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Oscilloscope and AC generator questions/observations

Discussion in 'Electronic Basics' started by Eric R Snow, Feb 15, 2006.

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  1. Eric R Snow

    Eric R Snow Guest

    Greetings All,
    I am learning how to use an oscilloscope. The one I have is a TEK
    465B. It sure is amazing what this device can measure. I have some
    questions that I haven't been able to find answers for yet. I decided
    to measure the frequency of a small (850 watt) AC generator. It's
    useful for powering lights, drill motors and the like. To make sure I
    understood what the display should look like the first thing measured
    was the 60Hz from the power company. Using a step down xmfr with a 9
    volt secondary the 'scope displayed slightly over 8 divisions with the
    divisions set at 2 mS. That works out to just over 16 mS which is 60
    Hz. So I knew that the scope was showing what I wanted. The wave form
    wasn't perfect though. On the rising side of the wave the form was
    distorted slightly instead of being a perfect sine wave. It looked
    sort of like a little part of the curve was a straight line. Could
    this be caused by the step down Xmfr? When measuring the wave form
    from the generator it appeared to be outputting 45Hz frequency. The
    wave was 11.5 divisions long. The 'scope settings were the same as
    when measuring the line power. Also the wave wasn't a smooth sine
    wave. It looks like the wave is made from a rippled string. Are these
    harmonics? What determines the frequency of an AC generator? The label
    on the generator says it has a 60 Hz output. The 2 stroke engine
    driving it seems to be operating at the proper RPM just by listening
    to it but it may be running too slow. I will use an optical tachometer
    to measure the actual RPM. Another test that is going to be done is to
    power a small induction motor and measure it's rpm. This motor is a
    1/8 HP 1725 RPM motor. If the 45 HZ measurement is accurate then the
    motor should spin about 1300 RPM (I think). I have used this generator
    with this motor in the past to power a pig roasting spit and the motor
    worked fine and ran cool so I know the starting windings were not
    energized while it was being powered by this generator. But it's
    geared down to spin the spit at about 1 RPM so if it spun slower it
    would be hard to tell.
    Thanks for reading and any answers.
    Eric R Snow
  2. Google for "how to use an oscilloscope" and you can get a ton of
    information in regards to questions like this, I downloaded and printed
    a great PDF from HP I believe which talked about scope basics and
    various things, I did just last month and found that signals don't
    always appear on the scope as they do in books, etc.
    What I did is google how to use a 555 timer to create a little circuit
    in which I could test the scope, meter etc, it took about 30 minutes and
    I was satisfied that the frequency was correct on the scope and meter.
    I still think my scope should be calibrated but have not found anyone
    local to do it
  3. Dan Hollands

    Dan Hollands Guest

    You should be able to look directly at the 120Vac line voltage with the
    scope. You can also usually just grab onto the end of the scope probe and
    get enough 60Hz pickup to verify the approximate sweep freq.

    From your description it is almost certain the the motor powered generator
    is running slow. The power company waveform will be very close to 60Hz

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  4. unbiased

    unbiased Guest

    Scoping the alternator under no load conditions will look different
    that half or full load.

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