# AC source magnitude same as its RMS value?

Discussion in 'Feedback and Suggestions' started by gbell12, May 29, 2010.

1. ### gbell12 Thread Starter New Member

May 29, 2010
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it is stated that when we do power calculations for a circuit driven by an AC source specified as 120$\angle$0°, we use its magnitude because that's its RMS value.

But if the sinusoid is a vector of length 120 rotating around the origin, isn't its magnitude, ie. the greatest shadow it casts on an axis, 120? Meaning its P-P voltage is 120, not its RMS...

2. ### mik3 Senior Member

Feb 4, 2008
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Its peak value is 120, its peak to peak is 240 and the RMS=120/sqrt(2).

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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did i wake up from the wrong side of the bed

4. ### Ghar Active Member

Mar 8, 2010
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120 is the RMS value, not the peak.
The peak is 120 * Sqrt(2) = 169.7

In power systems you work with RMS off the bat usually.

5. ### gbell12 Thread Starter New Member

May 29, 2010
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Well, therein lies the rub. If we represent that AC voltage source as a phasor, the length of that phasor should really be the peak voltage so that it accurately traces out the sinewave it's representing as it spins...

Maybe it depends on whether you're drawing it or not? I understand why we use RMS for power calculations, its just that when you talk about that source as a phasor I think its length should be Vpeak.

I think this may be a collision between the convention of specifying AC voltage sources as Vrms, but working with vectors and phasors for the purposes of calculations (which I think should be in terms of Vpeak).

Someone back me up here!

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Sorry - have to disagree.

I usually work in RMS for AC calculations as probably do the majority of electrical / electronics people. Most AC instrumentation is configured to display in RMS.

In the end it's no big deal what you work in - peak, rms or peak-to-peak - as long as you stick with your convention and can relate between the various terms.

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
If you are working with phasors, usually they represent the peak values of sinewaves. However, you can use a phasor and say it represents the RMS value. It depends on the user, provided he knows what is going on.

8. ### Ghar Active Member

Mar 8, 2010
655
72
RMS for sinusoids is just a constant coefficient so it still works mathematically, you just need to keep in mind that your numbers aren't the peak of the actual waveform.