DC Motor Problem

What you have said is all OK, but check up whether you have any series regulator for limiting the starting current.

Though I don't know whether you will need for such a small motor. Still 3 fans burnt, and starting current is same whether motor is small or large.

Thanks for stepping in so quickly.

Not sure what a "series regulator" is. Forgive my ignorance, but is that something I can buy at radio shack?

I did buy 2 new fans of a different type today. these are 12 VDC-.16A. they also say "started voltage:6V". might this be related to what you are talking about?

Also, I failed to state that all of these fans I have purchased are computer case fans (not sure if that matters).

Thanx again

Izzbo

I have not powered these up yet until I am sure the circuit is right.

I agree that your wiring description seems perfectly correct. I am not sure whether small DC motors present an inrush current problem when starting. Maybe a 1.5-amp rated power supply would give a bit more reserve. Can you verify the voltages and amperages with a multimeter? Do the motors heat up and burn out quickly? These are just a few ideas. (I'm kind of on a fishing expedition into a problem that doesn't look as though it ought to be a problem, but I am more familiar with larger AC motors than with smaller DC ones.)

This makes me think of weird stuff like 6VDC motors incorrectly labeled as 12VDC, farfetched as that may sound.... Maybe you could do a little playing around with batteries just to see what happens.

I initially used a 1.5A power supply on the first go-round. it burned up with the first 2 fans. I think that one was a bad supply as it was old and untested. the last 1 burned up using the current 1A supply that is tested and new. I am really starting to wonder about this start-up voltage thing.

Also (and this is probably normal) when the last fan burned up, the RPM on the remaining fan instantly jumped up a little.

More info: the first 2 fans ran for about 30 sec's before burning up. the last 1 ran for about 5 sec. I purchased these from a student that for some reason has about a dozen of these. all of the packages had been opened but taped shut. none of the contents actually seemed disturbed so I went ahead. maybe these were just defective motors.

I definitely want to explore the start-up voltage possibility before I power these new fans up as I would rather not spend more than the $50 i already have on a $100 unit!

Izzbo

Put some sort of a fan regulator between the fan and the supply and then slowly step up and see.

Looks like the fan motors are not 12V, may be less. Got a meeting, will come back, there seems to be some interesting points on your mail. Add up any other observation between the start and boom. Also what is burning? coil or the brush ?

burning after 30 seconds take out the inrush (ie starting current problem)

Meeting over as usual some blah blah and no result.

First power unit one burnt up with 2 fans - likely excess current drawn

Second one - (unit not damaged?) - fan burnt up - excess current, unit with protection, voltage dropped, but not enough to save the fan, fan burnt out, voltage rose, second fan accelerated.

Almost sure starting current - put an good old fan regulator, start at low speed and gradually increase. That will definitely protect the supply and the motor.

once the experiment is successful, you can plan for some sort of starting resistant.

In case you are getting sufficient speed check the voltage and current if possible (in case you are getting it at mid regulator level itself or lower) that case the motor may be a lower voltage motor and not a 12V one (my children play around with small 6V motors too)

No marking on motor ?

motors are marked 12VDC; .16A. I have a 12VDC regulator that came with the fans. its just a knob to control speed. I can use that for testing but I cannot use that in the finished unit. what would I use for starting resistance in the finished circuit? I imagine I would place it between the switch and motors?

Izzbo

Yes it will be just before the motor. Once you get the vfull speed, check the resistance of the regulator. This can be putpermanently in series.

However normally the starting resistance must be removed when the motor has achieved speed.

I think I do need the resistance removed after start-up. what would i use to acheive this?

Izzbo

May be a small push switch (two way) to make it simple- or an eletronic timer to make it complicated. In fact there are fans - where the starting capacitor (single phase system) gets disengaged by centrifugal action as fan picks up speed.

_{(Don't ask me I don't know the mechanical details of it, just the concept)}

BTW: did you experiment with regulator?

I'll look into that...

Haven't tested yet. gonna get to wiring it in and testing in about an hour. I'll let you know what happens

Izzbo

Kindly please tell me about the mechanical load ,because your supply circuit is correct But u may be provided a fuse of 0.5A, If this fuse is burnt that means motor carrying a large amount of power than there rating current.

Robin /INDIA

not sure exactly what you mean by a mechanical load, but the motor is simply turning an 80mm fan. The units are simple computer cooling fans. no fuses in them at least that are visible.

Izzbo

I've read all the posts and, like everyone, agree that there is no obvious problem....which means that the problem is something left field like the fans are AC not DC, the speed regulator is not simple and is really a step motor controller or combined rectifier thyristor control, some sort of third harmonic issue, etc. etc. none of which are likely to apply. I really can't see it being an in-rush current issue.

The one thing I'd have done "yesterday" and which might really help get to the bottom of things, and seems not to have been done, is to actually measure the supply voltage/s and load current/s. If for example the power supply is not 12 Volt as thought, or if the fan motors are really only 6 volt ones, then the input current to the fan motor/s will be too high.

It is assumed at this stage that you are not using the fans "in anger" and that the issue is not one of very hot air is passing over them causing failure that way.

I will do this shortly! How do I measure the actual voltage of the fans?

Izzbo

I would guess that the fans are suffering from overvoltage. These fans are brushless DC, and because they are brushless have built in electronics to do the commutation. If your power supply is not well regulated when under no load, when the device is first switched on, the voltage could be too high for the electronics.

what would your suggestion be for power supply then?

