12V DC Fan controlling

[sravan_rikka]

Hi Sir,

I want to control 12V DC Fan using PWM. IRF540N MOSFET is used to drrive the DC Fan. DC Fan is what type of load is it inductive.which type of equivalent model can use either small signal or large signal model. what is the equivalent model for attached circuit.
how can i know operating point of the MOSFET, how to calculate the drain current. DC Fan is not running upto 60% duty cycle applied to the gate of MOSFET, WHY. how to know how much current the Fan wants to start and how much the MOSFET gives.

 

[chuckey]

Some DC computer type fans are switched DC motors, they contain a chip which switches the current from coil to coil on a PCB and the rotor is just a magnet attached to the fan shaft. Apart from the voltage these motors need a very low impedance supply because the current is taken as pulses. I would experiment with your fan by running it from a variable PSU first.
Frank

 

[goldsmith]

Dear sravan
Hi
Can you attach your file in pdf format , please . i have a problem to opening it .
Best Wishes
Goldsmith

 

[sravan_rikka]

Hi sir,
I sent my circuit in PDF format plese help me..............

 

[goldsmith]

Hi again
If the voltage across the GS junction is between 12 volts or 15 volts you can be sure that it is on and if the voltage is lower than threshold , you can be sure that it is off .
BTW : don't forget to add a freewheeling diode across your motor to protect your mosfet from damages .
Notation : has your gate driver a totem pole in it's out put ?
Best Wishes
Goldsmith

 

[sravan_rikka]

Hi sir,
when i applied 60% duty cycle to the MOSFET the fan will run, untill 60% it wont be ON, i cant understood why it could be ON for 60% duty cycle only, this thing happens when PWM frequency is 1.22KHz. when i use 19.61KHz the fan is running for 95% duty cycle and when i use 4.90KHz it is running for 45% duty cycle. And can u tell me the electrical model of the previously posted PDF circuit, how to find the how much current the fan needs from modelling of circuit.

Thanks & Regards
Sravan Rikka

 

[goldsmith]

Hi Sravan
Do you have access to the oscilloscope ? if yes , can you show me the voltage across your motor , please ?
BTW : do you have access to a variable power supply too ? if yes , try to connect it across your motor and then change it's voltage from zero up to 12 volts and tell me the result .
Best Wishes
Goldsmith

 

[dselec]

U have torque problem ...its a need to apply more power at start then les pwr will get it going.
its like moving heavy load on ice first it will need more power and once it moves u can reduce it.
u will need to start at 80% at startup reducing gradually to 50-60% and use slower freq.

- - - Updated - - -

U have torque problem ...its a need to apply more power at start then les pwr will get it going.
its like moving heavy load on ice first it will need more power and once it moves u can reduce it.
u will need to start at 80% at startup reducing gradually to 50-60% and use slower freq.

 

[chuckey]

This randomness in the fans speed is due to the oscillator within it not being able to run with a pulsing power supply. if you put a large capacitor across the fan it should smooth out the pulses, resulting in an increasing DC with an increasing pulse width which will mimic the effect of a variable DC PSU. It would be interesting to see the results.
Frank

- - - Updated - - -

This randomness in the fans speed is due to the oscillator within it not being able to run with a pulsing power supply. if you put a large capacitor across the fan it should smooth out the pulses, resulting in an increasing DC with an increasing pulse width which will mimic the effect of a variable DC PSU. It would be interesting to see the results.
Frank

 

[sravan_rikka]

Hi sir,
I have oscilloscope but unable to capture picture. I incresed voltage from 0V to 12V and observed that speed of the fan is increased as the voltage increases from 0V to 12V.

 

[dselec]

https://www.smps.us/smpsdesign.html
use a pi filter ..that's parallel cap series coil and parallel cap then motor.

 

[chuckey]

"I have oscilloscope but unable to capture picture. I incresed voltage from 0V to 12V and observed that speed of the fan is increased as the voltage increases from 0V to 12V." So you have fixed your problem then?

Frank

 

[goldsmith]

Hi again , i'm completely agree with "Dselec " . the LC filter , is inseparable for motor controlling speed . and a thing for , fun ! when i controlled the speed of a 10KW DC motor (about some years ago ) when i didn't use LC filter it's core gone to be warm ! and it created pretty strange noises ! and when i used LC filter , all of the problems removed !
Best Wishes
Goldsmith