Principles of Electronic Instrumentation
The operating principle of electrical measuring instruments involves the movement of the pointer from its initial position to the measured value, indicating the quantity being measured. This movement is facilitated by various deflecting torques generated through different effects such as magnetic, thermal, chemical, electrostatic, and electromagnetic induction.
List of Electrical Measuring Instruments
Table-1: List of instruments with their operating principle
| S.No. | Instruments | Effects | Suitability for type of measurement |
|---|---|---|---|
| 1 | Ammeters, voltmeters, wattmeter, integrating meter and other electrical instruments | Magnetic effects | Current, voltage, power, and energy on both AC and DC systems |
| 2 | Ammeter and voltmeter | Thermal effects | Current and voltage for both DC and AC systems |
| 3 | Integrating meter | Chemical effects | Measurement of DC ampere-hour |
| 4 | Voltmeters only | Electrostatic effects | Voltage only on both AC and DC systems |
| 5 | Voltmeter, ammeters, wattmeter, and energy meters | Electromagnetic induction effects | Measurement of voltage, current, power, and energy in AC systems only |
Electronic Effects
Followings are the list of effects,
Magnetic Effect
When a current-carrying conductor is placed in a uniform magnetic field, it undergoes a force that causes it to move. This phenomenon finds extensive application in various instruments such as moving iron attraction and repulsion type instruments, as well as permanent magnet moving coil instruments.
Thermal Effect
The measurement of current involves passing it through a small element that heats up, resulting in a temperature increase. This rise in temperature is then converted into an electromotive force (e.m.f.) by a thermocouple attached to the element. By connecting two dissimilar metals end to end to form a closed loop and maintaining different temperatures at the junctions, an e.m.f. is induced, leading to the flow of current through the closed-circuit, which is commonly known as a thermocouple.
Electrostatic Effects
When two charged plates are in close proximity, they experience a force of attraction or repulsion, causing one of the plates to move. This effect is harnessed in electrostatic instruments, which are typically utilized as voltmeters.
Induction Effects
By placing a non-magnetic conducting disc within a magnetic field generated by electromagnets excited with alternating currents, an electromotive force (emf) is induced in the disc. When a closed path is provided, current flows through the disc. The interaction between the induced currents and the alternating magnetic fields creates a force that impels the disc to move. This mechanism is known as the induction effect and is primarily employed in energy meters.
Hall Effect
The Hall effect occurs when a bar made of semiconducting material is positioned within a uniform magnetic field while carrying a current. As a result, an emf is generated between the two edges of the conductor. The magnitude of this emf is dependent on the flux density of the magnetic field, the current passing through the conducting bar, and the Hall effect coefficient, which remains constant for a given semiconductor. The Hall effect finds significant application in flux meters.