How do we know if voltage is RMS or PEAK?

Thread Starter

enesene

Joined Mar 3, 2020
29
nnne.png
It's a AC circuit. Voltage is 150V as we can see but how do we know if it's Vrms or Vpeak?
Because if it's Vrms, I'll use S=V.I* to calculate powers
But if it's Vpeak i should divide it by 2
 

Thread Starter

enesene

Joined Mar 3, 2020
29
But if it's peak, we should use S=(V.I*)/2 formula to find powers right?
And if it's rms, we should use S=V.I* , are you agree with me?
 

Papabravo

Joined Feb 24, 2006
21,159
When you express an AC quantity as a Phasor e.g. 150∠0°, the 150 is understood to be the amplitude of a sinewave, the frequency is not specified and the initial phase is 0°. The diagram is in a fuzzy font and I can't see where Ix comes from but the source is 39 times that, whatever it is,
 

MrAl

Joined Jun 17, 2014
11,389
Hi,

Yes when we see this converted it all starts with something like this:
100*cos(2*pi*100*t-30) where '30' is in degrees
and this transforms into the phasor:
100 angle -30 degrees
and since the '100' was peak voltage in the original it must be peak in the result.

It is a little interesting though that when we deal with JUST current or voltage and NO power, it does not matter if it is in RMS or Peak, as long as we express the result in the same units. When it comes to calculating the power however then we need to know which it really is.
 

WBahn

Joined Mar 31, 2012
29,979
When you express an AC quantity as a Phasor e.g. 150∠0°, the 150 is understood to be the amplitude of a sinewave, the frequency is not specified and the initial phase is 0°. The diagram is in a fuzzy font and I can't see where Ix comes from but the source is 39 times that, whatever it is,
I've seen many diagrams of three phase power systems where the phasors were given with RMS values.

In general, you don't know for sure unless there is some note or indication that lets you infer it or back it out. The best bet, if you can't find something in the context of the problem, is to pick one and make a very clear note what assumption you are making and work the problem under that assumption. You could also then include the answer under any other reasonable assumptions or give an explanation of how to find them from the answer using your assumption. In this case it will be a simple matter to just include a second set of answers based on the first set.
 

Papabravo

Joined Feb 24, 2006
21,159
I've seen many diagrams of three phase power systems where the phasors were given with RMS values.

In general, you don't know for sure unless there is some note or indication that lets you infer it or back it out. The best bet, if you can't find something in the context of the problem, is to pick one and make a very clear note what assumption you are making and work the problem under that assumption. You could also then include the answer under any other reasonable assumptions or give an explanation of how to find them from the answer using your assumption. In this case it will be a simple matter to just include a second set of answers based on the first set.
There is nothing wrong with doing it that way of course, but if you've never seen it done it probably looks and feels a bit disconcerting. I've never done it that way because I was not specifically required or taught to do it that way. As always your mileage might differ.
 

MrAl

Joined Jun 17, 2014
11,389
I think it pertains to poly phase systems which are given in terms of RMS values.
The way the phasor concept is described though starts with A*cos(wt+ph) and there 'A' is always peak. That could be because peak is natural while RMS is derived.
 

Papabravo

Joined Feb 24, 2006
21,159
I think it pertains to poly phase systems which are given in terms of RMS values.
The way the phasor concept is described though starts with A*cos(wt+ph) and there 'A' is always peak. That could be because peak is natural while RMS is derived.
I have to admit that my knowledge of polyphase systems is anecdotal at best.
 

WBahn

Joined Mar 31, 2012
29,979
I think it pertains to poly phase systems which are given in terms of RMS values.
The way the phasor concept is described though starts with A*cos(wt+ph) and there 'A' is always peak. That could be because peak is natural while RMS is derived.
It's natural to introduce phasors using the amplitude of the signal because in all three of the common approaches taken you are mapping the time domain functions to the phasors as directly as possible. But the mapping is scalable and when dealing with power systems it just makes the most sense, for most things, to work with RMS values.
 

