Confused

Neither, or both.

First do the work of looking up the definition if "rms voltage", then ask this forum specifics about what you find.

What you will find here is that we usually won't do the basic homework for you. It's bad for retention while learning, and this subject is important.

Sir- It is not a homework question rather a question from a text book I am reading. I thought some of the more learned people on here wcould assist me with the comprehension of the subject. I have searched other websites but still I can't seem to grasp the concept.

I am a mechanical engineer trying to broaden my knowledge.

Dear Mechanical Engineer,

RMS value means Root of mean value of a variable function whose value increase from zero to Max and then decrease from E max to zero and zero to (- Vmax) to zero back. Now find square of it , Or effective value. Think of a sine wave whose value do vary from zero to max and Back. Read the link below.

http://images.google.co.in/images?hl=en&q=RMS+value&um=1&ie=UTF-8&sa=X&oi=image_result_group&resnum=4&ct=title

Rms voltage is the "root mean square" of the peak to peak voltage. It has nothing to do with whether the voltage is single phase or multiphase. For a pure sine wave (something very seldom encountered in the real world), the rms voltage is the peak to zero voltage times the square root of two. Theoretically, the rms value is what is comparable to DC voltage in power equations, Ohm's law, etc (i.e., 120 V ac rms would give the same power as 120 V dc in the same resistive circuit).

In a three phase system, it is generally the rms voltage one is looking at, whether measuring from line to line or from line to neutral. The relationship between line to line voltage to line to neutral voltage depends on whether one is looking at wye or delta connections; life gets more complicated when one looks at center-tapped delta. As an example, on a 480 V AC wye connected generator, the 480 V AC rms is measured line to ground; the same circuit will measure 277 V AC rms measuring line to line (approximately). Note, for a 3 phase circuit, this works out to line to neutral being the square root of three times the line to line measurement, which results from the three phases being 120 degrees out of phase with each other.

Note to the electricians of the world- this is a mechanical engineer's perspective on the terminology. Please don't shoot me for over-simplifying...

Thank you both for replying. Its a star connected system and I was getting confused wether the RMS voltage would be the line voltage or the phase voltage?

As I noted, in the star configuration (also called "wye"), rms voltage can refer to either line voltage or phase voltage. Generally, when one specifies line voltage, one is referring to the rms line voltage, and when one refers to phase voltage, one is referring to rms phase voltage. I can never remember if line voltage is measured phase to phase and phase voltage measured line to ground, or vice versa, so I always make sure I know which one I am measuring and go on from there.

So would I be right in assuming that in this problem V_line = 300 x .707 = 212 V and

then

V_phase = 212/√3

Not necessarily. Show me a diagram...

Hi CW

haven't got any scanning equipment here. This is just a question thrown in, in this book but it does not give an answer. My reckoning was that like in a 415V system then the peak to peak voltage would be 585V in such a system. So the phase voltage is then 415/√3 = 240V

On a 3-phase system there's phase-phase and phase-neutral voltages. Phase-neutral voltage is phase-phase voltage divided by √3. Both are always quoted as RMS values.

E.g. in Europe it's usually 400 volt phase-phase, 230 volt phase-neutral.

Cheers.........Codey

Dear Guest,

First, I did answer you specifically in post #1: RMS is neither phase or line voltage as a definition, but both phase or line voltage can be expressed in RMS values.

I then suggested that you need to have an understanding of what RMS actually means by definition.

Now you have given us more of an understanding of where your confusion lies, and I will address specific points to assist in your clarity of understanding.

You have mentioned in post #7 both the factor .707 and the factor √3. These are apples and oranges - they do not apply to the same features of voltage, as you found by your example equations.

I will re-state your equations correctly, and if you look closely at the differences, you will see how the two factors should be used.

V_{Line RMS} = V_{Line Peak}(300V) x 0.707 = 212V_{Line RMS} (Line voltage expressed in RMS). The important thing to note here is that 0.707 does not apply to the difference between Line and Phase measurements, but between peak and RMS of either of those.

V_{Line RMS} (212V)/√3 = 122V_{Phase RMS} (WYE voltage only, for DELTA applies to Amps only). So then

V_{Line Peak}(300V) x 0.707 /√3 = 122V_{Phase RMS (Wye volts, Delta amps)}

Regards, CJM

There also seems to be a confusion over "peak to peak" and "peak" voltage. For example, if the peak to peak voltage is 300, then the peak voltage is 150 volts (assuming a symmetrical pure sine wave), in which case the rms value would be 150 divided by the square root of 2 (or about 106 volts). If the PEAK voltage is 300 volts (peak to peak now being 600 volts), then the rms value would be 300 divided by the square root of two (or about 212 volts). This is true whether you are measuring line to line or line to neutral.

So, let us say, using your 300 Volt peak to peak in your original post, that this is a line to line voltage. In this case, your rms voltage line to ground will be 106 volts times the square root of 3, or about 183.6 Volts. On the other hand, if the 300 volt peak to peak is a line to neutral specification, then your line to line voltage will be 106 divided by the square root of 3, or about 61.2 Volts.

Now, considering the case of the 300 volt PEAK, in which case the rms value is about 212 volts. If this is a line to line specification, then the line to neutral voltage will be 212 times the square root of 3, or about 367 volts. On the other hand, if the 300 volt peak refers to the line to neutral measurement, then your line to line voltage will be 212 divided by the square root of 3, or about 122 Volts.

As CJ has pointed out, I have reversed the "line to Line" and "line to neutral" measurements. I apologize. To correct my error:

If the specification is line to line voltage for a 300 volt peak to peak, the rms voltage would be 106 volts measured line to line, or 183.6 measured line to neutral.

For 300 Volts peak line to line specification, the rms voltage measured line to line will be 212 Volts and line to neutral would measure about 122 Volts.

Sorry for the carelessness...

Thanks everyone that has helped a lot.