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#61612 - 01/29/06 03:17 PM Peak Voltage  
ShockMe77  Offline
Joined: Jun 2005
Posts: 821
Rahway, New Jersey

I'm currently taking a class called AC Principles and have a few questions. Our instructor has been teaching us about sign waves and I'm having a hard time understanding peak voltage.

How do we know that the peak voltage of a 120VAC sign wave comes to 170 volts?

And how does the figure ".707" relate to the 170 volts?

We'll be getting into 3-phase in the next class and I don't think I have grasped this concept yet.


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#61613 - 01/29/06 03:24 PM Re: Peak Voltage  
RobbieD  Offline
Joined: Feb 2003
Posts: 231
Peak Voltage:
Peak voltage tell you how far the voltage swings, either positive or negative, from the point of reference. Peak voltage is only a moderately useful way of measuring voltage when trying to express the amount of work that will be done when driving a specified load.

RMS Voltage:
RMS voltage is absolutely the most common way to measure/quantify AC voltage. It is also the most useful. Because AC voltage is constantly changing and is at or near the highest and lowest points in the cycle for only a tiny fraction of the cycle, the peak voltage is not a good way to determine how much work can be done by an AC power source.

The RMS voltage of a pure‡ sine wave is approximately .707*peak voltage. If you read voltage with a voltmeter you are generally given the RMS voltage of the wave form.

So for your example the sine wave goes from 0 to 170V in the positive and then goes down to 170V in the negative. To find RMS (peak Voltage) x (0.707) this will give you 120V.

If you hooked up an osiliscope to a 120volt receptacle you would see the sinewave and would notice that the highest voltage that is recorded is 170volts. This is Peak Voltage. Its only there for a really, really short time. So use the RMS (RMS is .707 of Peak Voltage)

[This message has been edited by RobbieD (edited 01-29-2006).]

#61614 - 01/29/06 07:46 PM Re: Peak Voltage  
ShockMe77  Offline
Joined: Jun 2005
Posts: 821
Rahway, New Jersey
Thank you for that explanation. I really do appreciate the help. So if I know 170 volts is the peak voltage level for a 120 volt sine wave, would I also use the .707 figure to find the peak level of a 24OV sine wave? How 'bout 480?

#61615 - 01/29/06 07:52 PM Re: Peak Voltage  
RobbieD  Offline
Joined: Feb 2003
Posts: 231
Yes. The .707 is a constant. If you read 240Volts with your multimeter you are reading RMS Voltage. So the peak voltage would be 240/0.707 so Vp=340

If Vrms=480 480/0.707 so Vp=679

#61616 - 01/29/06 08:13 PM Re: Peak Voltage  
Radar  Offline
Joined: Apr 2004
Posts: 349
Los Angeles, CA
The .707 is a constant

This is true for a sine wave, not necessarily for other wave shapes. It has to do with the square root of 2, a bit of trig trickery (not unlike smoke & mirrors).

Hook up a scope to an AC circuit and see the peak values are + & - 170V. Then divide 170 by the square root of 2 (1.4142) and you get 120V (approximately) RMS value. RMS is the effective, or DC equilivant value, work wise.


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#61617 - 01/30/06 04:08 AM Re: Peak Voltage  
RODALCO  Offline
Joined: Dec 2005
Posts: 854
Titirangi, Akld, New Zealand
The RMS value of a sine wave voltage generates the same amount heat for a resistor at the same DC value.

e.g. 120 VoltsRMS AC generates the same amount of heat as 120 Vdc, regardles of the higher peak value. ( and lower values near the zero crossing )

Robbie gives you a very good explanation for it anyway.

In case of 3Ø power the value V3 or root 3 (1.73) becomes more important as you will learn at tech. later on.
Phase voltage between phase and neutral. is 120 volts in your case.
Line voltage is 120*1.73=208 volts, between 2 phases.

Hope it helps a little
regards Ray.

The product of rotation, excitation and flux produces electricty.

#61618 - 01/30/06 11:49 AM Re: Peak Voltage  
SolarPowered  Offline
Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
The .707 is the square root of the integral from 0 to 2π of sin(θ)^2 dθ, divided by 2π. Which is, as RobbieD said, the RMS value of the waveform. That integral comes out to 1/sqrt(2), which is, rounded to three digits, 0.707.

(Edited to turn off smilies, which were showing up in the middle of the math.)

[This message has been edited by SolarPowered (edited 01-30-2006).]

#61619 - 01/30/06 07:59 PM Re: Peak Voltage  
ShockMe77  Offline
Joined: Jun 2005
Posts: 821
Rahway, New Jersey
I love this forum!

Thanks to all of you.



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