"Cooling Fan Manufacturer" AC fan works: driven by AC power, voltage alternating positive and negative, do not rely entirely control circuit, generates a magnetic field. Fixed frequency power pole change speed silicon produced is determined by the power frequency, switching frequency, the higher the magnetic field, the faster, the faster the speed will theoretically. However, "Cooling Fan Manufacturer" frequency can not be too fast, too fast will cause difficulty in starting. A lot of people have been very concerned about this point, I do not know the difference between real and flow fan DC fan somewhere. That "Cooling Fan Manufacturer" to analyze the first very important from the fan bearings.