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Thread: VA vs. Watts

  1. #11
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    I use the Beer and Foam analogy when I have to explain why the utility wants bigger customers to correct power factor. The PoCo is the barkeep, he buys beer and resells it, foam or no foam. The user is the pub patron. He buys beer, not foam, so he expects the bartender to pour off the foam. That pour off is a loss of revenue to the barkeep, but for the occasional tourist who comes in for a beer once in a while and doesn't ask for a discount, it doesn't matter much. But he insists that his biggest drinker local patrons who expect a better price bring their own beer mugs that reduce foaming, or pay a premium for the beer to make up for the expected loss.

    For the end user's perspective, I like to use the analogy of horses pulling a rail car. If the horses are walking right on the track in front of the rail car, all of their power is being used to pull the car along the track, so that is like having a PF of 1.0 or Unity. But if the horses move away from the rail, an amount of the energy they exert is wasted on pulling the rope sideways, not in advancing the car down the track. The difference between the total exertion by the horses vs the force used in moving the car along the track is the Power Factor. If the PF = .80, that means that 80% of the horses' strength is used to pull the car along, 20% is just keeping the rope tight. The extreme is if the horses move out to where the rope is 90degrees from the rail car; all of the energy is pulling sideways, the car cannot move. That's like having a PF of zero. PFC capacitors are basically like a teamster that guides the horses back to being closer to the track again so that more of the force they exert is used to do useful work.

    Then when the "energy saver" guys try to tell me that their teamster (capacitor) saves energy, I can point out that the number of horses hasn't changed, nor has the amount of hay they expect to eat.:grin:

  2. #12
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    Quote Originally Posted by ibew501ed View Post
    Excellent and accurate and I'm gonna steal it (with permission)

    Permission granted.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  3. #13
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    Quote Originally Posted by charlie b View Post
    [FONT=Times New Roman][SIZE=3]
    From me, you may or may not get "interesting," but I hope you at least get "clear." However, "concise" is not in my nature. :wink:
    It's a concept I have understood for many years so it is difficult for me to judge the clarity from the perspective of someone who is not entirely familiar with it but, given the positive feedback from others, it has obviously been received well. Kudos for that.:smile:

    However, I do think you should correct the last paragraph, Specifically:

    The units of measure for the three lines described above are fundamentally the same. They all relate to the rate of use of energy, and can all be expressed in terms of “joules of energy per second of time.”
    As I'm sure you know, Joules/second is power in Watts. There is no power associated with the vertical line. VArs are Wattless, thus can't be correctly expressed in terms of "joules of energy per second of time”.

  4. #14
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    Quote Originally Posted by MarkyMarkNC View Post
    Concise for an engineer!!!:grin:
    Engineers are concise. Salesmen are not.

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    Actually I believe charlie b is right.

    V = (kg*m^2) / (C*s^2)
    A = C / s

    V*A = (kg*m^2) / (s^3) = (kg*m^2) / (s^2) * (1/s) = J / s

    Broken down into it's most fundamental pieces, a Volt-Amp is still Joules per second.

    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

  6. #16
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    Quote Originally Posted by Besoeker View Post
    However, I do think you should correct the last paragraph

    No need. It is correct as written.
    Quote Originally Posted by Besoeker View Post
    As I'm sure you know, Joules/second is power in Watts.

    Not quite true. It is like the difference between saying “a dog is an animal” and “an animal is a dog.”

    A “Watt” is a “Joule per second,” but a “Joule per second” can be a “Watt” or it can be one of at least two other things. Specifically, a “Joule per second” could be a “Volt-Ampere Reactive,” which is a measure of reactive power, and a “Joule per second” could also be a “Volt-Ampere,” which is a measure of apparent power.
    Quote Originally Posted by Besoeker View Post
    There is no power associated with the vertical line. VArs are Wattless, thus can't be correctly expressed in terms of "joules of energy per second of time”.
    If you take each type of power down to the very fundamental units (i.e., length, mass, time, and charge), then,

    · “Real Power” is measured in units of “Kilograms x (meters squared) / (seconds cubed)"
    · “Reactive Power” is measured in units of “Kilograms x (meters squared) / (seconds cubed)"
    · “Apparent Power” is measured in units of “Kilograms x (meters squared) / (seconds cubed)"
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  7. #17
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    “Joule per second” could also be a “Volt-Ampere,” which is a measure of apparent power.
    So you want to redefine the Joule?
    A Joule per second is a Watt which power, not apparent power.
    Take a reactive load like a capacitor.
    The current it takes times the applied voltage gives it's VA.
    Ignoring losses (a close approximation at power frequencies), there will be no power and thus no Watts. It will consume no Joules of energy in any one second period.
    In short, in an AC circuit, Joules per second is not VA.

  8. #18
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    Quote Originally Posted by Besoeker View Post
    It will consume no Joules of energy in any one second period.
    You missed the point. No one is saying that a Watt is the same as a Volt-Amp. When boiled down to their fundamental units of measure, both are quantified the same way. They are applied very differently, yes, but both have the same building blocks.

    The sentence you questioned in charlie b's dialog is correct as written. You are reading too much into it. Your points are valid, but you are correcting something that he didn't say.
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

  9. #19
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    Quote Originally Posted by drbond24 View Post
    The sentence you questioned in charlie b's dialog is correct as written)
    Since he is dealing with AC circuits, it is incorrect.
    VA and Watts are not the same thing except for the specific case of unity power factor.
    Specifically, a “Joule per second” could be a “Volt-Ampere Reactive,”
    That's just wrong.

  10. #20
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    Quote Originally Posted by Besoeker View Post
    VA and Watts are not the same thing except for the specific case of unity power factor.
    You're going to have to show me where he said "a VA and a Watt are the same thing." That is what you are arguing against, but that isn't what he said. Of course they are not the same thing.

    Quote Originally Posted by Besoeker
    Specifically, a “Joule per second” could be a “Volt-Ampere Reactive”
    Break it down to fundamental units. Watts, VA, and VAr all break down to (kg*m^2) / (s^3).

    I think I would be correct in saying that the point here is that Watts, VA, and VAr all describe the same quantity of energy. They all describe different kinds of energy, of course, but not different amounts.

    Imagine you're on the USS Enterprise and you can convert mass and energy back and forth at your whim. You tell the computer to convert one Watt of energy into a mass. Then you tell the computer to convert one VA of energy into a mass. The computer will create you two identical masses.
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

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