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PWM Cooling Fan Control

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I run a ViPec V88 ECU. I use one of the Aux outputs in PWM mode to control my cooling fan via a solid state relay from IO-Power. I have a flyback diode installed.

I have the software setup so that the cooling fan comes on somewhat early, but at a low duty cycle, like 30%. The duty cycle is gradually increased to 100% as the coolant temp increases. I figured this way there would be less chance of the coolant temp oscillating as the fan came on and off at full power.

I have been having issues with cooling fan vibration. At first I was running a Spal fan. I assumed the fan had gone out of balance or something, so I replaced it with the same model. It still vibrated. Somewhat discouraged with Spal, I recently swapped over to a Derale fan. And it still vibrates. What the heck? That seems like too much coincidence so I started to think about what they all had in common. And the common factor is that they all came on at a low duty cycle, which is where the vibration is manifest.

I just tested this with the Derale fan. At 30% duty cycle it vibrates like crazy and makes a racket. at 50% duty cycle no vibration, no noise. But even when I set things up at 100% duty cycle for all coolant temps, it still makes bad sounds as it spools up.

So at this point I am left wondering, is this a problem with the PWM signal, the relay, or the fan motor(s)? I thought a DC motor should be ok with lower duty cycles, but not sure.

If anyone has any thoughts I would love to hear them.

Thanks - Gustave

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what frequency are you running the PWM at as it sounds like it could be to low causing the fan to pulse (vibrate)

Thanks. You're perhaps thinking the fan is hitting one of its resonant modes? That is likely. I do not know the PWM frequency. I did not enter it into the ViPec software. I am not sure it is adjustable. I'll have to check on the ViPec forum.


I stand corrected. The frequency for the PWM control IS adjustable.

It had been set to 20 Hz.

I am going to play with it to see if I can get better fan behavior. Will try 50 Hz to start.


Just finished testing various frequencies for the PWM control of the cooling fan. I ended up setting it at 300Hz (the maximum possible value) and now the fan operates as smooth as silk all the way down to 30% duty cycle. Voila.

glad to hear it :)

Thank you Chris for your help. I would not have solved this without you prompting me to look at frequency. Gustave

You're more than welcome, that's what we're here for

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