Fan Types and Considerations
Fans are generally categorized by the way air enters and leaves the fan, which comes in two common forms: the axial fan where air is drawn in from one side and expelled from the other on the same plane or the centrifugal fan design where air is drawn in and expelled in a different direction. This style of fan, most commonly found as a blower, effectively compresses the air, allowing for the delivery of airflow at 90° to the intake under different pressures. The application requirements will determine which design is more suitable as axial fans deliver greater airflow in systems with low static pressure, while centrifugal fans offer lower airflow, but can deliver it against higher static pressure.
Audible and electrical noise are also important considerations when selecting a fan. In general, the greater the airflow required, the greater the audible noise which means that careful design to optimize airflow and reduce system impedance will reduce the required CFM and minimize noise. Axial fans will also tend to be quieter than their centrifugal fan counterparts. In addition to noise, electromagnetic interference (EMI) generated by the dc motor in a fan should also be considered. This unwanted system effect is usually limited to conducted EMI in the power leads and can be effectively suppressed with ferrite beads, shielding, or filtering.
Fan Bearing Types
Depending on the application, fan bearing type should also be a consideration with sleeve and ball bearings being the most common. Sleeve bearings, which are the simpler and lower cost option, have proven to operate as well as ball bearing fans at consistently lower temperatures. However, at variable or high temperatures sleeve bearings tend to degrade more quickly and can begin to experience wobble, noise, and friction issues commonly found in the sleeve bearing design. On the other hand, ball bearings address many of the uneven wear and friction problems found in sleeve bearings resulting in a significantly higher operating life. They can also be operated at any angle for use in portable applications, but at the same time are less impact resistant, more complex, and costlier than sleeve bearings.
As a third option, CUI Devices has developed the omniCOOL™ system, an advanced sleeve bearing design incorporating a magnetic structure that enables rotor-balancing to minimize tilt, wobble, and friction, allowing for operation at any angle. This magnetic structure, along with either a specially hardened bearing or a specialized groove bearing, combine to make the omniCOOL system a more reliable and cost-effective design compared to traditional bearing technologies.
omniCOOL™ System Bearing
Fan Control Options
Many fans now come with additional features that provide greater control over fan speed and operation to improve system performance. Having a fan run constantly even when maximum cooling is not required does not result in an efficient system and can reduce the operating lifetime of a fan. That is why many systems now monitor the temperature within an enclosure and only operate a fan when it is required. However, this can present problems with thermal lag or a fault condition if the fan was unable to start due to an obstruction. To address this, most modern dc fans feature auto-restart protection that detects when a fan motor is prevented from rotating and automatically cuts the drive current. Additional fan control options include:
- Detects rotational speed of fan motor and provides a pulsed output
- If motor stops, output stops pulsing and stays at logic high or low
- Doubles as a lock sensor where output remains at logic low during normal operation, but is driven to logic high if fan motor stops
PWM Control Signal
- Gives the ability to control the speed of the fan
- Duty cycle of this input determines speed of fan’s rotation, relationship between duty cycle, and whether fan’s speed is linear
- When used with a microcontroller one can create a sophisticated thermal management solution used to adapt to system conditions and provide more efficient operation