Width Modulation Fans.
Width Modulation (PWM) is a common technique for speed control which
can overcome the problem of the poor starting performance of a motor.
for motor speed control works in a very similar way. Instead
of supplying a varying voltage to a motor, it is supplied with a fixed
voltage value (such as 12V) which starts it spinning
immediately. The voltage is then removed and the motor
'coasts'. By continuing this voltage on/off cycle with a
varying duty cycle, the motor speed can be controlled.