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Old 10th July 2011, 8:08 PM   #1
the Fan-Man Thread Starter
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Default finding the amps draw of a DC motor

Hi guys, I bought some radiator fans from a wrecker for a wall mount fan extraction system at home. But i can't find out the watts/amps of these fans.

They both only display volts (12v) but i need to know the amps for the appropriate powersupply. The only label on the units is one with the voltage and model number

1) Bosch IPL 12V 0 130 703 215
2) "power motion" 12V SLM 1209-02 64.54-8 351 577

These are pretty big-ass fans, there are 2 boschs from the V8 7 series bmws and one cyclone fan (power motion) from the M5

But i dont know the amps yet :/ I have a cheapo voltmeter from tandy, is there a way that i can use resistance to get the amps?

thanks in advance

tFM
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Old 10th July 2011, 8:17 PM   #2
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you may find you need >20A

http://www.zeebuck.com/bimmers/tech/...ectricfan.html

edit: you can use ohms law to find stall current, but i don't know what the running effects have on draw.

Last edited by bonox; 10th July 2011 at 8:43 PM.
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Old 10th July 2011, 10:13 PM   #3
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There is no way to measure the standalone fan to find out how much power it uses, as that depends on how much air resistance on the blades there is to push against. Really need to power it up and measure it
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Old 10th July 2011, 10:19 PM   #4
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No multi-meter I take it just volts. OK, I am no electrician but highschool physics taught me a few things

The voltage drop across a resistor in series with the fan will let you figure out the current: Volts (V) = Current (I) x Resistance (R). So with a known resistor and a known voltage across the resistor you can figure out how many amps are flowing (I = V / R). The resistance should be very low and able to dissipate lots of energy (resistors rated at 10W are easy to get at Jaycar). It's role is to work as a shunt.

This is basically what an ammeter is http://en.wikipedia.org/wiki/Ammeter

Be very careful though. Depending on the values involved, if the fan is drawing a lot of current the resistor may get extremely hot or even catch fire.

There is no substitute for the right tool and multimeters from Jaycar start at $20, this is the only method I would recommend. Even then it is easy enough to blow a multimeter up (I have a fuse on order for mine ATM after a silly mistake). Make sure you understand how to use it.

Using a lower supply voltage may make things less exciting but many (all?) fans will draw more current at lower voltages.

The motor's 'start up current' may also be an issue when it first turns on. Serious current may be involved to start things moving.

Finally, if 20 Amps is involved it may be more than the multimeter can handle!
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Old 10th July 2011, 10:21 PM   #5
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A good place to start would have been looking at the fuses for the fans.

what are you planning on powering them with? i'm sure 12 volts seems like a great idea but it is pretty impractical outside a car for high power stuff, also depending on your ducting design they may not work the way you want them to, radiator fans are low pressure high flow beasts wheras you would probably want something that is more moderate flow and higher pressure like a centrifugal fan.
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Old 10th July 2011, 10:29 PM   #6
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Jaycar sell shunts, which are basically very low value resistors this one should do the job:
http://www.jaycar.com.au/productView.asp?ID=QP5412

I'm guessing 6-50 amps should cover what the fans draw.
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Old 10th July 2011, 10:35 PM   #7
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Quote:
Originally Posted by RussellK View Post
Jaycar sell shunts, which are basically very low value resistors this one should do the job:
http://www.jaycar.com.au/productView.asp?ID=QP5412

I'm guessing 6-50 amps should cover what the fans draw.
Having had a longer look at Bonox's link, I think my methods above are better suited to regular computer fans. The Ohms law stuff is OK but the rest of the post can be safely ignored. Soz.
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Old 11th July 2011, 12:05 AM   #8
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Quote:
Originally Posted by bonox View Post
you may find you need >20A

http://www.zeebuck.com/bimmers/tech/...ectricfan.html

edit: you can use ohms law to find stall current, but i don't know what the running effects have on draw.
the fans are muuuuuuuuuch bigger than that one :P

the cyclone fan probably wont be appropriate for its blade design as an exhaust fan:
in this pic, the fan on the left: http://farm2.static.flickr.com/1294/...63889520_o.jpg

but the other fans are condensing fans and produce high pressure, with 7 blade rotor design fan blades

the car fuses it off to 40a, but this is shared with instrument cluster
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