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RMS varia
Subject:
RMS varia
Date:
Wed, 9 Apr 97 14:53:33 EDT
From:
pierson-at-gone.ENET.dec-dot-com
To:
mail11:;;;;;-at-us4rmc.pko.dec-dot-com-at-us4rmc.pko.dec-dot-com-at-unknown.pupman-dot-com;;;;;;;;
(-at-teslatech)
CC:
pierson-at-gone.ENET.dec-dot-com
[these are out of order...]
[And the attributios are gone, 'coz i am discussing the ideas, not the
people.]
>Whoa there LT! Most meters measure RMS.
'most meters' is hard to define. most AC meters (in my
experience,
esp. voltmeters) are 'rectifier type. They measure AVERAGE,
and are MARKED ont he face to INDICATE RMS. This works ASSUMING
the waveshape is a sine wave.
>You'd better know p-p to rate your caps.
Yep. Or a little over. Maybe a LOT ove.
>P=1.414*RMS
>P=.5*P-P
>P-P=2*P
>RMS=.707*P
Yep. Assuming 'P' equals peak....
>VA Ratings (or Volt-Ampere ratings) refer to power capabilities.
>Power=Volts*Amps
Sort Of.
But VA is not equal to watts is not equal to power in AC work.
SOMEtimes VA == power. Sometimes not. (in AC work).
Specicially, for a reactive load (inductive, capacitive, VA is
more
than Watts. Measuring, unless a scope is to hand is nontrivial.
===============================================================================
>RMS stands for "Root Mean Square" to find it, take your peak to peak
>voltage and multiply by .707.
This assumes a sine wave. And
RMS is .707 the Vpeak.
>or take RMS voltage and divide by .707 to get back to the VP-P
This would get U to Vpeak. Vp-p would be twice that.
>that most meters measure.
See above. Common Meters _measure_ average, _indicate_ (by
means of
calibration). in RMS. (Digital meters, well, read the manual.
SOME are
true RMS, some have little corrections built in.
========================================================================
>I need to know about RMS of my xformer when calculating the capacitor
>value needed.
The questions not answerable, as posed.
'RMS' is a modifer of volts (or amps (milliAmps...))
>What is the RMS and how do I find it?
RMS voltage and RMS current are stated on the nameplate or spec
sheet.
In general a stated value in AC work is in RMS volts or amps,
unless
stated otherwise.
>Also volt amps is something I need to know about
See Above, also RH explanation.
=================================================================
>Root Mean Square or RMS can refer to voltage, current, power, etc.
My first thought was 'RMS power'? My second was Richard is
right,
as usual.
>Scince you stated you needed to know this for capacitor purposes I'll
>assume you are talking about RMS voltage. This is actually the name
>plate voltage. It is the voltage measured by most common AC voltmeters
>for a sine wave.
Fine finicky point. MOST meters measure something else, usually
_average_ and then play games with the calibration to _indicate_
RMS ASSuming a sine wave. When dealing in large blocks of
current and voltage and 'unusual' loads, as in coiling, this can
be a BAD assumption, as the waveforms get distorted.
This is critical because the peak-rms conversion for 'some other
waveform' will be different than the .707 for a sine wave. Most
tesla coil related waveforms are not sufficiently well behaved
to
use a 'canned' conversion like '.707'. A true RMS meter is the
way to go, tho they can be costly.
>>I am starting to get into more serious coiling and I am going to go for Q and
>>preformance so I guess Ill have to do some stinking math.
A very Knowledgable and practically skilled man of my
acquantance (
his specialty was mesuring microvolts, one at a time..., in
production
quantity instrumentation) once said, praising a coworker:
He understands that the numbers are REAL and have real
meaning.
In some cases, as in coiling, much of what is done involves
paramters which are too complicated to be conveniently reduced
to
numbers, but much else can be. And dealing with the smallest
set of
unknowns, at any one time, is generally a good thing.
regards
dwp