DC-DC Converter and BLDC motor behavior question.!!!

Thread Starter

silence00

Joined Aug 28, 2019
2
Good day! I'm coming slowly back to electronics after 35 years, and am rusty.

At the moment the project at hand is limiting the speed and power draw of a pair of brushless DC fans, ca. 2.25A ea. with a stated range of 6-14V. First attempt was a 12V PWM fan controller, which works, but results in a lot of noise, and according to Comair-Rotron, a shortened FET life. They suggested varying a smooth DC voltage instead.

I am looking at adjustable DC buck-boost converter modules to provide the fans with linear-appearing voltage, and see that a large percentage of those on the market have both voltage and current control. I know how to figure out the usual power law math, but I am not sure what behavior to expect out of one of these controllers.

Additionally, I would simply use a buck converter, but the commonly available modules have too much voltage overhead, and with the AGM battery source I didn't want to have a range of only 6-9V available.

My speculation is that if I leave the "current limit" trim pot set wide open, the fan motors will draw the current they need, up to about 4.5A at full draw. Would this be correct?

I am intending to remove the voltage trim pot and replace it with a remotely mounted single-turn pot for fan speed control.

Is there a reason to pay any attention to current limiting? To be relatively safe, how much overhead should I leave; e.g., if I know the fans are rated at 4.5A, would the 5A converter be safe?

Hope the question is appropriate, and thanks!
 

Tonyr1084

Joined Sep 24, 2015
3,949
Welcome to AAC.

Can't answer all your questions, but at 4.5A I bet your buck is close to its capacity. You could try (or I would try) a large capacitor across the outputs and see if that quiets things down. As for amp capacity, if you have more than you need then your load will only draw as much as it will (according to ohm's law).

Like you, I've been away from the hobby for a long time. And I was never that advanced. The people here are friendly and willing to help anyone who is putting forth the effort. I've seen some people come here looking for someone to engineer a solution to their problem without even giving it a try. We don't much care for handing out projects for others to build. So as long as you put forth the effort, the folks here will be happy to help. Advice, direction, clarification, understanding. We may even ask a question or two about what you're attempting to accomplish. I think you're going to enjoy this website. I know I do.
 

shortbus

Joined Sep 30, 2009
7,248
First attempt was a 12V PWM fan controller, which works, but results in a lot of noise, and according to Comair-Rotron, a shortened FET life
As there are so many PWM modules and motor controls working in the world at any one time, I'd ask why they aren't blowing their mosfets? If you are having a noisy motor it is usually due to too low of a PWM frequency, not anything to do with mosfet life at all.

When you say you are using BLDC fans, are you sure they aren't already, internally PWM controlled?
 

Thread Starter

silence00

Joined Aug 28, 2019
2
Can't answer all your questions, but at 4.5A I bet your buck is close to its capacity. You could try (or I would try) a large capacitor across the outputs and see if that quiets things down. As for amp capacity, if you have more than you need then your load will only draw as much as it will (according to ohm's law).

Like you, I've been away from the hobby for a long time. And I was never that advanced. The people here are friendly and willing to help anyone who is putting forth the effort. I've seen some people come here looking for someone to engineer a solution to their problem without even giving it a try. We don't much care for handing out projects for others to build. So as long as you put forth the effort, the folks here will be happy to help. Advice, direction, clarification, understanding. We may even ask a question or two about what you're attempting to accomplish. I think you're going to enjoy this website. I know I do.
tweakbox appvalley https://vlc.onl/


thankyou
 

Tonyr1084

Joined Sep 24, 2015
3,949
@silence00 You don't need to cut and paste to quote someone. Just click the "Quote" button, then on your reply, click the orange box below where it says "Insert Quotes..." and it will automatically insert the quote and notify the person you quoted so they can respond further. You can select a section of someone's post and click quote from the dropdown and you will be able to quote just the part you want to quote.

You're welcome. I hope I've been of some help.

Can I ask why you linked in those apps?
 

MisterBill2

Joined Jan 23, 2018
4,600
The motor will not draw more current than it needs, just keep the voltage under control. Conside that a typical 12 volt automotive battery can supply several hundred amps for a time, but each connected device only draws the current it needs.
 
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