AC cooling fan, and, DC cooling fan distinction. 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 Fan" due to a power frequency is fixed, so the rate of change of silicon steel pole generated by the power supply frequency is determined, the higher the frequency of the magnetic field to switch the faster, the faster the speed will theoretically like DC fan speed faster the more the number of poles as the principle. However, the frequency can not be too fast, too fast will cause activation problems.