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Understanding Electrical Measurements: Galvanometers, Ammeters, and Their Sensitivities

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miriam-stark
9 diapositives · 260 Views · Dec 01, 2014

This presentation explores electrical measurements with a focus on galvanometers and ammeters. Galvanometers, particularly the sensitive moving-coil type, are vital for detecting low currents, employing techniques like taut-band suspension and light-beam reflections. We delve into the torque equations, current and voltage sensitivity, and the use of shunt resistors for large currents. The importance of minimizing resistance variations through temperature-stable materials, such as manganin and constantan, in both galvanometers and ammeters is highlighted. Gain insights into multirange ammeters and their switching mechanisms.

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  1. Presentation on Electrical measurements & measuring instruments

  2. Galvanometer • is a PMMC instrument designed to be sensitive to extremely low current levels. • The simplest galvanometer is a very sensitive instrument with the type of center-zero scale. • The torque equation for a galvanometer is exactly as discussed in the previous section. • The most sensitive moving-coil galvanometer use taut-band suspension, and the controlling torque is generated by the twist in the suspension ribbon.

  3. With the moving-coil weight reduced to the lowest possible minimum for greatest sensitivity, the weight of t he pointer can create a problem. The solution is by mounting a small mirror on the moving coil instead of a pointer.

  4. The mirror reflects a beam of light on to a scale. This makes light-beam galvanometers sensitive to much lower current levels than pointer instruments • Current sensitivity galvanometer • Voltage sensitivity galvanometer • Galvanometers are often employed to detect zero current or voltage in a circuit rather than to measure the actual level of current or voltage.

  5. DC Ammeter • is always connected in series • low internal resistance • maximum pointer deflection is produced by a very small current • For a large currents, the instrument must be modified by connecting a very low shunt resister • Extension of Ranges of Ammeter • Single Shunt Type of Ammeter

  6. Example 4.1: An ammeter as shown in Figure 3-9 has a PMMC instrument with a coil resistance of Rm = 99 and FSD current of 0.1 mA. Shunt resistance Rs = 1. Determine the total current passing through the ammeter at (a) FSD, (b) 0.5 FSD, and 0.25 FSD

  7. Swamping Resistance • The moving coil in a PMMC instrument is wound with thin copper wire, and its resistance can change significantly when its temperature changes. • The heating effect of the coil current may be enough to produce a resistance change, which will introduce an error. • To minimize the error, a swamping resistance made of manganin or constantan is connected in series with the coil (manganin and constantan have resistance temperature coefficients very close to zero.

  8. The ammeter shunt must also be made of manganin or constantan to avoid shunt resistance variations with temperature. • Multirange Ammeters • Make-before-break switch • The instrument is not left without a shunt in parallel with it. • During switching there are actually two shunts in parallel with the instrument.