finding the amps draw of a DC motor

Discussion in 'Electronics & Electrics' started by the Fan-Man, Jul 10, 2011.

  1. the Fan-Man

    the Fan-Man Member

    Joined:
    Jun 16, 2009
    Messages:
    1,568
    Location:
    Bronte Beach, Sydney NSW
    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 :tired:
     
  2. bonox

    bonox Member

    Joined:
    Jul 14, 2001
    Messages:
    3,337
    Location:
    Sydney
    Last edited: Jul 10, 2011
  3. dakiller

    dakiller (Oscillating & Impeding)

    Joined:
    Jun 27, 2001
    Messages:
    7,460
    Location:
    Gippsland
    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
     
  4. Mickatroid

    Mickatroid Member

    Joined:
    Jun 8, 2002
    Messages:
    1,563
    Location:
    Canberra
    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!
     
  5. oculi

    oculi Member

    Joined:
    Aug 18, 2004
    Messages:
    9,187
    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.
     
  6. RussellK

    RussellK Member

    Joined:
    Jun 11, 2003
    Messages:
    493
  7. Mickatroid

    Mickatroid Member

    Joined:
    Jun 8, 2002
    Messages:
    1,563
    Location:
    Canberra
    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. :)
     
  8. OP
    OP
    the Fan-Man

    the Fan-Man Member

    Joined:
    Jun 16, 2009
    Messages:
    1,568
    Location:
    Bronte Beach, Sydney NSW
    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/1294/536904753_4a63889520_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
     

Share This Page