The Ultimate Mechanical and Electrical Thread!

Discussion in 'General Science & Technology' started by Thoreau, Nov 19, 2008.

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  1. Thoreau Valued Senior Member

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    As a generator technician, I figured we needed a thread for all those out there that have a passion for mechanical and electrical engineering; a place where we can get together, question, and discuss the various aspects of M&E engineering and its components.

    So have fun!
     
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  3. Thoreau Valued Senior Member

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    (and yes, it is a science and technology, but believe since science is the basis of all mechanical and electrical theory and applications, that it belongs in the Science sub-forum)
     
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  5. MacGyver1968 Fixin' Shit that Ain't Broke Valued Senior Member

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    Can you go over why certain electrical devices us VA instead of watts . You mentioned something in the generator thread..but a more detailed explanation would be nice.

    Thanks

    Macgyver1968
     
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  7. jpappl Valued Senior Member

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    Damn MacG.

    I was hoping to read another one of your witty one liners.

    Keep em coming.

    MZ3,

    What are you thoughts on spending the extra money for say a honda compared to one you might get at home depot. There is a huge price difference, for someone like me who runs into a few days without power a year, can you tell me why or why it is not worth spending the extra dough. Anything I should look for specifically other than power limit.

    Thanks.
     
  8. leopold Valued Senior Member

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    it could be because of power factor.
    there are two ways you can measure power. one is the straightforward ohms law, the second takes into account the reactance of the circuit elements and is called the apparent power this probably accounts for the discrepancy.

    transformers are usually rated in VA whereas motors and generators are measured in watts.
     
  9. Carcano Valued Senior Member

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  10. MacGyver1968 Fixin' Shit that Ain't Broke Valued Senior Member

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    Ok..I'm quite familiar with Ohm's law, but the power factor bit I'm still a alittle foggy on. VA's are the same as watts, right?
     
  11. cato less hate, more science Registered Senior Member

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    you are on the right track. although ohms law does not calculate power. when you calculate power (P=I*V) it is Watts law. but that's just being knit-pickey

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    the answer to the question (remembering back to my classes): basically, when something is limited by current, a lower power factor will make it draw more current per unit of real power. therefore, if you rate something in VA (instead of W), then you are making sure they don't draw too much current. if you rated something in W that has an effect on PF, then you could be within spec for real power (W) but blow it up because your PF got too low.

    so, VA is a better way to tell when your stuff is going to blow up. but if your device is passive, then Watt works also.
     
  12. leopold Valued Senior Member

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    power factor is almost always applied to circuits with reactive elements (capacitors, inductors). the apparent power of these types of circuits is almost always higher than the actual power expended.
    it's worth noting that electric companies charge you for the apparent power not real power used. this is why manufactures are always trying to keep their power factors as low as possible.
     
  13. MetaKron Registered Senior Member

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    They'll put you in jail for manipulating your power factor in your favor but they have no problem with taking your money if the power factor falls in their favor.
     
  14. Vkothii Banned Banned

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    The difference is RMS; root-mean-square output is for reactive (time-dependent) frequencies, like generators have. VA is generally used for battery output - a 7VA battery can deliver 1 amp for about 7 hours.

    You calculate RMS output for electronic AC circuits (I can't remember the exact details why) but it has to do with frequency dependence. A DC circuit doesn't have an AC output, so these are 'simpler' power delivery systems. Transformers I think have a 'steady' flux through them. Or the back-emf is balanced maybe, by either winding. Something like that (it's been a decade or two).
     
  15. leopold Valued Senior Member

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    it seems it should be the same as watts doesn't it?

    the best way i can think of to visualize power factor:
    on a grid label the x axis with the voltage of the circuit in question.
    now label the y axis as watts.
    in a purely passive circuit (no inductors or capacitors) the "watts" line will be at 90 degrees to the x axis.
    in a reactive circuit the "watts" line will be at some angle other than 90 and the hypotenuse is the reactance in ohms. using this reactance to calculate power will yield the apparent power of the circuit. the power factor is the relationship between true and apparent power

    like stated in the post above it's dependent on the applied frequency.

    yeah, it's starting to come back to me now.
     
  16. MacGyver1968 Fixin' Shit that Ain't Broke Valued Senior Member

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    Ok...I see what your saying. Inductance and/or capacitance have a part in it.
     
  17. leopold Valued Senior Member

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  18. Thoreau Valued Senior Member

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    Quite honestly Honda, from my personal experience, are quite unreliable and poorly built from a mechanical perspective. Many of thier models have horrible fuel injection problems, fragile valve springs, bad timing (from the factory) as well as block failure. Honda's have more recently moved to using aluminum blocks instead of iron/steel (for cost saving purposes), whereas many of the other major genset manufacturers such as Briggs and Statton, Detroit, and Generac have stayed with the iron/steel blocks. And though the aluminum blocks transfer and thus release heat much more efficiently than the other, aluminum blocks are much less structurally sound and have a tendency to expand and contract more around vital locations, such as the injectors and filters.

    It all comes down to what your willing to pay for, how much load your pulling, and what you expect.

    Also keep in mind that when you buy a Honda, you are paying more for the name. Most common houseowners that know very little of generators are more familiar with the name of Honda than with those that specifically manufacture generators. It's a marketing tool. Everyone has heard of Honda at some point in thier life, but not everyone has a clue who Onan or Generac are.

    If you want something a bit more dependabe, longer lasting, able to hold up longer under heavy loads, go with either B&S, Generac or Cummins/Onan.
     
    Last edited: Nov 20, 2008
  19. jpappl Valued Senior Member

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    Thanks for your insight MZ3.
     
  20. orcot Valued Senior Member

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    when I was 11 we got a whole hour on electronics and 've first heard abouth the basics of electronica at the end the teacher took out a box with some simple switches and a light on it and give the entire class 5 minutes to figure out what was wrong with it and nobody figured it out.
    It wasn't plugged in.
    That and a other experience when I was electrecuted by the christmas lights tought me the most importend lessons abouth electricity
     
  21. cato less hate, more science Registered Senior Member

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    I think you need to do some reading. the simplest explaination of RMS is that its's a way of calculating the voltage and current such that you can multiply them together and get the real power for AC signals, since the instantaneous voltage/current is always changing you can't simply do V*I like with DC circuit. (unless you put them in RMS).

    secondly, reactive does no mean it's AC. reactive means that it has reactive elements (capacitance and inductance).

    you are on the right track, but you had some details messed up.
     
  22. leopold Valued Senior Member

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    brain freeze.
    inductors cannot generate a moving magnetic field with DC. inductive reactance depends on a moving feild.
    capacitors do not conduct DC.
    ah yes, chopped DC. this will work for inductors. i'm not so sure about capacitors though.
    also, the formulas for Xl and Xc both have frequency as a factor.
    Xl is directly related to frequency while Xc is inversely proportional.
     
  23. cato less hate, more science Registered Senior Member

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    its not the AC presence that makes a circuit reactive. you can (theoretically) have a purely resistive AC circuit, thus passive (non-reactive). adding a reactive element (C or L) makes the circuit reactive.

    additionally, you are right, a capacitor and inductor will do nothing with DC, that's why they call it reactive, it reacts to changes in voltage.
     
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