+ Post New Thread
Results 1 to 20 of 22

24th May 2019, 15:14 #1
 Join Date
 Oct 2018
 Posts
 10
 Helped
 0 / 0
 Points
 210
 Level
 2
explaining about RMS
HI
i have a question about rms
rms = 0.7 vp
where (0.7) come from
is it depend on frequencies 50/60 hz ?
like PWM when we vary duty cycle we obtain different value of volts
but here we vary frequencies
thanks

Advertisement

24th May 2019, 15:55 #2
Awards:
 Join Date
 Apr 2014
 Posts
 15,611
 Helped
 3557 / 3557
 Points
 77,053
 Level
 67
Re: explaining about RMS
Hi,
0.7 > more exactly 0.707... = 1/sqrt(2) or 0.5 x sqrt(2)
(only for pure sinewave).
It does not depend on frequency.
It just depends on waveform.
****
PWM:
with DC voltage PWMīd:
V(average) = V_DC x duty_cycle
V(RMS) = V_DC x sqrt(duty_Cycle)
KlausPlease donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
1 members found this post helpful.

24th May 2019, 16:04 #3
 Join Date
 Dec 2009
 Location
 USA
 Posts
 54
 Helped
 15 / 15
 Points
 2,053
 Level
 10
Re: explaining about RMS
RMS  tutorial ...
https://www.electronicstutorials.ws...svoltage.html
1 members found this post helpful.

24th May 2019, 17:55 #4
 Join Date
 Jan 2019
 Posts
 357
 Helped
 86 / 86
 Points
 1,893
 Level
 10
Re: explaining about RMS
Vrms = 1/2 (square root of 2) Vpeak applie ONLY to sin waves
if you any other waveshape, you have to do the math as in the link in post 3
1 members found this post helpful.

25th May 2019, 01:51 #5
 Join Date
 Nov 2012
 Posts
 3,128
 Helped
 768 / 768
 Points
 17,084
 Level
 31
Re: explaining about RMS
i have a question about rms; rms = 0.7 vp; where (0.7) come from
Consider an arbitrary but periodic waveform. It can be sine or square or any random graph but it should be periodic; because we need that period later.
Any arbitrary periodic graph can be described in terms of moments; let me try to explain.
The first one is the mean: Σx(t) taken over one period. To get the mean you divide the sum by T, the period. x is the voltage at any time t. You need to perform this sum over as many points as you can.
If the waveform is symmetrical (e.g., a sine or a cosine wave), the mean vanishes. In fact, all odd moments do vanish for a symmetrical waveform.
The next moment is the sum of squares: Σxē(t); this is related to the variance or standard deviation and RMS value. RMS value is √((x^2(t))/T)
Whether the waveform is symmetrical or not, the sum of squares is always positive (can be zero only if all the points are zero).
There are higher moments: the third moment is Σx^3(t) is related to skewness (it also vanishes for a symmetrical wave). The fourth moment ∑x^4(t) is related to kurtosis.
In general, they are called generalized mean: generalized mean of the nth order will be (1/T)Σx^n(t).
Now you see why RMS is so important (the simple mean disappears for a symmetrical wave).
For a sine wave, RMS value is peak value /√2 (or, more simply 0.707 x peak value).
But for a general waveform you must calculate all the moments manually.
1 members found this post helpful.

25th May 2019, 12:16 #6
 Join Date
 Oct 2018
 Posts
 10
 Helped
 0 / 0
 Points
 210
 Level
 2
Re: explaining about RMS
HI
@ c_mitra I see now there are more and more to learn
and I appreciate every post

26th May 2019, 02:45 #7
 Join Date
 Nov 2012
 Posts
 3,128
 Helped
 768 / 768
 Points
 17,084
 Level
 31
Re: explaining about RMS
I see now there are more and more to learn ...
monopole moment:
dipole moment:
quadrupole moment:
Octopole moment:
Wikipedia has a somewhat boring description of the multipole expansion: see, for example, https://en.wikipedia.org/wiki/Multipole_expansion
Coming back to the original question; if you specify the RMS value of a given but random waveform, you get some idea about the waveform.
But if you know all the values of the generalized means (sum over one cycle (x(t))^n) you get to know the complete waveform.
But in electrical engineering, the RMS value is the most important  it is related to the power or energy.
But that is a different area altogether.
1 members found this post helpful.

26th May 2019, 05:20 #8

26th May 2019, 12:28 #9
 Join Date
 Oct 2018
 Posts
 10
 Helped
 0 / 0
 Points
 210
 Level
 2

26th May 2019, 12:52 #10
Awards:
 Join Date
 Apr 2014
 Posts
 15,611
 Helped
 3557 / 3557
 Points
 77,053
 Level
 67
Re: explaining about RMS
Hi,
True RMS means there is a "true" RMS measurement and calculation...with squaring the signal, averaging and taking the square root.
In opposite there are some (how I call them) "fake RMS" calculations.
Often used:
* peak value measurement ... multiplied with 0.707 (for pure sinewave)
* rectified average measurement .... multiplied with 1.11 (for pure sinewave)
* some odd measurement .... multiplied with just a factor to get the expected output value.
You see the "measurements" and calculations are not true RMS. The measurement has nothing to do with RMS, none of them use "square" and "square root". Thus the output is fake.
Only if you have a known, never changing, reliable waveform .... the the results "seem" to be RMS values.
My personal opinion: It's like measuring the size of your feet to find out how tall you are. For many people you may find a good relationship. But for some people the result simply is wrong.
KlausPlease donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
1 members found this post helpful.