Izzbo

Like everyone else I can't see anything obviously wrong.

Just a suggestion

How much room have you got in the final product for the PSU. Why not ensure compatibility of PSU and fans by using all PC parts. You can probably find a scrap PC with working PSU (and maybe a chassis fan). If you buy them new, you should be able to get the PSU for less than $20, and the fans for less than $5.

You can then use the power on INPUT to the PSU to turn it and the fans on:-

The above pictures are from:- http://pinouts.ru/Power/atx_v2_pinout.shtml

Just put two wires on ground and the PS_ON pins on the motherboard connector, and, use the peripheral (4 wire) leads to power the fans.

thanks. I had considered that but wanted to avoid the extra bulk. those pix will help if i decide to do this in the future. I just might too because I think that with some brands/types of computer cooling fans, they have extra leads for all sorts of control options and i do not know how those will affect operation bypassing them. I did some testing and then went ahead and wired up the unit with the new fans. All is working great. thanx a billion guys! Those fans i got initially from that guy were just junk. the fourth one burned up when i tested it with the good speed controller i had!

Izzbo

I personally would go for a regulated supply, "Wall warts" are usually not and have a very high voltage when partially loaded. Find a "Wall wart" that is regulated, check with a voltmeter that its unloaded volatge is no more than 12 volts.

Some are adjustable with a rotary switch, set it down to say 9 volts, the fans will run quieter and live longer....

Get a "wall wart" that can suppy 2 amps. It should not run continuously near its maximum load....

The fans you are buying may be shoddy Chinese products. Go for quality.

Hello izzbo,

I was going to say, if the resistance of the motor is less than 24 ohms then it is not a 12v .5 AMP Motor, go with a different motor.

Regards,Tim

Have you looked at the loaded output waveform from your wall wart? By far most of these produce a rectified AC waveform with little or no filtering. This means that your fans are seeing a vaveform which still contains a lot of AC components...

I'm thinking that on startup, it needs more than the rated voltage to kick the first revolution out of the fan (larger motors have starting circuits with big capacitors in them so they can get (for example) 250 volts for a second or less into the wiring of a 120 volt fan. Why? because initial torque has to overcome friction and inertia. if the fan slowly spins up, each rotation will be generating heat. if it doesn't get up to speed rapidly enough to start efficiently cooling itself, the wiring is toast. Therefore, I would give it a lot of capacity beyond the rated capacity--if it will run at one amp or half an amp, have (just top of my head here) four or six amps available to it. It'll only take what it needs, but if it can't get what it needs, that results in lost efficiency which translates into heat. Other things that can burn out a little fan--expecting it to move too much air, running it to draw on or blow into a restricted air space (these fans were not designed to deal with a vacuum or a pressure head), or the fan body might be twisted in some way, resulting in deadly friction in the journal bearings. It would run for a while but when the lubricant evaporates due to heat, it would sieze and burn up. The plastic housings around a cpu fan is not designed to deal with any twisting. Check your voltages at every step. A transformer might be putting out less than 12 volts because the wall voltage is less than 120 volts. If possible, check your amperage at every step and make sure it's within design limits. Most computer fans are 12 volt, so you might assume they all are. But some might be 24 and 48 volts. Some might be 6 or 3 volts.

Everyone is addressing the electrical side which is not my area of expertise.

I do know a bit about fans, though, and your problem may be there. It would be nice if you could get a fan curve but that isn't likely for a packaged product.

From experience I've witnessed many times a cooling tower being started to test the fans only to see the starters trip after a few seconds or burn out the motors if there is no overload protection.

The problem is the fan is attempting to to produce too much work. In the case of a cooling tower, the water has to be flowing first as it provides a back pressure that lessens the CFM and the amount of work the motor produces. This problem is especially prevalent with centrifugal (AKA squirrel cage) fans. Prop fans, on the other hand, are more akin to positive displacement devices where blocking air flow increases the work load.

Centrifugal fans can have foreward inclined or backward inclined blades. Each has different characteristics. If your fans are foreward inclined, I can almost guarantee you excess CFM is the problem. Build a little box for it with a damper and watch the amps go down as resistance to air flow is increased.

Remove the fan from the motor. Then, run the motor to check if the fan is imposing too much load.

I'm far from being an expert on anything, but since the fans are from a computer, it sounds to me that the voltage is too high. 5V sounds like a computer voltage to me not 12V.

Check out this spec for a typical $15 300 Watt PC PSU:-

from:- http://www.octigen.com.tw/DB/Download/PSU%20+barebone/300W%288007CCOTG%29.pdf

That's 144 watts at +12V, 53 Watts at 3.3V, and 100 Watts at 5V

And this is a typical $4, 3½" (80mm) case or heatsink fan from Akasa at

http://www.akasa.com.tw/update.php?tpl=product/product.detail.tpl&no=181&type=Fans&type_sub=Classic%20Black&model=DFS802512L

The cheap unregulated 12v wall-wart puts out a peak voltage of around 18v. This is more than likely to fry the 12v brushless fan electronics.

Try a switch mode wall wart (regulated). You could scavenge one from a old external USB disk drive or purchase one separately (about $20).

Digikey or Jameco are possible sources on the net.

Like post #21?

Yea! Beat me to it.