pugwash Posted September 6, 2006 Share Posted September 6, 2006 I have got myself totally confused about electrickery at the moment. say i had a 12v fan from a computer that pushed a huge amount of air (useful for cooling oil coolers) but was rated to 1amp what would happen if i connected into the main wiring loom? would it only draw the maximum current it needed, or would it spin uncontrollably ntil it blew itself up trying to run at huge amps? this is such a dumbasss question- no taking the tiddle just because i is a little confused! Quote Link to comment Share on other sites More sharing options...
Turbocharger Posted September 6, 2006 Share Posted September 6, 2006 The fan has a certain resistance so if you supply 12v that is what decides the current, not whether you have a lunking great car battery or a stack of watch batteries on the other end (so long as the batteries can supply that current). It's slightly more complicated because motors have inductance too, which limit the speed they work at, but the principle is the same. Quote Link to comment Share on other sites More sharing options...
02GF74 Posted September 6, 2006 Share Posted September 6, 2006 The fan has a certain resistance so if you supply 12v that is what decides the current, not whether you have a lunking great car battery or a stack of watch batteries on the other end (so long as the batteries can supply that current).It's slightly more complicated because motors have inductance too, which limit the speed they work at, but the principle is the same. I'm not sure about the second part re: inductance. what causes the motor to turn in the first place? magnetic force - there needs to be two manets and as like poles repel, it is this that turns the rotor. most likley there is a winding that becomes an electro magnet and there'd be a permanent magnet. the power of the magnets will be the driving force plus other stuff like the restistance to rotation, mainly that of the fan spinning in air - remove the fan and the motor will go faster. If you decrease the voltage, the current is less, so less magnetic force so slower spinning. the motor shouldn't burn out as the thin wire making up the coil will have resistance so limiting the current (actually it may as voltages in a computer are well regulated at 12 V; in your LR the laternatro will be chucking out up to 14 V - that may be enough to fry the fan). Quote Link to comment Share on other sites More sharing options...
simonr Posted September 6, 2006 Share Posted September 6, 2006 Or to put it simply, it will draw about 1A unless it's broken! Use a fuse that is a little higher than 1A - say 3 or 5A Not a particularly dumb question. It's only a pitty motors dont just run uncontrollably fast - it would be good on a skateboard! Si Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 6, 2006 Share Posted September 6, 2006 A DC motor with a standard commutator will draw the current it needs. If you increase the load the current will increase. Its resistance will determine the stall current, and usually start a fire. Inductance doesn't affect a DC current, as the motor runs it acts as a generator and produces a voltage to oppose the driving voltage, this is called the back emf. If you try to crank an engine with a duff battery the starter won't spin fast enough, not enough back emf to oppose the current which rises and overheats the motor. This is why a low battery will burn a motor out, whereas a good one wont. If you drive a motor with AC, the above applies, but the inductance will also oppose the current. If you get the windings right the inductance will prevent the current rising enough to burn the motor out, these are sometimes called "inductance protected", a common example being the flat AC fans used in computer equipment. Edited - Correction should be "impedance protected" impedance is basically the resistance due to inductance. Quote Link to comment Share on other sites More sharing options...
BogMonster Posted September 6, 2006 Share Posted September 6, 2006 If you try to crank an engine with a duff battery the starter won't spin fast enough, not enough back emf to oppose the current which rises and overheats the motor. This is why a low battery will burn a motor out, whereas a good one wont. I'd always wondered why you could burn out a starter with a flat battery - interesting Quote Link to comment Share on other sites More sharing options...
landmannnn Posted September 6, 2006 Share Posted September 6, 2006 I doubt a pc fan would draw enough air to assist an oil cooler. Maybe if you were stationary for long periods it would be better than nothing I suppose. The other problem with automotive use is damp and dust which would destroy a pc fan very quickly. Quote Link to comment Share on other sites More sharing options...
pugwash Posted September 6, 2006 Author Share Posted September 6, 2006 I doubt a pc fan would draw enough air to assist an oil cooler. Maybe if you were stationary for long periods it would be better than nothing I suppose.The other problem with automotive use is damp and dust which would destroy a pc fan very quickly. you'd be surprised- i have 3 80mm fans which between them will pull 240cfm- not far off what a 14" kenlowe will do in front of a rad. these fans are dust proof, running on ball bearings; whilst they might indeed go wrong- at only 3.20 i can swap an awfully large number of these before i get to the price of one kenlowe! they will be protected out of the way of mud and carp anyway and they are very veyr far from being situation critical. Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 6, 2006 Share Posted September 6, 2006 FWIW, i've been using an 80mm 12v fan to ventilate my cubby box and cool the PC inside it, and that's not suffered from dust, though obviously the cab is (less) dirty than anywhere an oil cooler is going to be. They do seem to shift a lof of air, be interesting to see how they perform. Luke Quote Link to comment Share on other sites More sharing options...
