Another variable in fan selection is EMI (Electromagnetic Interference) and EMC (Electromagnetic Compatibility). EMI, by definition, is any electrical imposition that can interfere with the normal operation of equipment. There are two broad areas of EMI interference: conducted interference and radiated interference.
Conducted interference refers to any undesired signal conducted through power and signal lines. Radiated interference refers to any undesired signal that radiates from a source and may affect the normal operation of equipment. Conducted EMI is usually more of a problem than radiated EMI. In fact, when dealing with brushless DC fans, conducted EMI is normally the only concern.
Typically, AC induction motors running sine wave voltages do not present EMI concerns. There may, however, be small magnetic interference present close to the motor and its input leads. DC motors, either mechanically or electronically commutated, and AC motors powered by electronic controllers have EMI signatures. EMI is produced by the switching of the DC voltage, which is necessary to produce rotation of the magnetic fields in the motor.
EMC (Electromagnetic Compatibility) can best be described as the ability of equipment to operate without generating unwanted electromagnetic interference that can affect the operation of other electronic equipment, as well as its ability to not be negatively affected by unwanted interference generated elsewhere.
In summary, fan selection is a very important part of liquid-to-air and air-to-liquid cooling applications. It takes more than just airflow and static pressure calculations to size the proper fan for an application. As discussed in part 1 and 2 of this article, there are some other very important factors that must be considered by a designer when sizing a fan to provide a reliable system. These include air density effects, noise, life expectancy, and EMI/EMC interference.
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