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Baffled with experiment

  1. Jan 2, 2010 #1


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    I have a dc motor which I am not sure of it's power and was checking the volts and amps. I used a drill to rotate the dc motor.

    I have a multimeter and I was measuring the motor output, the outcome I got was 40v and when I checked the amps the motor suddenly slows down then stop after the multimeter reads 150amps.

    Why this happens every time I tried to measure the amps, it suddenly stops?

  2. jcsd
  3. Jan 2, 2010 #2
    The ampmeter, ideally, has a very low resistance, so if you measure the current like you would the voltage, you are redirecting the current from/around the whole or part of the circuit.
  4. Jan 2, 2010 #3
    You might like to compare how fast the motor slows down unconnected versus short-circuited (noting the principles of how voltmeters/ammeters were constructed, say from an introductory physics textbook). Cute classical example of the observation influencing the results.
  5. Jan 2, 2010 #4


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    I was wondering if there are any ways to measure the amps since I don't know where I got the DC motor from and there are no labels? I am trying to find a way to measure the motor's power output.
  6. Jan 2, 2010 #5
    The proper way to measure current is to make the ampmeter part of the circuit, connected in a series to the motor.
  7. Jan 2, 2010 #6
    The electric-drill experiment isn't helpful because the voltage will vary depending upon how fast you spin the drill. You need to have a real load on the output of the motor, not just an amp-meter on the leads with low resistance. Meaning, you need load on the output shaft that's variable and you need a variable voltage input. As you increase the load on the input shaft, you'll notice that the amperage-draw increases (voltage will drop due to internal-resistance of motor). The input voltage will dictate the RPMs and the load will determine the amps drawn.

    It really comes down to heat generation really. The size of the wire in the windings will dictate how many amps you can pump through that motor and if you try to get too much power out of it, it'll overheat.
  8. Jan 3, 2010 #7
    A meter set to measure amps has to be inserted into the circuit in SERIES, if you took a meter set to amps and touched the probes to the motor leads, you have effectively shorted the motor and you are lucky if you didn't wreck your meter. You can either get a clamp on meter or series your meter with one of the motor leads. (Most meters are 10amp max)
  9. Jan 3, 2010 #8
    lol, I missed this before I replied.
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