MrAl

Joined Jun 17, 2014
11,389
It's natural to introduce phasors using the amplitude of the signal because in all three of the common approaches taken you are mapping the time domain functions to the phasors as directly as possible. But the mapping is scalable and when dealing with power systems it just makes the most sense, for most things, to work with RMS values.
Wikipedia's take: https://en.wikipedia.org/wiki/Amplitude

Under the heading "Root mean square amplituce" is found: "For alternating current electric power, the universal practice is to specify RMS values of a sinusoidal waveform.", and even when the waveform isn't perfectly sinusoidal (which it usually isn't these days) RMS does what is wanted.
Yes it is more or less tradition, so it is like comparing theory vs practice again.
I've never seen a 3 phase system described in terms of peak voltages and i worked in the power industry for many years back in the day.

But in most cases it is a recognizable system too, such as 120vac, 230vac, etc. These are recognized i bet by almost everybody here as RMS specifications and they arent even 3 phase systems or at least they dont have to be. So working in scaled amplitudes is not uncommon.

In one of my reference books a well known author uses peak values to introduce the phasor concept, but as soon as they get to poly phase systems they provide a footnote that says that they will be using RMS values.

I still think the TS's first post is a little ambiguous though when taken at face value. That's because we have no history in which to judge the context of his/her coursework. If they regularly work in RMS values for these kind of problems then that would tell us right away. If they regularly work in peak values then we'd know that right off. 150vac is not a common system so i think the TS should provide more information about recent coursework. Without that i think it is a guess.
 

MrAl

Joined Jun 17, 2014
11,389
I have to admit that my knowledge of polyphase systems is anecdotal at best.
Well i think it is more about tradition then any real requirement, except it does make things simpler to work in RMS values when dealing with power systems of any kind.
 

Papabravo

Joined Feb 24, 2006
21,159
Well i think it is more about tradition then any real requirement, except it does make things simpler to work in RMS values when dealing with power systems of any kind.
You have to experience something before it has a chance to become a tradition. Sorry about the hole in my experience you could drive a truck through.
 

MrAl

Joined Jun 17, 2014
11,389
You have to experience something before it has a chance to become a tradition. Sorry about the hole in my experience you could drive a truck through.
Well i would bet you are familiar with your own power line specifications. Probably either 120vac, 230vac, or less common 100vac. There are actually DC power systems out there in the US also but very uncommon although there had been somewhat recent talk about going to all DC for everywhere as amazing and antihetical to what most of us believe about power transmission. It has to do with reducing losses due to the skin effect and probably proximity effect in light of the more recent low loss switching converters power components.
What would be cool though is if the power company actually installed separate lines to homes in the US that carried 5vdc power so everyone could charge their phones right from the power line with no converter :)
 

Papabravo

Joined Feb 24, 2006
21,159
Well i would bet you are familiar with your own power line specifications. Probably either 120vac, 230vac, or less common 100vac. There are actually DC power systems out there in the US also but very uncommon although there had been somewhat recent talk about going to all DC for everywhere as amazing and antihetical to what most of us believe about power transmission. It has to do with reducing losses due to the skin effect and probably proximity effect in light of the more recent low loss switching converters power components.
What would be cool though is if the power company actually installed separate lines to homes in the US that carried 5vdc power so everyone could charge their phones right from the power line with no converter :)
I am familiar with the mains power and I've (successfully) built power supplies, I've just never used phasors in conjunction with designing a power supply. I certainly understand Pk-Pk, Amplitude, and RMS in great graphic detail. Never having worked with polayphase systems was what I was referring to.
 

MrAl

Joined Jun 17, 2014
11,389
I am familiar with the mains power and I've (successfully) built power supplies, I've just never used phasors in conjunction with designing a power supply. I certainly understand Pk-Pk, Amplitude, and RMS in great graphic detail. Never having worked with polyphase systems was what I was referring to.
Oh i see, thanks for the clarification. I worked with industrial power in the past so i was used to seeing something like 3 phase 240vac (rms), 208vac (rms), etc. line to neutral. But even at home the line is almost always referred to as "120vac" and taken to be RMS. Many appliances quote that in the specs or right on the back panel but they dont usually state "RMS" with it, although i guess some could.
The power most homes in this area have is a dual 120vac system where the two are 180 degrees out of phase, and share a common neutral and a separate ground wire.
For a long time i wished we had 230vac (rms) because a lighter gauge wire is required to deliver the same level of power.
Thanks for the reply.
 
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