From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: AC panel voltmeter for RMS readings Date: Thu, 07 Sep 2000 02:53:34 -0400 sbourg@my-deja.com wrote: > In article <39B665CD.E3728F0D@bellsouth.net>, > Neon John <johngd@bellsouth.net> wrote: > > > > > Absolutely incorrect. Moving vane meters respond to the RMS value > > of the incoming waveform. You can verify that by either hitting the > > books and learning how they work (I did, long ago) or you can > > compare the instrument's reading to a true-RMS DVM (I have.) > > > > John > > You can say it until you're blue in the face, John, and it will make no > more sense. These meters have DC movements which do not respond to AC at > all, and of course rms is a meaningless term without a waveform. The > simple full-wave rectifier that converts the input to the DC current > that these meters DO respond to does not produce the rms equivalent to > any arbitrary waveform. Iron vane meters do NOT have rectifiers, do not have permanent magnets and respond equally accurately to AC and DC with one minor exception. Because the iron vanes have a small amount of retentivity, on DC, the current must be reversed, a second reading taken and the two averaged. The amount of retentivity-induced error is small - a few minor divisions at most - so it can be ignored for casual use. > So what reference tome did you pull your misguided concepts from anyway? Oh my, where to begin. This is like asking a civil engineer where he learned of concrete. Let's see. At about the age of 10, the Coyne Electrical Institute's Encyclopedia of Electricity was my daily companion. Wonderful set of practical electrical books - you really ought to try to find a set from a rare book dealer - you obviously need a good course in electricity. Later in college, the "Process Instruments & Controls Handbook", usually known as the "Considine book" by those of us in the trade. Later I gained intimate working knowledge of these instrument when I first worked in and then headed the Sequoyah Nuclear Plant's metrology section. Why don't I quote from Considine? Sect 13.16 - "Moving Iron Instruments" "An iron vane instrument provides a means for measuring AC or DC current using an attraction-repulsion method. A field coil is wound through which the current to be measured is passed. In the center of this coil are placed two iron vanes. One vane is stationary, and the other is made movable by connecting it to an axis, pointer, and torque spring. The magnetic field setup by the current passing through the coil induces like polarities in the iron vanes, causing them to separate. The amount of separation is indicated by a pointer. The principle is illustrated in Fig 16, and an early magnet vane mechanism is shown in Fig 17. (the URL below has an illustration close enough to Fig 16). "The energy stored in the magnetic field can be written: E = 1/2*I^2*L Where L is the inductance of the system, and its derivative, the torque T, is: T = dE/d<theta> = 1/2*I^2*(dL/d<theta>) "The system's response is proportional to I-squared. (I'm going to type this very slowly, Steve, so you can be sure and embrace every word) >>>>> "The instrument indicates root mean square (RMS) current or voltage. <<<<<< "By suitably shaping and arranging fixed and movable irons in the field of the coil, the instrument scale can be made very nearly linear over as much as 80% if its range. Alternatively, the upper range of the scale can be greatly compressed, and a small portion of the total range can be expanded to cover much of the scale. (Don't miss this part either, steve) The latter arrangement is particularly appropriate in a voltmeter used to monitor voltage which is nearly constant most of the time, such as line voltage. Over the years, various vane configurations have been developed, two of which are shown in Fig. 18." (sorry, you gotta buy the book to see that one.) Oh, I might add, the above section was written by B. P. Gropp, R&D Director, Weston Instrument Division, Sangamo/Weston, Newark, NJ. Kinda like getting the 10 Commandments directly from Moses. If you happen to disagree with Dr Gropp, perhaps you could take it up with him directly? If you want an on-line reference, here's the very first hit from the search engine: http://www.tpub.com/neets/book3/7e.htm I'm sure there are better on-line references but this is enough to demonstrate that you don't have the faintest idea of what you speak. > I'm sure there are dozens of manufacturers of expensive rms voltmeters > who will want to quash this publication so they can continue to foist > their overpriced instruments on a naive society of engineers! I can't imagine what sort of conspiracy theories you might cook up in your little mind but I can speak to the power utility industry. Every switchboard voltage or current indicator - AC or DC - made in the last, oh, 100 years that I've ever seen is a moving iron instrument. They are equally accurate on AC, DC, 25-500 hz, square wave inverter waveform, distorted small generator output, distorted saturable reactor output. In other words, just about anything you want to toss at it. And there are no semiconductors to get damaged by transients. Because a well-designed