RMS and true RMS

----------- Yes, Note that the average power for half wave rectified signal will be half that at full wave or AC.

For a symmetrical periodic function such as a sine wave, average voltage, velocity etc, is really sort of a useless measure. For a non-symmetric wave, it is the DC component.

Reply to
Don Kelly
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I apologise. I was going to do so anyhow if you hadn't responded. After I answered, I realised that you were referring only to the statements regarding sinusoids. Note also that , in the first response, I did clearly say your results were correct for a sinusoid.

Reply to
Don Kelly

Thank you, and bless you and the horse you rode in on.

ns

Reply to
northstar

Sorry I was so bitchy, I have been under the weather.

Reply to
Don Kelly

Don't know which kind of 'old meters' you're referring to here.

Those that use a simple D'Arsonval movement are *average* meters that are scaled for RMS (as another poster mentioned). D'Arsonval movements have a moving coil of wire suspended in a magnetic field from a permanent magnet. Either jeweled bearings with tiny springs on each end of the shaft that double as current-carrying to the moving coil and restoring torque on the pointer. Or 'taut-band' suspension that uses a tiny flat band of metal for the 'shaft'. The band in front carries current into the coil, while the band in back carries the current out. The restoring force for the needle comes from the elastic twisting of the bands.

The ones with a non-linear scale are usually 'moving iron vane' movements where there is no moving coil, just a stationary one. It induces a magnetic field in an iron vane (hence the name) that repels from a stationary piece of iron next to it. Restoring torque is provided by tiny springs. The iron-vane movement is very non-linear and some folks have mistaken it for a square root scale, but it isn't purely square root. In fact, most such meters are initially calibrated by hand-marking the meter face while applying a variety of test signals. Then the face is given to a graphic artist that puts permanent markings. So the meter faces of iron-vane movements are *not* interchangable.

Iron-vane were/are quite popular with large switchboards for ammeters. This is for the simple reason that most such switchgear use current transformers that typically are rated for a 5A secondary. And iron-vane coils are often rated for 5A directly, so you don't need to mess around with shunts (like you do with D'Arsonval movements).

The forces created in both types of movements are related to the instantaneous current flow through them. The inertia of the pointer coil/vane and any dampening vanes causes the pointer to respond to the

*average* of these forces, not the RMS value. When using one designed for sinusoidal currents on other waveforms, one can correct the reading *if* one knows enough about the waveform and you remember to 'descale' the faceplate reading from the RMS marking to get the 'average' reading.

daestrom

Reply to
daestrom

Moving vane meters are relatively cheap compared the more accurate dynamometer meter. Instead of a permanent magnet, as used for a d'Asrsonval movement, an electromaget is used. This can give accurate true rms results. The same is true for electrostatic voltmeters.

-- Ferme le Bush

Reply to
Salmon Egg

Moving iron meters (not necessarily moving vane) have a torque depending on the square of the current in the coil and, as with other mechanical meters, the meter inertia does the averaging to get true RMS. As far as I recall, moving iron, dynamometer and the really old "hot wire" meters all inherently read RMS and that is why they were used. Hot wire meters were too bulky with their meter long tubes, dynamometer movements were generally too expensive except where absolutely needed for power/var/pf devices so the bulk were moving iron. Darsonval meters, as you indicate, read average values. Many modern moving iron meters are nearly linear, with some compression at each end of the scale (more at low end) and are still sold for switchboard use. (see reference below)

"Meters with a moving iron are intended for measurement of AC currents or voltages of frequencies from 15 Hz to 100 Hz. They measure rms values independently of the signal form of current or voltage. "

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Reply to
Don Kelly

Yet, I *know* that meter scales for iron-vane movements are individually hand-calibrated as I described (been there *done* that). So if it's supposed to be sqrt(), why are so many such meter faces *not* a perfect sqrt()? Must be some irregularities that creep into it. Go get an old switchboard meter, mark off the face in perfect sqrt() scaling and compare it to the actual markings. Not going to match.

daestrom

Reply to
daestrom

Agreed. In fact the meters that are shown in my reference are definitely not sqrt. In fact they are nearly linear over a wide range. However, this appears to be built into the design of the magnetic system (i.e. vane spacing or, if solenoidal, the shaping of the plunger, or in the linearity of the spring). About 60 years ago, I had a book detailing the construction and operation of different meters and about all that stuck was that certain moving iron meters were true rms. Among the other things that fall by the wayside over time. The meters that I am particularly thinking of were made by Weston for lab work -about 4-5 inches square, and 3 deep, typically 1/2 of 1 % of full scale (the 1/4% ones were generally dynamometers) but were true rms but unlikely to be iron vane. As to scales, If a voltmeter is average reading the torque will be directly proportional to the voltage. If the spring is linear, then the scale (for rms based on a sine wave) should also, in theory, be linear. If this is so and the panel meter is actually average reading, why is its scale not linear? Hmmm.:)

Reply to
Don Kelly

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