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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
A simple current meter is a coil of wire that is energized within a permanent magnetic field. The deflection will read proportional current (I believe the average current). An iron vane meter is a piece of iron that is attracted to an energized coil of wire and this reads the RMS current.
I have tried searching for the physics behind why one is average and one is RMS. Does anyone have a reference or explaination?
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Mechanical current meters measure average current, in reality. Example: Connect your moving coil milliammeter across an AC (lets say 50-60Hz) current source of RMS value 1mA, and it will read 0mA. Why? Because the average value of a purely AC signal is 0. It does not read the RMS value. A typical mechanical VOM reads RMS because the AC signal is rectified and the meter senses the average value of this rectifed current, which for a full-wave rectified sinusoidal signal, is
Iavg = 2 * Ipeak / pi
The scale of the meter can be constructed to measure RMS from the average value because the RMS value and the AVG value are related by
Irms / Iavg = 0.7071 / 0.6366 = 1.111
Therefore, by calibrating the tics on the meter, you can get the RMS reading.
Note that the meter scale will not give correct RMS values for a square wave, for example. A measurement of RMS current for a square wave can be made by taking the indicated RMS meter value (which is calibrated for a sine wave) and divide by the form-factor 1.111 to get the RMS value for a square wave (because Iavg = Irms for a square wave).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I don't know.. I was (and still am, in fact) under the impression that an iron vane meter reads the true RMS value of a signal. The reason being that the attraction between iron and coil is proportional to the square of the current.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Steve, I think you are partially right about the iron-vane meter..deflection will be proportional to the mean square of the current (not root mean square).. The force on the vane will be proportional to the square of the field (as long as the current is not high enough to begin saturating the vane)... The tics on the meter face need to accout for a "square-root" operation to read RMS current correctly. Iron-vane meters also need to be corrected for frequency effects..
In my previous blathering, I was speaking about the moving coil meters, which seem to be the most common..
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
The really nice thing about moving iron meters is that you can calibrate them with DC- which is easy- then use them to measure AC true RMS currents quite accurately, even over a fair range of frequencies and waveforms. The "fudge factor" of 1.111 only works for sine waves. The big problem is they are inductive as anything and not very sensitive. Last time I saw one was in an old battery charger where- ironicaly, you want average current not RMS.
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iron vane meters & current meters