instrument responds equally to DC and the RMS value of AC, the detailed 5 point calibration can be done with simple DC instruments and then checked at a point or two on AC as a cross-check. To summarize, Steve, you don't have the faintest idea what you talk about. But as usual, you speak your mistake authoritatively and you caustically ridicule those who DO know and dispute what you say. Pretty typical. The only thing that makes this instance remarkable is that you're so starkly wrong about a basic, fundamental electrical principle that even a freshman EE student knows that it only took a minute to completely refute what you said and to post authoritative references. Other than myself, of course. Never has that old saying, "better to remain silent and be thought a fool than to open your mouth and remove all doubt." been more apropos. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: AC panel voltmeter for RMS readings Date: Thu, 07 Sep 2000 19:29:32 -0400 sbourg@my-deja.com wrote: > In article <39B73B6E.46F2FCE5@bellsouth.net>, > Neon John <johngdNOSPAM@bellsouth.net> wrote: > > > Sect 13.16 - "Moving Iron Instruments" > > > > "An iron vane instrument provides a means for measuring AC or DC > > current using an attraction-repulsion method. A field coil is wound > > through which the current to be measured is passed. In the center > > of this coil are placed two iron vanes. One vane is stationary, and > > the other is made movable by connecting it to an axis, pointer, and > > torque spring. The magnetic field setup by the current passing > > through the coil induces like polarities in the iron vanes, causing > > them to separate. The amount of separation is indicated by a > > pointer. The principle is illustrated in Fig 16, and an early > > magnet vane mechanism is shown in Fig 17. (the URL below has an > > illustration close enough to Fig 16). > > > > "The energy stored in the magnetic field can be written: E = > > 1/2*I^2*L Where L is the inductance of the system, and its > > derivative, the torque T, is: T = dE/d<theta> = > > 1/2*I^2*(dL/d<theta>) > > > > "The system's response is proportional to I-squared. > > > > (I'm going to type this very slowly, Steve, so you can be sure and > > embrace every word) > > > > >>>>> "The instrument indicates root mean square (RMS) current or > voltage. <<<<<< > > > > Ok, I admit, I'm confused. Explain to me please what any of this has to > do with the Simpson taut-band (rectified) panel meters you provided a > link to? Silly me, I assumed this was your recommendation. If you go to the first URL in my original article, which points to Simpson's web page, and go to the Wide-Vue meter's data sheet, here: http://www.simpsonelectric.com/pdf/webpdfe/Wide-Vue%20VAC.pdf , You'll see that the Wide-Vue format is available in both rectifier and iron vane types. The instrument I specified, #10220 is an iron vane, 0-150 volt AC voltmeter. If you look at the top of that page under "Specifications", you'll note that both types of movements are jewel-pivot and not taut band. I certainly prefer the taut band-style movement but I thought it more important to specify an instrument that can be easily purchased at Graingers from stock. Graingers web page appears to be down as I write this note so I can't double-check the URLs I provided to the catalog pages. Since I double-checked them when I wrote the article, I have confidence in their accuracy. > Note too the contradiction in adjacent sentences of what you claim to > have found which states that the response is proportional to the power, > or I^2, and then to rms - which is the square root of that. Can't have > both. Get your money back. I^2*R = power. R is the coil resistance. Power is what does work to move the pointer. He was simply stating that the pointer movement is proportional to (real) power. The scale is linearized via the shape of the iron vanes. Since the RMS value of a complex waveform IS the real power, the two are equivalent. Moot discussion unless you plan on making your own movement, since the manufacturers provide you a black box with two terminals to hook the voltage or current to and a fairly linear scale. Think I'll keep the book. I suggest you drop the $150 for a copy too. I might add that in a rectifier-type meter, the RECTIFIER and not the meter movement is what causes the instrument to not read the RMS value of the input. If the polarity of the field magnet could change in sync with the input AC wave, that type meter would also read RMS value. And indeed the field magnet can change - in an electrodynamic (ED) meter movement. The ED movement looks quite similar to the conventional movement except that the field is a wound coil that is connected to the input terminals. It responds equally to DC or the RMS value of AC. Indeed, before the development of the fancy computing standards, the ED meter was the usual transfer standard between AC and DC. The standard meter was calibrated against a DC standard and then used to calibrate AC instruments. The movement is too large and fragile for use in an RV, of course, but it is of interest nonetheless. At least to me. John |
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