What do you call this voltage

[Jim Dungar]

That's I'm just trying to understand the new terminology and concepts.

Average RMS is what most electrical test meters read, basically it comes from the relationship between DC and a pure AC sinewave (i.e. Vdcpeak * .707 = Vacrms). Most electrical calculations assume a pure sinewave.

True RMS comes from performing the integration of the actual AC waveform in order to calculate the "exact" RMS value.

This is not new math, even though they may be new concepts to you.

 

[Gilbert W]

Average RMS is what most electrical test meters read, basically it comes from the relationship between DC and a pure AC sinewave (i.e. Vdcpeak * .707 = Vacrms). Most electrical calculations assume a pure sinewave.

True RMS comes from performing the integration of the actual AC waveform in order to calculate the "exact" RMS value.

This is not new math, even though they may be new concepts to you.

Jim,



I’m not trying to get you or Dan PO’d but it seems as though that’s the case. I’m sorry if I have. I won’t use the phrase new math anymore.



It’s been a while since I went to school, so I’m a little rusty. What I remember on this subject is that the AVERAGE value of a sine wave is calculated by taking each peak and multiplying by .636 . You then add them together and the result will be zero since you have a positive and negative peak. The old Simpson 260 analog meters used a rectified average value to give their readings. The scale was graduated to take this into account. The RMS value (NOT Average RMS) of a sine wave is .707 times the peak. The formula does not average the positive and negative to zero because it takes the mean value. If you plotted a sine wave on a piece of paper and drew a vertical line from each zero crossing straight up, then drew a horizontal line at the positive peak to create a box, you’d find that the sine wave portion that was in the box filled 70.7 %



Average and RMS are 2 different ways of looking at an AC wave. Like I said before, to me using them together is like asking for the distance in Kilometer-Miles.



That’s the way I remember it anyway. Have at it; I’m sure I’ve got it all wrong.





Regards,

Gil W.

 

[leitmotif]

The point is RMS is a calculation to give an average that correlates to DC. 10 amps x 260 volt = 2600 W whethr AC or DC (ruffly)

If you just averaged AC you would come up with a different value ie only one set of peaks (pos or neg) If you did both the average would be zero.

IF you went to school and learned this once break out your old textbooks and look at them again - OK you are rusty but like a bicycle you will get it back.

Dan

 

[Jim Dungar]

Average and RMS are 2 different ways of looking at an AC wave. Like I said before, to me using them together is like asking for the distance in Kilometer-Miles.

I believe, the correct term is "RMS, average".

I am as guilty as others in using slang.

Average/routine/typical "RMS, average" devices often measure one point along the waveform (not always the value at 8.33ms) and then apply a multiplier to create a reported RMS value. These devices do not change their reported value even if the waveform is distorted.

True "RMS, average" devices take many samples of the waveform and calculate an almost true RMS value regardless of the waveform.

 

[nightex]

one circuit 2 main wires = 1 phase (one sinusoid)
if more than 2 wires = 3 phase

 

[Gilbert W]

IF you went to school and learned this once break out your old textbooks and look at them again - OK you are rusty but like a bicycle you will get it back.

Dan

You're right Dan. It hit me like a ton of bricks after looking at some reference books. I'm afraid it's going to take a little more than a wire brush to remove the rust.


Thanks,
Gil W

 

[Gilbert W]

I believe, the correct term is "RMS, average".

I am as guilty as others in using slang.

Average/routine/typical "RMS, average" devices often measure one point along the waveform (not always the value at 8.33ms) and then apply a multiplier to create a reported RMS value. These devices do not change their reported value even if the waveform is distorted.

True "RMS, average" devices take many samples of the waveform and calculate an almost true RMS value regardless of the waveform.

Jim,

Thanks for explaining. You're averaging many cycles of the line, maybe seconds or minutes.

Also see my post above to Dan. I'm a lot rusty. Thanks for taking the time.

Gil W

 

[keithkyll]

A typical meter uses a bridge to rectify the AC into DC. The result is Average. The meter is then scaled to read RMS. As long as the sine wave is pure, you can get away with this. It becomes a problem when you're trying to read the output of a Phase Angle Controller (SCR or Triac). A basic lamp dimmer is a good example. Because the sine wave is 'chopped off', a standard meter will give you a large error.
A meter with a 'True RMS' feature will read the voltage properly, because it digitizes the signal and calculates the true RMS. The same rule applies when measuring AC current.