difference between ac and rms ac

Discussion in 'The Projects Forum' started by embpic, May 4, 2014.

  1. embpic

    Thread Starter Member

    May 29, 2013
    187
    3
    what is difference between Normal ac and rms ac???
    In India,in home which voltage is in board? whether simple ac or rms ac??
     
  2. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Check the top of your page of this forum. Free downloads that will explain every question you may have. VOL.I - DC, VOL.II - AC etc.

    Your utility power is measured as r.m.s. AC, but contain peak AC etc. Depends what you want to measure. Usually the voltage that you measure on a good meter, will be r.m.s. value. Please read/study the difference from the free downloads.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
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    Rms (root mean square) is simply a measure of the effective voltage of the (normal) AC found in your home.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In simple terms the RMS is the value of the AC voltage that would offer the same power equivalent to a resistive load supplied with an the equal voltage value in DC.
    Max.
     
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  5. MrChips

    Moderator

    Oct 2, 2009
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    Hope you follow what all the others have said.

    The AC voltage coming into your home is a basic sine wave signal. There is no difference between what you call "normal ac" and "rms ac".

    The RMS value is simply the number that is a measure of the amplitude of the signal.

    RMS stands for root mean square of the signal.

    It is the square root of the average voltage x voltage, i.e. the voltage squared.

    Why do we do this?

    Because voltage x voltage represents power, the RMS voltage is the value of a DC voltage that would have the same power as the AC voltage.

    Hence when comparing voltages of different waveforms and shapes, comparing the RMS value is a fair comparison with respect to power.
     
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  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    The answer is in the cards or how expensive the AC voltmeter is.

    non-TRMS meters use an averaging technique and they multiply the result by a "fudge factor" so it reads RMS for a sine wave input only with a certain frequency range.

    TRMS meters use other techniques that do measure the effective DC voltage for almost any waveform. This function has been easier and cheaper to do as technology has advanced, TRMS has a specific mathematical definition.

    Without a TRMS meter the voltage measured at the output of an electric light dimmer would not be RMS.

    The current drawn from a PC power supply will not be sinusoidal.

    P=v*I or Power = voltage * Current is ONLY VALID for resistive loads.
     
  7. embpic

    Thread Starter Member

    May 29, 2013
    187
    3
    so RMS is pure sine wave??
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
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    Not necessarily, Wiki definition.
    In mathematics, the root mean square, also known as the quadratic mean, is a
    statistical measure of the magnitude of a varying quantity.
    Saw tooth, square wave, sine wave etc.
    Max.
     
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  9. wayneh

    Expert

    Sep 9, 2010
    12,137
    3,054
    "RMS" is a term applied to a measurement to indicate it is time-averaged, as opposed to instantaneous or peak. Integration of a sine wave over time shows that the time-average voltage is the peak voltage divided by 1.41. This time-averaged voltage is also referred to as RMS or root-mean-square. Any voltage profile can be integrated or averaged over time, but only for certain waveforms, including sine, would you technically call this a root-mean-square.

    [edit] After some reading, I'm less certain there is any distinction between RMS and the integrated time-average voltage. I would have sworn learning there was, but I can't see it.
     
    Last edited: May 5, 2014
  10. MrChips

    Moderator

    Oct 2, 2009
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    No. Definitely no.

    RMS is a method of consistently measuring the power of any waveform including DC.
     
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  11. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    If RMS(any wave) = 120 and lets say R=10, then P=(120^2)/10
    so, we can say 120 DC generated the same power as 120 V RMS into a 10 ohm load.

    It's also true that 10 VDC = 10 VRMS, BUT we generally think of taking the RMS value of a time varying signal. Those TRMS meters may have options of TRMS(ac) and TRMS(ac+dc).

    The proper name for the AC meters that are non TRMS is average responding, TRMS reading. Those meters basically precision rectify and average and multiply by a fudge factor. That fudge factor assumes the input is a sine wave.
     
  12. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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  13. MrChips

    Moderator

    Oct 2, 2009
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    Correction:

    RMS is the square root of the average of the voltage squared.
     
  14. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    (square of the average voltage) = (Ave(v(t))^2 = average of the voltage squared

    Same difference.

    Now put the sqrt on the outside.
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    Not the same.

    The average voltage of an AC sine wave = zero.

    The square of the average voltage of AC sine wave = zero.

    The average of a sine wave voltage squared = RMS squared.
     
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  16. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    I won't argue, but it just "depends on where you put the parenthesis"

    So, the only way to describe it, is to use the mathematical definition and not words. It's like the glass half full or half empty problem.

    I know what RMS is. You know what RMS is. Maybe your description is words is better. Maybe I like Reverse Polish Notation and you like Algebraic. BTW, I do like RPN.

    v(t)
    Push Square
    Push Average
    Push Sqrt

    Happy now or do I need to do the LATEX representation?
     
  17. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,281
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    True: square of the average voltage = Ave(v(t))^2 ≠ RMS^2
    True: average of the voltage squared = Ave(v(t)^2) = RMS^2

    True for some waveforms, but not all:

    Ave(v(t))^2 = Ave(v(t)^2)
     
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