Fan control PWM

[rakeshm55]

Hi
attached fan spec ....618 NM
Nominal voltage range for fan specified as 36-56 V....
My supply rail is 48V...
 No I want to apply a PWM for fan speed control.....I will be using TC646B ..... (this chipset has mechanism to overcome initial inertia)....

Say for a 50% duty cycle the effective dc voltage is 24V .....will the fan continue rotation at a lower rpm if I apply a 50% duty cycle PWM....
Since the minimum dc voltage specified is 36V , Does it mean that the minimum Duty cycle should be 75%??.....
Pls help

[Zero999]

Going by the datasheet, the fan isn't specified to work below 36V because it won't meet the specifications or the driver electronics inside the motor won't start reliably at lower voltages.

The fan contains a brushless motor with an electronic speed controller and the datasheet doesn't say anything about PWM control, so it might not work properly.

The datasheet mentions an option with a speed signal so your best bet is to use that, rather than PWMing the supply voltage.

[bktemp]

When applying PWM to a load, the voltage seen by the load during on time stays at the same level. So it won't have problems with a too low voltage.
But most fans don't like PWM. I couldn't find anything from ebm papst, but other manufacturers recommend not to use PWM on a fan not designed for PWM operation.
PWM often increases acoustic noise and depending on the PWM frequency and the fan it could damage the bearing or the electronics used in the fan and reduce its reliability.
The best solution is using a fan with a dedicated PWM input. Or ask the manufacturer about using PWM on this particular fan.

[Zero999]

The application schematic TC646B datasheet, uses PWM on the power supply of the FBA06T12H, a fan with a brushless DC motor.
http://ww1.microchip.com/downloads/en/DeviceDoc/21755C.pdf
https://media.digikey.com/pdf/Data%20Sheets/NMB-MAT/FBA06T.pdf

[rakeshm55]

 FBA06T12H Specifies a voltage range of 7V to 13.8V .... And doent speak of speed control.....
But in tc646B data sheet they suggest a typical example for 40% duty cycle operation .... @12V supply rail...

In the below link I could find
https://www.maximintegrated.com/en/app-notes/index.mvp/id/1784
""During PWM control, the voltage applied to the fan is either its rated voltage (12V in the case of a 12V fan) or 0V. However, because the fan is spinning at something less than its rated speed (remember, that is the whole idea), its back EMF is reduced. This causes higher-than-nominal current flow through the windings during the on period of the PWM cycle. Although fans are designed to handle increased currents, such as during startup, heightened currents at 30 times per second for the life of the fan can entail negative reliability issues. But even with these negatives, PWM control can be the appropriate solution in low-cost noncritical applications.""