Hence only deflecting and damping toque are present. That indicates controlling torque is absent. Given that an instrument gives maximum deflection for any amount of quantity passed through it. So before coming to rest the pointer always oscillates due to inertia, so to bring the pointer rest within a short time we will use damping torque without effecting controlling torque (or) inertia.
The deflecting torque is used for deflection, the controlling torque acts opposite to the deflecting torque. After removal of moving mechanism, the pointer has to come back to its initial position, but in absence of controlling torque, the pointer won’t come back to its initial position.In absence of controlling torque, the pointer will swing beyond its final steady-state position and the deflection will be indefinite.It is to control the pointer to a definite value which is proportional to the quantity being measured.The torque needed to move the pointer over a calibrated scale is known as deflecting torque and it is capable of overcome inertia of the moving system, controlling torque and damping torque. AC bridges are used to inductance and capacitance.DC bridges are used to measure the resistance.Kelvin bridge is used for measurement of low resistance.Harmonic distortion analyzer, used as a notch filter, used in audio and high-frequency applications Used to measure relative permittivity, dielectric loss Suitable for medium Q coil (1 10), slowest bridgeĥ-point bridge, accurate and fastest bridge (Q < 1)Ĭampbell’s modification of Heaviside bridge The loss of charge method is used for the measurement of high resistance.A potentiometer is used to measure the small voltages.Megger is used for the measurement of high insulation resistance.Schering bridge is used for the measurement of capacitance, permittivity, dielectric loss and leakage resistance of the capacitor.PMMC and induction type (which are used permanent magnet) Moving iron (MI) and EDM (Electro dynamometer) instrumentsĮlectro-static voltmeter (ESV) and Induction type instruments There are two common forms of eddy current damping devices:ġ) A metal former which carries the working coil of the instrumentĢ) A thin aluminum disc attached to the moving system of the instrument, this disc moves in the field of a permanent magnet The disc should be of conducting and non-magnetic material This torque is proportional to the strength of the magnetic field and the current produced.This current interacts with the magnetic field to produce an electromagnet torque which opposes the motion.When a conductor moves in a magnetic field an emf is induced in it and if a closed path is provided, a current (known as eddy current) flows.In the moving coil instrument or PMMC type instruments, damping is provided by eddy current damping. To be effective the damping torque should be proportional to the velocity of the moving current but independent of the operating current. The damping device should be such that it produces a damping torque only while the moving system is in motion. The following mechanism can be used for producing damping torque. So before coming to rest the pointer always oscillates due to inertia, so to bring the pointer rest within a short time by reducing oscillations, we use damping torque without affecting controlling torque (or) inertia. >Example: A potentiometric type of recorder used for monitoring temperature records the instantaneous temperatures on a strip chart recorder. >Any electrical quantity like current, voltage can be recorded by a suitable recording mechan>ism. In principle, these are indicating instruments but so arranged that a permanent continuous record of the indication is made on a chart or dial.
Recording Instruments: These instruments record continuously the variation of any electrical quantity with respect to time. >Example: Ammeter, Voltmeter, Wattmeter, Speedometer These instruments indicate the instantaneous value of the electrical quantity being measured at the time at which it is being measured.
Indicating Instruments: These indicate the quantity being measured by means of a pointer that moves on a scale.
These instruments give reading for a specific period of time but no indication of reading for a particular instant of time.Įxample: Ampere-hour meter, Energy meter, kilovolt ampere-hour meter. >Integrating Instruments: These instruments record the consumption of the total quantity of electricity, energy, etc.