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Why is there a limit to power supply?

  1. Jun 1, 2013 #1
    As stated:

    every now and then you see maximum power output to a motor, a turbine, but why is there a limit to the power output to these electrical/mechanical devices?

    isn't Power just equals to F*v for mechanical devices and VI for electrical devices? what are these "ratings" and restrictions coming from?
  2. jcsd
  3. Jun 1, 2013 #2


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    That is exactly equivalent to asking "why is a wire rated at 10 amps? Why can't I put 1000 amps through it?"

    Things melt.

    (That's only one problem but it should suffice to answer your question)
  4. Jun 1, 2013 #3
    but other than that there's really no limit to how far i want to push a power source right? (assuming i take off all the things like current regulator and things that prevent me from melting the parts)
  5. Jun 1, 2013 #4


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    uhh? wot about friction, air resistance, vibration, liability to break, etc? :redface:

    get real!! :rolleyes:
  6. Jun 1, 2013 #5


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    Absolutely wrong. I can't imagine how you think that would work. Do you have any idea how power supplies / generators / etc work? There are ALWAYS limitations.
  7. Jun 1, 2013 #6
    Well there are many considerations. The first is how much power can you device say a motor handle? The second is how much power can your power supply supply without its fuse blowing out. The third is how much power can your source of energy supply, for eg., the generator.
  8. Jun 1, 2013 #7
    in an very very ideal frictionless case, wouldn't i be able to continue draw power from a source until i destroy the component tho?
  9. Jun 1, 2013 #8


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    Which part of "THINGS MELT" do you not understand? I'm NOT talking about heat due to friction, I'm talking about heat due to excessive current. Also, electrical power sources have, themselves, power sources (hydroelectric dams, nuclear reactors, coal fired boilers, etc). There is always a limit.
  10. Jun 1, 2013 #9
    there is no idea case in the real world. for electrical stuff, there is always resistance in the wiring that dissipate power and generate heat. AND heat melt!!! That's where super conductor comes in, it push the components more towards ideal, but still it's not.
  11. Jun 1, 2013 #10
    No in some cases it just would not work.That's why jumbo jets are not powered by model airplane engines.It's not just a question of turning up the juice and reducing friction.There are specific limits to the performance of devices depending on size, materials and fuel they use.
  12. Jun 1, 2013 #11
    In the mass spectrometers I work with before, those turbo pumps for high vacuum have the moving parts suspended in vacuum by magnetic force so they can spin without touching anything. When you turn off the spinning power and maintain the magnetic suspension power, the moving parts slow down and stop. Even suspend in high vacuum, there are enough friction to slow down object. There is no frictionless environment, just more or less.
  13. Jun 1, 2013 #12


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    There are many factors that can limit a power source. For example, If you have a simple DC power supply which runs off an AC current. It could be made from a bridge rectifier and filter circuit and voltage regulator. Now the main job of the regulator is not to prevent "melting" but to prevent overloading the supply. Essentially, if you draw current at too great a rate, the filter circuit can't keep up and you don't get the nice smooth DC you probably want.

    Another thing to consider is that there is no such thing as an ideal power supply (one that can maintain a constant voltage at any current level). Any real power power supply is really more like an ideal power supply that is in series with its own internal resistance.

    Thus the load and internal resistance act as a voltage divider. The greater the current draw by the load, the larger the voltage drop across the internal resistance, which leaves less voltage for the load. In addition, a larger and larger proportion of available power is dissipated by the internal resistance.

    For example, assume a 100v source with an 100 ohm internal resistance.

    Start with a 1000 ohm load. the load will see 90.9 V and 8.26 W

    If we decrease the load to 100 ohms, it sees 50 V and 25 W which is an increase in power to the load.

    However, if we decrease it again to 10 ohms, it sees 9.09 V and 8.26 W, which is down from the 100 ohm load

    Decreasing again to 1 ohm gives 0.99 V and .98 W

    In essence, once your load tries to draw current at a rate that lowers its effective resistance to below that of the internal resistance of the source, you actually see a decrease in power to the load.

    Thus the maximum power is delivered to a load when its resistance matches than of the source.
  14. Jun 1, 2013 #13
    relax....that's why I'm asking...
  15. Jun 1, 2013 #14

    that's why i said until I "destroy" the component....
  16. Jun 1, 2013 #15


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    No, your power source has to have something performing work on it to generate the power in the first place. Even if you had this ideal frictionless power supply, it would be limited by how fast something else could turn it.
  17. Jun 1, 2013 #16

    understood, and it's the power source limit usually from the limit of one of its power variable ?( ee: voltage, fluid : pressure gradient, thermal: temperature difference) ?
  18. Jun 1, 2013 #17


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    I'm sorry, I can't understand what you've asked, your grammar is confusing.
  19. Jun 1, 2013 #18
    My apologies for my poor grammar, English is my second language.

    I meant to ask: Is the magnitude of the power output of a power source usually limited but its across/through variable? (as well as the physical laws that associated with it?)

    such as the magnitude of a thermal power source is limited by its temperature difference and Carnot cycle(eg: geothermal plants),

    a electrical source is limited by its voltage (voltage in a battery), internal resistance and V^2/R ( as in your example).
    Last edited: Jun 1, 2013
  20. Jun 1, 2013 #19


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    You don't have this quite right. A power source COULD be limited by, for example, the amount of water pressure/power that is available to drive a generator OR it could simply be limited by design well below the amount of water pressure/power available because building it bigger would be expensive and unnecessary.
  21. Jun 1, 2013 #20

    But in both cases, the restriction is usually set on one of the variable that defines the power mathematically? (in this case : the P in water pressure power source is either limited by the natural source or the people who designed the system.) where P * Q (Q = volumetric flow rate, through variable, P = pressure is the across variable that defines the power value mathematically).

    I think I can see that mathematically, if one of those variables are restricted, the other one is automatically restricted through the physical laws that associate the two. Ohm's Law in electrical domain, and in the water pressure is probably by Benooulli's equation?
    Last edited: Jun 1, 2013
  22. Jun 2, 2013 #21

    another way to look at this is this. Take for example an internal combustion engine. Your car's engine.
    Say that everything is ideal. Take the conditions however you like them (no friction, no failure of the materials...whatever you like). In order for this engine to produce work, you have to burn fuel. Say that the combustion is perfect and also that there are no losses of any other kind (the chemical energy is totally converted to mechanical). Then the engine's output is directly dependent on the fuel's properties. So, you see that - even under all this imaginary assumptions - you can't squeeze as much power as you want out of this poor engine...

    The same applies to all applications.
  23. Jun 2, 2013 #22

    that's the answer i was looking for, I knew there must be some theoretical limit but I don't know what it is.

    thank you
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