Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Odd voltage reading from digital volt meter

  1. Jan 23, 2016 #1
    I have a three phase alternator and I used a digital voltage meter across the output terminal and the reading went from OL to 4000 volts to 3000 volts and quickly decreased to zero volts, then went through the range again starting from OL. It continued to do this, I thought the meter could have had a fault so I used another volt meter and the same thing happened. What would cause this?
     
  2. jcsd
  3. Jan 24, 2016 #2

    Svein

    User Avatar
    Science Advisor

    Maybe your voltmeter was set to DC?
     
  4. Jan 24, 2016 #3
    I had the volt meter set to AC volts.
     
  5. Jan 24, 2016 #4

    Svein

    User Avatar
    Science Advisor

    Then I suggest two tests:
    1. Set the meter to DC volts and measure an AA battery. It should show a bit above 1.5V.
    2. Set the meter to AC volts and measure the mains voltage. This should give you a stable reading.
    If both tests are OK, then there is something wrong either with the alternator or the measurement setup. One thing to check is the alternator load. Mostly those things have problems with the regulation if there is no load, so make sure there is a load of some sort (since you have not informed us on the nominal output voltage, I cannot tell you what kind of load).
     
  6. Jan 24, 2016 #5

    rbelli1

    User Avatar
    Gold Member

    What voltage should the alternator be outputting? Was this under a static RPM and load? What is the frequency that it outputs?

    BoB
     
  7. Jan 24, 2016 #6
    I tested all the meters and the DC and AC voltage measurements are correct with the appropriate supply. The load I am using is basically a short circuit, It drops 99.95% of the power across the load. The frequency remains stable at 240Hz with no fluctuation.
     
  8. Jan 24, 2016 #7

    rbelli1

    User Avatar
    Gold Member

    So the voltage you are trying to measure is a very small?

    You may be measuring the ambient em field rather than your alternator.

    BoB
     
  9. Jan 24, 2016 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Do you really mean that? Should it not be a resistance, suitable for the rating of the alternator? Is there any regulation for the field of the alternator?
     
  10. Jan 24, 2016 #9
    The resistance of the load is less than 0.0003 ohm. The voltage across the load is in excess of 100 volts. There is regulation for the field, it is supplied by a seperate DC source.
     
  11. Jan 24, 2016 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That makes about 300MW (X3?). That's a pretty fearsome alternator. What is the nominal output voltage per phase? You are not giving us the whole story here. Is there not some other metering associated with the alternator? What's the background?
     
  12. Jan 24, 2016 #11
    The source per phase winding had a resistance of 0.7ohm. I tried to get a current reading with a clamp meter and it was showing zero amps. I thought the value of voltage across the load was current x resistance. I tried to measure the load voltage many times with different multimeters and they would show the same results. The odd thing is the prime mover was a 1/3 HP motor. The results are odd, as well I ran the alternator several time and the results were the same.
     
  13. Jan 24, 2016 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    There is something very strange here. Have you a diagram and a photo of your setup? Why did you apply a dead short to the output? No alternator is designed to work into a short.
     
  14. Jan 24, 2016 #13
    Sorry, but I don't have a diagram or photo of this setup. I designed a circuit that interfaces into the alternator to collapse the magnetic field of the system. It seems to have worked, just odd output results. I used a short circuit as a load to provide a high counter torque on the alternator for testing purposes.
     
  15. Jan 24, 2016 #14

    Tom.G

    User Avatar
    Science Advisor

    Insufficient data!
    What was the original intended use for the generator?
    What is on the nameplate?
     
  16. Jan 24, 2016 #15

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    None of this is making any sense. Waste of time so far. No diagram and no decent description. We are told it is a 3 phase alternator but are not told how the phases are connected. Implication is a single load, but is it 3 loads so it is balanced for all 3 phases? Anyone who can
    should not need to ask a question about voltmeter readings. Nor should they provide a load of .0003 ohms with a winding resistance that is .7 ohms and claim that:
    and
    Technically we drop voltage, not power. How do you know it is stable at 240 hertz? What kind of device are you measuring this with? A 1/3 HP motor will at best get 150 to 200 watts out of the average alternator. What makes you think you can possibly get 100 volts across .0003 ohms for a power of well above tens of megawatts? Is your load actually a large inductive load?
     
  17. Jan 24, 2016 #16

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    i'd rather be fishing


    8051629-boat-fishing-trolling-panoramic-rod-and-reels-blue-sea-wake.jpg
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Odd voltage reading from digital volt meter
  1. Volt/meter to decibel (Replies: 6)

Loading...