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Oscilloscope and AC generator questions/observations

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. Cheers, Eric R Snow
Reply to
Eric R Snow
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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

Reply to
James Douglas

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

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Dan Hollands
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Reply to
Dan Hollands

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unbiased

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