02GF74 Posted September 7, 2006 Share Posted September 7, 2006 Inductance doesn't affect a DC current, as the motor runs it acts as a generator and produces a voltage to oppose the driving voltage, this is called the back emf. If you try to crank an engine with a duff battery the starter won't spin fast enough, not enough back emf to oppose the current which rises and overheats the motor. This is why a low battery will burn a motor out, whereas a good one wont. back emf, hmmm interesting, seems I need to remind myself about this. I am not convinced this is the reason though; putting a load on a duff battery will drop its output voltage so the current going through the motor drops too. I would propose the reason the starter motor dies is because the person is running it for a very long period of time trying to start the engine and this prolonged running is heating up the wires. Also the current to the motor is going to be the highest when the motor is cold and before it starts to move i.e. right after the igntion key is turned - that is when the motor should blow, (like light bulbs, fuses etc) but although I've never experienced this, I'll stick ,y neck out and say it doesn't go pop at the point - be interesting to find out though. Quote Link to comment Share on other sites More sharing options...
jules Posted September 7, 2006 Share Posted September 7, 2006 If this is any use to you I don't know The Freelander td4's have a pair of three speed fans which have a control box mounted to them, This is inturn controled by the ECU.... I don't know if its possible to make a box that can act as the ECU so then you can have fans which have three speed settings. I have wanted to fit these to the 90 for a while but don't have the tech no how to do it but it must be possible.. they have a large current draw I assum buy the size of the main power feed due to the fans being useless to any other car they should be cheep to buy, well my last set was.... Only a thought Quote Link to comment Share on other sites More sharing options...
Mark90 Posted September 7, 2006 Share Posted September 7, 2006 will pull 240cfm- not far off what a 14" kenlowe will do The 9" pacet I have on my cooler will do 500-600ish cfm, can't recall the exact figure. Quote Link to comment Share on other sites More sharing options...
edwardatherton Posted September 7, 2006 Share Posted September 7, 2006 Kenlowe Performance Chart give the specs, and a 13" standard fan running through a "4x4's thick radiator" is 761 CFM. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 7, 2006 Share Posted September 7, 2006 1) putting a load on a duff battery will drop its output voltage so the current going through the motor drops too. I would propose the reason the starter motor dies is because the person is running it for a very long period of time trying to start the engine and this prolonged running is heating up the wires.2) Also the current to the motor is going to be the highest when the motor is cold and before it starts to move i.e. right after the igntion key is turned - that is when the motor should blow, (like light bulbs, fuses etc) but although I've never experienced this, I'll stick ,y neck out and say it doesn't go pop at the point - be interesting to find out though. 1) Yes, but the lower current is still sufficient to burn a motor out. Obviously if the battery is near zero you'll probably be OK. When you struggle to start an engine a full battery is eventually discharged to bring us into the area where the motor will suffer. You'd be OK if you were using an external jump starter. Starter motors are a bit of a special case anyway because they're not designed for continuous use. 2) Starter motors don't blow like a bulb, they go slowly. The commutator may burn slightly due to overcurrent, this will make it less efficient, and so the effect snowballs. Certainly if the battery is not kept up to scratch the starter motor will have a shorter life. Quote Link to comment Share on other sites More sharing options...
02GF74 Posted September 7, 2006 Share Posted September 7, 2006 1) Yes, but ....... Starter motors are a bit of a special case anyway because they're not designed for continuous use. ...... that's effectivley what I said. If you, or like my neithgbour, try to run it for minutes non stop rather than the 3 or 4 seconds it is meant to be run for, they it is going to heat up and I still still my neck out to say that is why they blow. If it were not the case, and lack of back emf were the cause, then they would blow immediately the igntion key is put in with a low or bad battery. I have nothing to support this outlandish cliam though but I've never blown a starter motor with a or bad low battery. (If I do run the starter for any length of tim, I puse for a while to let it cool down). Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 7, 2006 Share Posted September 7, 2006 that's effectivley what I said. If you, or like my neithgbour, try to run it for minutes non stop rather than the 3 or 4 seconds it is meant to be run for, they it is going to heat up and I still still my neck out to say that is why they blow. If it were not the case, and lack of back emf were the cause, then they would blow immediately the igntion key is put in with a low or bad battery. I have nothing to support this outlandish cliam though but I've never blown a starter motor with a or bad low battery. (If I do run the starter for any length of tim, I puse for a while to let it cool down). OK in the case of starter motors there are two ways of blowing them up! They don't blow immediately you switch them on because they are robust enough to withstand the heating effect of the current for the time from turning the key until they start cranking. If you prevented them from rotating they would heat up & burn out after a short time. Also bear in mind that a back emf is generated by the inrush of current into the inductive windings (self inductance) so you wouldn't get the full short circuit current providing the motor wasn't jammed. Inertia starters are further protected because they are already rotating before the load is applied. Quote Link to comment Share on other sites More sharing options...
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