In article , jim beam wrote:
On 03/25/2013 06:59 AM, Scott Dorsey wrote:
In article , jim beam wrote:

well, you live and learn. apparently the reason they use a linear
controller is because it allows the fan to run near silently at low
speed. with pwm control the fluctuating magnetic fields in the motor
coils cause it to vibrate and make a humming noise at the pwm control
frequency.

Yes, this is why you put an integrator stage after the pwm stage, so that
the motor sees nice filtered DC with very little of the PWM leftover.

Problem is that the integrator stage costs money and big electrolytics
tend to have limited life, so auto folks don't like doing that.

you don't want to integrate the output, merely rub the shoulders off the
square waves to get the harmonics down. the whole point and benefit of
pwm is that you have full voltage full power available in each pulse.
that's how you can start and control a motor with high torque at low
rpm. if you integrate or smooth out the motor's supply, you effectively
lose that and the motor won't start or torque the same way or even at all.

The more you rub off, the quieter the motor is! You start throwing stuff
in the KHz range into the motor, and it whines from magnetostriction.
The more of that you remove, the quieter it goes. Of course, if you make
it too quiet, people complain....
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."