Refers to "Brushless Cooling Fan" rotating blades times per minute, the unit is rpm. Fan speed is determined by the motor inside the coil turns, voltage, number of fan blades, angle, height, diameter and bearing systems. Fan speed and quality are not necessarily linked. AC cooling fan speed can be measured by the internal speed signal can also be measured by the outside. With the application and ambient temperature changes, and sometimes require different speed AC "Brushless Cooling Fan" machines to meet demand. Some manufacturers specially designed adjustable fan speed cooling fan, manual and automatic modes. Manual is designed to allow users to use a low speed in the winter low-noise, high speed access to good cooling effect in the summer. Usually automatic type provided with a radiator thermostat thermostat sensor to automatically control the fan speed based on the current operating temperature, the temperature is high, the rotational speed is increased, the temperature is low, the reduced speed to achieve a dynamic balance, so that "Brushless Cooling Fan"cooling effect of wind noise and maintain an optimal combination of points.