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Forum Index : Electronics : Fans in parallel....
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9629 |
Hello everyone.  I have a thing here , that has two fans wired in parallel, but when powered they both vary their speeds quite noticeably, and the current and voltage dips and peaks all over the place. If I disconnect them, and run either one just on its own, the fan runs at a constant speed and is perfectly happy. I suspect it is simply cheap nasty fans used in this Chinese thing, cos I have run fans in parallel before and there has never been an issue. Can someone suggest what might be going on? I am thinking there is some kind of electrical noise being generated by one fan, which the second fan is then reacting to, and you have a kind of back-and-forth cascade type effect where one fan's noise upsets the other one and vice-versa. I have gutted this thing, and am going to totally re-wire it, cos the current consumption is supposed to be about 30A @ 12v, but they have used 10A wire, and while running it DOES work, but the connecting wire gets very hot - not good. So, I will be rewiring it with the correct gauge wire as one step, and will probably end up replacing the high-velocity fans too, but I am intrigued by the fans behaviour when they are connected together in parallel.  Smoke makes things work. When the smoke gets out, it stops! |
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
30A sounds like a lot of current. Does each fan have a controller? Are you running them off battery or a high-current power supply? Have you put a 'scope on the supply? |
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SimpleSafeName Guru Joined: 28/07/2019 Location: United StatesPosts: 351 |
A blocking diode before each fan might help. Have you run it with just the fans and not the heater? It would be interesting to see what happens then. |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
I should have clicked on the link, forget the high-current comment! If you are running these things off a battery try a high-value electro across the supply. |
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Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1156 |
If they were brush motors I could see the potential for that to happen, revs change and fight each other.  The fan motors with the hall sensor (I think) tend to rev up to the airflow available. I have had 12v fan fly off the table when the step down voltage converter shorted and give the fan 50v....fan survived.  Cheers Aaron Off The Grid |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9629 |
Yes, just fans and no heater, and they go a bit crazy. Diode on each of the positives, same result. 470uF cap across fan(after diode) and they BOTH settle down and run smoothly then. Removed diodes, and left the 470uF cap across the fans, and they both run smoothly - and faster then they were going before, so they move air better too.  So, I know now how to fix it, it's just curious why that was happening. Obviously some kind of noise there upsetting things, cos the cap is obviously filtering that crud out. I guess I could look on the scope, but I don't think I can be bothered so long as I have something that fixes it. Smoke makes things work. When the smoke gets out, it stops! |
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| SimpleSafeName Guru Joined: 28/07/2019 Location: United StatesPosts: 351 |
Hmmm, that's interesting. I had wondered if a cap would help but it seemed counter-intuitive. Nice catch. :) |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9629 |
I will probably use a 1000uF cap just for good luck. Smoke makes things work. When the smoke gets out, it stops! |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
High frequency energy that a cap filters out but the battery doesn't. The 'scope should provide the answer. |
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