Brushless DC fans are usually available at three nominal voltages: 12V , 24V and 48V. If the system has regulated power supply in one of these, then a brushless DC fan may be selected which will give the exact performance required, regardless of the AC input variables which plague AC fans.

Because the speed and airflow of a typical DC fan is proportional to the voltage supplied, a single product may be used to meet different applications by setting the supply voltage to what will give the desired airflow.

Figure 9 describes that results of varying the DC voltage supplied to a given fan. If, for example, a fan supplies 110 CFM of air at free delivery, 28V may yield 127 CFM, should that be needed. On the other hand, 24V operation may provide too much flow; the supply voltage might then be reduced to a level that yields the desired airflow.

The voltage range that may be applied to the fan to assure satisfactory operation is dependant upon the individual fan design and may be as small as 10-14V for 12V units and up to 12-56V for 48V units.

Current

Brushless DC fans do not draw constant currents. The choice of the power source along with the addition of other peripheral devices will be affected by the type and number of DC fans and their motor current characteristics. Throughout the rotational cycle and particularly at commutation, the currents will fluctuate from minimum to maximum.

The wave form and level of ripple current will vary significantly between fans and motor designs, marking specifications in narrow terms difficult. An understanding of the power source limitations and how they may be impacted by various brushless DC fans early in the design phase will help prevent problems and allow maximum system flexibility.

Current ripple may be analyzed by wave form measurements using a laboratory-quality oscilloscope and a suitable series resistance (r), typically 1 ohm. Note that the current excursions vary from a minimum of 20 mA to a maximum approaching 800 mA while the motor is in a running condition. ( see figures 10a,10b and 10c)

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