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  • Easy question for someone (Measuring Amps)

    Is this the correct way to measure amperage draw on a DC motor:

    You put the meter in series with the motor, and allow the current to pass right through the meter?

    If that's not correct, what is the best way to measure a DC motor's amperage?

    I'd like to measure the amps drawn by my auxillary cooling fan. When the fan comes on there's a slight, but noticeable, drop in the engine RPM at idle. IOW, it seems as though the aux fan is drawing a lot of amps.

    I'd like to compare the aux fan amps with the published amperage of the Spal fans. It'd seem that using the most efficient cooling fan is desireable.

    Thanks!
    The day ain't over yet....

  • #2
    Are you using a regular VOM (voltmeter) or using an Ammeter? If you are using a Ammeter, there should be a switch for AC or DC source. Either way, yes, the probes have to be connected both with the load. Meaning, you have a positive and negative test probe, you measure on both terminals. If you let the Ammeter to measure one of the load side, then you will get nothing. You are just disconnecting the line. But a dedicated Ammeter is highly recommended. But if using a Clamp-On, then you measure one line at a time. Personnaly, I hate working with DC. I am more on the AC. I almost failed DC when I was in my engineering school daze. They are a pain. Hope that helps and my .02 worth.
    Last edited by Das Boots; 10-14-2009, 03:04 AM.
    Lawrence

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    • #3
      My OEM fan was doing same thing. On 2 different cars no less. The initial/startup current draw was excessive. It would pull my battery voltage from 13.6-8 down into high 12.x volts as well.

      You are correct about the amp meter wiring...just make sure the meter is capable of high amps. As mine was "dying" it must have been pulling way more than 25A.

      I'd also recommend looking at Flex-a-lite fans. Their cfm/amps ratio was better than spals when I last checked and they costs half as much through places like jegs, etc. I've had mine for 2+yrs as the only fan (no A/C though) with no issues.


      Nick

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      • #4
        Originally posted by ///mike View Post
        Is this the correct way to measure amperage draw on a DC motor:

        You put the meter in series with the motor, and allow the current to pass right through the meter?

        If that's not correct, what is the best way to measure a DC motor's amperage?

        I'd like to measure the amps drawn by my auxillary cooling fan. When the fan comes on there's a slight, but noticeable, drop in the engine RPM at idle. IOW, it seems as though the aux fan is drawing a lot of amps.

        I'd like to compare the aux fan amps with the published amperage of the Spal fans. It'd seem that using the most efficient cooling fan is desireable.

        Thanks!
        Pull the speed and low speed fuses and put your meter across the terminals - less
        hassle.

        Comment


        • #5
          Originally posted by Nikos View Post
          My OEM fan was doing same thing. On 2 different cars no less. The initial/startup current draw was excessive. It would pull my battery voltage from 13.6-8 down into high 12.x volts as well.

          You are correct about the amp meter wiring...just make sure the meter is capable of high amps. As mine was "dying" it must have been pulling way more than 25A.

          I'd also recommend looking at Flex-a-lite fans. Their cfm/amps ratio was better than spals when I last checked and they costs half as much through places like jegs, etc. I've had mine for 2+yrs as the only fan (no A/C though) with no issues.


          Nick

          Definitely make sure you’re meter can handle what you’re trying to do. I can tell you for sure my Fluke 88 will blow the fuse in the meter if I tired that. That’s why my meter came with a current clamp for applications like this.


          Originally posted by drinaldis
          I dated a girl who used to do the reverse grind. I kinda liked it.

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          • #6
            It's a Sperry digital VOM. I'll set it to high enough that I don't have to worry about burning it out.

            The load when the aux fan is on must be enormous -- my belt was loose enough that it'd start squealing when the fan went on, and stop squealing when the fan shut off.

            Thanks for the suggestions! I'll be interesting. I'm sure the fan motor is on the way out, so this might be a good test in any case.
            The day ain't over yet....

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            • #7
              FYI* I just looked at Sperry’s website and all their DVM’s, which can measure amperage, have a 10AMP limit. If you hook it up in SERIES to the 30 AMP fan circuit, you WILL ruin the fuse if not your meter. Since it sounds like you don’t have the proper device to measure what you’re trying to do, swap out the 30A fuse with something smaller and keep going down until you blow the fuse, that way you have a ROUGH baseline for the amperage the fan is pulling.


              Originally posted by drinaldis
              I dated a girl who used to do the reverse grind. I kinda liked it.

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              • #8
                Originally posted by Jeremiah View Post
                FYI* I just looked at Sperry’s website and all their DVM’s, which can measure amperage, have a 10AMP limit.....
                Thanks for checking! That's actually what led to my original question -- it didn't seem "right" to put a lot of amps through a digital device. I may just make the investment in a simple ammeter. If I recall correctly, they're not expensive and are pretty simple to use.

                Thanks again!
                The day ain't over yet....

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