Advertisement

26th May 2019, 14:12 #11
 Join Date
 Nov 2012
 Posts
 3,128
 Helped
 768 / 768
 Points
 17,084
 Level
 31
Re: explaining about RMS
It's like measuring the size of your feet to find out how tall you are. For many people you may find a good relationship...
https://www.improbable.com/2016/01/1...ownewpatent/
Of course this is a joke but the point should not be missed.

26th May 2019, 15:20 #12
 Join Date
 Dec 2012
 Location
 New Delhi
 Posts
 1,209
 Helped
 182 / 182
 Points
 6,247
 Level
 18
Re: explaining about RMS
Hey C_Mitra !! Here are your 2 lines again. I may be nitpicking, but you have clearly written that "rms value.... get an idea of waveform" I paraphrase.
Your 2nd line (quoted above) doesn't make any sense, so I ignored it. If you meant that if we know all the points in the signal, then yes of course we know the signal ?!? That's a tautology.
   Updated   
Try to stay on topic please. Thanks !

26th May 2019, 16:23 #13
 Join Date
 Aug 2016
 Posts
 493
 Helped
 79 / 79
 Points
 2,788
 Level
 12
Re: explaining about RMS
It is not dependent on frequency.
Note that 0.707 is only for sine wave. Other waveforms yield different factors.
Actually the factor is the ratio of integrated voltage waveform in one cycle to the total area in one cycle. This is to say the that the factor is the ratio of the area taken up by the voltage waveform over the sum of the area taken up and that not taken up.
   Updated   
ADD:
Note that the total area is (Vpk * one_period).
Akanimo.

26th May 2019, 16:29 #14
 Join Date
 Nov 2012
 Posts
 3,128
 Helped
 768 / 768
 Points
 17,084
 Level
 31
Re: explaining about RMS
Actually the factor is the ratio of integrated voltage waveform in one cycle to the total area in one cycle...
see, for example, https://www.wolframalpha.com/input/?...from+0+to+2*pi
In fact, both these are ZERO for any symmetrical waveform.
1 members found this post helpful.

26th May 2019, 16:35 #15
 Join Date
 Aug 2016
 Posts
 493
 Helped
 79 / 79
 Points
 2,788
 Level
 12
Re: explaining about RMS
No, remember that when we say negative, we imply the direction of flow of current. Current flowing in opposite directions every other half cycle doesn't cancel out. In fact the same heating consequence is produced irrespective of direction.
So when integrating, integrate the absolute values. This would give 2 times what you get for a half cycle but not zero.Last edited by Akanimo; 26th May 2019 at 16:40.

Akanimo.

26th May 2019, 16:51 #16
 Join Date
 Dec 2012
 Location
 New Delhi
 Posts
 1,209
 Helped
 182 / 182
 Points
 6,247
 Level
 18
Re: explaining about RMS
Are you implying that the power transferred/ carried by a symmetrical wave (around zero) is always Zero? This is obviously a flawed and misguided statements. And the flaw is selfevident.
Power (and RMS) is an integral over squares. Hence doesn't cancel.
Akanimo made a slight error. Nothing to get so excited aboutLast edited by kripacharya; 26th May 2019 at 16:59.

26th May 2019, 16:59 #17
 Join Date
 Nov 2012
 Posts
 3,128
 Helped
 768 / 768
 Points
 17,084
 Level
 31
Re: explaining about RMS
Note that the total area is (Vpk * one_period). ...
Unless, of course, you are defining area as above. But then could you care to explain the same using a sine wave? (say the peak is 1v, the period is 1s)
   Updated   
No, remember that when we say negative, we imply the direction of flow of current...

26th May 2019, 17:02 #18
 Join Date
 Dec 2012
 Location
 New Delhi
 Posts
 1,209
 Helped
 182 / 182
 Points
 6,247
 Level
 18
Re: explaining about RMS
C_mitra ..... you are forgetting what the 'S' stands for in RMS. It's 'Square'.
Power is proportional to SQUARE of voltage. It's proportional to SQUARE of current. When driving a resistive load where V and I are in phase.
When V and I are not in phase the situation is different, and power too flows back and forth.
Get over it.Last edited by kripacharya; 26th May 2019 at 17:28.

26th May 2019, 17:03 #19
Awards:
 Join Date
 Apr 2014
 Posts
 15,611
 Helped
 3557 / 3557
 Points
 77,053
 Level
 67
Re: explaining about RMS
Hi,
So when integrating, integrate the absolute values. This would give 2 times what you get for a half cycle but not zero.
> For RMS you need to integrate square_of_voltage .... which can never be negative.
Integrating the absolute value of voltage gives (ONLY for pure sinewaves) a value (precisely: Pi/4) that has to be multiplied with 1.11 to look like the RMS value.
With any altered waveform, the factor need to be adjusted, too, else the resulting value is wrong.
Without knowing exact waveform an RMS calculation with the use of integrating absolute values is mathematically and physically (generated heat in a resistor) wrong.
KlausPlease donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.

Advertisement

26th May 2019, 17:13 #20
 Join Date
 Aug 2016
 Posts
 493
 Helped
 79 / 79
 Points
 2,788
 Level
 12
Re: explaining about RMS
Yes Klaus, thanks.
I just noticed I had made that blunder just before your notification came in.
I had been describing the average voltage all along. RMS is obtained from squaring each sample in a period, summing up all the squared results, dividing by the number of samples and then finding the squareroot of the quotient.
The higher the number of samples, the more accurate the result.
Akanimo.
1 members found this post helpful.
+ Post New Thread
Please login