Ugly Sine Wave

Discussion in 'General Electronics Chat' started by qitara, Dec 12, 2013.

  1. qitara

    Thread Starter Member

    Jan 18, 2013
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    Hi guys

    Was taking some measurements on a Genset yesterday using my handheld oscilloscope and noticed that its producing a weird sine wave, what is happening here ?

    [​IMG]

    [​IMG]


    Note the two pictures is from the same output, just different resolutions.


    The generator is 1 year old, i am listing the specs below

    15 KW

    50 Hz 4 pole 1500 RPM

    Single phase

    Brushless alternator


    Thanks in advance guys
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    If it's ugly, it aint a sine wave ! ;)

    That looks more like a RC charge/discharge profile. I think you'll need to supply more detail about your probes, setup, etc. for the pros to diagnose your problem.
     
  3. qitara

    Thread Starter Member

    Jan 18, 2013
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    these measurement where taken with no load on the genny, and the same result is when i connect loads to it Inductive/Resistive , and it is not causing any problems but i am curios to know why i am getting a such wave
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
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    It isn't an inverter/generator by chance?
     
  5. qitara

    Thread Starter Member

    Jan 18, 2013
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    no, it is a purely generator
     
  6. Sensacell

    Well-Known Member

    Jun 19, 2012
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    I would check the probes.
    The rotating mechanical nature of a generator makes it unlikely that it's really outputting that waveform, try the CAL square wave test point that most scopes have for probe compensation adjustment, see if you really get a clean square wave.
     
  7. qitara

    Thread Starter Member

    Jan 18, 2013
    87
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    is there any way that the AVR can be causing this ?
     
  8. tcmtech

    Well-Known Member

    Nov 4, 2013
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    To be honest very few gensets put out a perfectly shaped sine wave. What you have is completely normal.

    In fact I would be pleased if our utility power looked that clean most days. :p
     
  9. qitara

    Thread Starter Member

    Jan 18, 2013
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    Hmm.... i don't fully agree with you, the utility power is producing a very nice and clean sine wave, comparing that to the one i am getting is a big diffrence.
     
  10. tcmtech

    Well-Known Member

    Nov 4, 2013
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    The wave forms around my area vary greatly from hour to hour and at times moment to moment. ;)

    Is the slight distortion of the generators wave affecting anything or are you just being picky about it?
     
  11. qitara

    Thread Starter Member

    Jan 18, 2013
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    To be honest i am being picky about it, reason is that to my experience the wave of an brush less AC alternator should be looking like that
     
  12. tcmtech

    Well-Known Member

    Nov 4, 2013
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    There are very good medications that make concerns like that far less concerning. ;)

    Theory, hope and expectations tend to fall rather far from what life actually is and how it actually works.

    What you are seeing is the result of the generators relative rotor velocity fluctuating in response to the mechanical give and take of energy in the engine every time a cylinder goes through a compression stroke followed by a power stroke.

    Nothing you can do about it. Its just physics doing what the laws of physics dictate over physical motion and conservation of energy.
     
  13. soulripper

    New Member

    Sep 21, 2009
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    Hey there,
    for the "ugly" sine wave, I think there are even harmonics (2th-100Hz, 4th-200Hz and so on) presented in the sine wave that are causing this distortion. These harmonics distort the wave in a similar way. Any induction motor/generator is designed, in particular the coil schematics and the rotor/stator slots, to avoid the presence of even harmonics.
    I also agree with tcmtech that the AVR may cause these distortions, because if there are any pulsating moments on the shaft of the generator, a back EMF is generated and the generator in a very brief moment act like a motor.
    This is what I can say about your sine wave :).
    (So maybe if there is a way to tune somehow the gasoline engine, you can smooth the voltage, but I have to say that this is not my field of knowledge.)
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Does the generator have on-board voltage regulation. If so (I assume so), what method is used to regulate AC voltage?
     
  15. qitara

    Thread Starter Member

    Jan 18, 2013
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    I don't fully agree with you on that because there is a flywheel installed between the engine and the alternator that filters the fluctuating out.
     
  16. qitara

    Thread Starter Member

    Jan 18, 2013
    87
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    excitation voltage control and voltage stability control
     
  17. tcmtech

    Well-Known Member

    Nov 4, 2013
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    The flywheel sees a lot of velocity variations during one revolution depending on whether it's gaining energy from a power stroke of losing energy to a compression stroke on top of its varying torque loading occurring from the AC generator itself.
    Its no different than the voltage fluctuations that a capacitor sees when being supplied by a intermittent power source like rectified AC. During the peaks it's gaining velocity (Voltage) during the troughs (zero crossing) its losing velocity (Voltage).

    Its average rotational velocity is not the same as its instantaneous rotational velocity just like how the primary capacitors in a power supply have a small amount of ripple voltage superimposed on its main DC voltage.
    The average voltage of the power supply may be 12 volts DC but there is some ripple voltage on top of it as well which depending on how large the capacitor is and how low of high its ESR is that ripple could be micro volts to several volts.

    In a AC generator systems that velocity ripple in the rotating mechanical parts can show up as waveform distortion or skewing like what you see when the combination of mechanical and electrical conditions are just right. ;)
     
  18. qitara

    Thread Starter Member

    Jan 18, 2013
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    How does it come that other generators doesn't produce the same distorted wave ?
     
  19. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    If you have an FFT function, you can see the harmonic content of that sine wave.

    Below is a panoramic view of the 60 Hz at my shop when I was in western Oklahoma, about 125 miles N of Amarillo TX. The generating plant was in Amarillo.
     
  20. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Because they are designed different than yours. :rolleyes:

    What exactly are you expecting from a brushless type 15 KW genset that runs at 1500 RPM anyway?

    Are the loads you are running that sensitive to the wave distortion or is this just a case of someone having a O scope and too much time on their hands to worry about what they are finding with it because it does not match an assumed perfection? :confused:
     
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