Blower Questions
 

I bought my first blower today. Its a beast compared to the fans I'm currently working with. When I'm done it will be mounted inside a radiator box. I've only tried it at 5V at which it pulls 4.25A. I have no solid number for CFM but there is a definate breeze coming from it. Now for the questions...

To test it I used an old AT 250W PS. It didn't seem to like a cold start. It would jerk slightly then stop, I'd have to shutoff the PS then try again. My guess is that it requires more startup Amps then the PS can handle so the PS shuts off. It will start as long as I manual spin it before I apply power. Other than a larger PS can I add a cap or something to help it start?

Secondly this blower is "naked". Is a shroud required for it to operate well? I will be building a radiator box around this so I was planning on cutting a 5" diameter hole for in the intake and just let it suck air into the box. The only escape for the air would be through the radiator.

What is it rated for? how many volts?
my guess 12 volts?
what if you apply 7 volts? connector from the 12 and 5 volt lines...
or even 12 volts... will it power up...
if that works you may want to come up with a system that it starts up at 7 or 12 volts but once it starts up it automaticly or manualy switches to 5 volts.
my 2¢ worth

At 12V the same thing happened. Thats another reason I beleive the PS just cant handle the start up draw. The PS is "rated" at 25A@5V 10@12V also keep in mind this is a salvaged AT PS thats had its fair share of abuse.

If there is not a sticker on the blower, you may have to try and locate the specifications for it. This presumes you know the brand and the company has a web site with product specifications on it.

Knowing the specifications will allow you to judge the maximum amprege draw.

If you can't find the specifications... and if you have another, more modern power supply (350 W or larger), I would try that first. Last resort is to try a car battery... no lack of amps at 12VDC on that baby. :D

If you still see the problem, and you are sure you are applying plenty of amps, then you may have a bad blower.

As for needing a shroud, are we talking a squirrel cage fan or traditional fan? A picture would help describe the "nakedness" of the fan for us.

That doesn't seem surprising to me.

The amp draw is "average", but the startup is going to be tremendously high, as the brushes essentially short on the first position of a rotor coil.

I use a blower from a Ford Mustang, and I can't get it to start at 12 volts (same prob; the PSU shuts down). I haven't taken any measurements yet, but I'll try to "pre-spin" it, and see if it'll go. Mine starts just fine at 5 volts.

An inductor coil might help your fan get started, but don't ask me which one... For sure if you can't start it normally, it's gonna be useless to you.

That aside, I'd recomend that you use the protection circuit outlined in the "Johnson pump" thread (in my sig), to protect your CPU from spikes and negative voltage throws.

The brushes don't short on a motor at any point. The problem is when you switch on a device with a high power coil you have to form the magnetic field, which takes a lot of energy. Once it's running you're basically just powering the resistive loss in the coils. And yes spike protection is a good idea - if it has a huge inrush current you're going to get a badass spike off it when you power down.

ah, thanks for the clarification Butcher!

I always thought that the brushes, in a stopped position, would contact one of the rotors coils, which is essentially the same as a short, as opposed to taking an average amp reading when the motor is running.

My cheap "opening a bank account" digital camera died. So no pics fo now. I can't overstress the naked of this blower. The motor section is enclosed and includes a mounting plate. While the blower wheel itself has no shroud at all.

The model is Unimotor 11588 12V 03281. I couldn't locate it on the companys website.

If it was shorted at stop you'd never get it moving as the coils would never energise. :p

i would think you could use a cap on it to start it, but my guess it'd have to be a fairly large one. a large enough cap would be able to deliver more than enough cranking juice to start it, especially since you're talking about probably less than 1-2 seconds of startup time.

A capacitor won't help with the start-up unless you also use a timer & relay to delay starting the blower until after the PSU is on and your cap's have charged.

Easier is to add inductance in series with the blower. On mine I've recycled a big inductor from a dead PSU.

The diode in the diagram is absolutely necessary if you add inductance (and a good idea even if you don't). Use a diode that can handle 100 Amp surges, preferably a Schottky diode.

The cap in the picture is to cut down on the electrically noise produced by the brushes. Use mylar or polyester or film or ceramic caps and have a close to the blower as possible. Different values handle different frequency noise, so use a combination of a couple. I used 10 pF, 1 nF, 100 nF, 10 uF.
http://pages.sbcglobal.net/water.gro...es/blower1.bmp

I think you'd need a really large inductor to make any difference. (0.5H or more? based on a quick dI/dt calc).
A capacitor would work, but only if you devised a current limiting system to charge it, with a timer, like Groth says. Even so, you'd probably need a few 10000µF.

I would just try adding some series resistance: between 0.2 and 0.4Ω would limit startup current to 15A or so if you use the 5v rail. The blower would obviously run slower, as you would be dropping about 1v across the resistor.
You would need it to be rated for at least 5w, unless you put it in the air flow....

If you want to calculate the resistor value yourself, you ideally need to know the blower motor winding resistance. I'm just guesstimating 0.1-0.2Ω.

Edit: dont even think about running that thing on 12v, it would need well over 20A nominal, let alone startup :shrug:
And yes, it does needs a "shroud" (volute may be the correct word), which can just be box with a circular inlet hole :dome:

If it's 4.25A @ 5V, then you're looking at around 10A (where'd you get 20A from?) for it nominal - inrush current could easily be 40-50A.
The diode is definitely a good idea to avoid reverse spikes blowing your psu up, I tend to use a rectifier diode usually (1N4001 or such).

BTW to handle 40-50A of surge you're going to need some badass caps - most have a ripple current rating of less than 10A. :p

That's the part I'm talking about, Butcher.


That aside, the cap alone definitely won't work: it's just going to suck up more power to get charged, because it'll be discharged at startup. The coil is what would make it happen, but it'd have to be hefty (as iroc pointed out), and that's going to have to involve a diode (as pointed out by Groth).

Maybe a PWM circuit would do the job here. Ideally, it'd have a low voltage startup sequence, or something... (but that's AWM).

Fan laws state that power rises as cube of rpm .
rpm is roughly proportional to voltage.
thus current will vary as square of voltage.

Hence you would need (12/5)^2*4.25 (=25A)
edit: i forgot about dry friction. so 15-20A may be closer to the truth

that bit I agree with:D

Sevisehda, I suggest you try powering the motor (on 5v) through a few meters of thin speaker cable as an experiment.

easy - leave PSU on, insert switch into circuit and turn it on with that.
no need for complicated timer/relay.....

Thanks for all the ideas. I'll have to go over them again to see what looks best.

After thinking about it, I realized that I would need to put a PS in the rad box itself. Since that I only need ~5V I may look into using a non-AT(X) PS. Then use the protection elements you've recomended here.

Some possible PS I've found

http://www.excess-solutions.com/Spec...SP280-60FS.pdf
http://www.mpja.com/productview.asp?product=14066+PS

Take it out to the garage, open the hood on your car and hook it up to the battery terminals. You can test it that way to see if it works before spending any more time on it.