DC VS AC

AC 400 HZ or 28 VDC brushless?

In the high reliability aerospace market AC 400 Hz fans are less expensive  than 28 VDC brushless fans with the same performance. On a jet aircraft where presumably both types of current are available some people ask why go with 28 VDC. Obviously it may just be a matter of convenience. Transformer rectifier units typically use  AC 400 Hz fans because the TRU has AC 400 hz input.  On the other hand the fan may be cooling an electronics box which is running on 28 VDC and there is 400 Hz AC available nearby. Another reason would be the need for a signal to indicate the speed of the fan or whether it is turning at a minimum speed necessary to cool the box.These signals, commonly known as fan performance sensors or low speed warning require DC current. You can have a speed sensor on an AC fan but you have to power it with DC normally. AC fan speed is controlled by the frequency. DC fan speed can vary with the voltage. DC fan electronics can have all kinds of built in features like soft start to limit inrush current.