Re: What is Rms?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Stephen Wille Padnos
<spadnos@s...> wrote:

>
> On Sat, 2005-11-26 at 07:05 +0000, Joel Hagen wrote:
> > RMS stands for root mean square, it is used to compute the real value
> > of what you are measuring.
> > If you run 100% for 50% of the time and 0 for the other 50% you

have a

> > effective rms of 50% of what you are measuring.
>
> Actually, that's not accurate. The RMS value of a 50% duty cycle square
> wave is 0.7071 (sqrt(1/2)). You forgot to take the square root :) The
> DC average voltage is 50% of peak though.
>
> > It is used in math analysis to determine the real effective value of
> > what you are measuring, power, voltage, amps etc.
> > That is why there are voltage meters that are true rms, and those

that

> > are not. In ACV it makes a big difference what the freqency is, ie
> > 60hz or 400hz. Ever wonder about the great stuff from the airforce
> > that you can't use.
>
> Also, not quite right. It makes no difference what frequency a sinewave
> source is, the RMS value is still 1/sqrt(2)* peak. The reason that
> cheap meters aren't accurate for other frequencies is that they assume a
> 60Hz sinewave, so they average several readings with timing appropriate
> for a 60Hz source. True RMS meters can either use much faster sampling
> (and actually do RMS calculations, rather than assuming a sinewave and
> taking the 1/sqrt(2) shortcut), or can use an analog front end that
> gives RMS output (to either the analog or digital front end).
>
> - Steve

If you are running your spindle with a variable freq drive, it changes
frequency, (hence the name) and your typical Amprobe is designed for
something like 45 to 75 Hz and that's it.

A older VFD can easily run 400hZ, and the newer ones much higher.

I think the origional poster needs to give us make an model and
(hopefully) someone can explain the 5A RMS (I think we all know RMS
voltage to some degree.

I was under the impression that RMS only applied to sine wave type
power (or half wave) but not to square wave. After all, isn't duty
cycle used to calculate current ?

Dave