Electronics - ISO 42v DC Brushless Fan to 110AC with Speed Knob MOD HELP
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Red is +48VDC
Black is negative
Blue is freq (rotation and lock signals)
Yellow is PWM 


All you need is a 5V PWM signal to control it and a 48v motor driver/power supply. Duty cycle should be 20KHz and to start it you should use a 30% PWM signal. If the yellow line disconnected it will rotate at full speed. everything else is in the datasheet.

The Data Sheet has some good info on how to drive it correctly too.

on a side note you can use a 555 timer and a potentiometer(speed knob) to create the PWM.