, so the rate of change in magnetic silicon steel produced is determined by the power frequency, the higher the frequency of the magnetic field is switched, the faster, the faster the speed will theoretically, the more the number of poles as a DC fan speed sooner principle same. However, the frequency can not be too fast, too fast will cause activation problems.