Subject : ELECTRICAL MEASUREMENTS AND MEASURING INSTRUMENTS Group No. 5

ELECTROSTATIC INSTRUMENTS TOPIC:- ELECTROSTATIC INSTRUMENTS Group Members:- Enrollment No:- Name 130990109023 Mohit Singh 130990109024 Montu Patel 130990109025 Nimesh Nagar 130990109026 Mayur Panchal 130990109027 Mihir Panchal

In electrostatic instruments, the deflecting torque is produced by the action of electric field on a charge conductor. Such instrument are essentially voltmeters, but they may be used with the help of external components, to measure current and power. Their greater use in laboratory for measurement of high voltages. There are two way in which forces acts: Attraction disc type electrostatic instrument. Quadrant electrometer.

Attraction disc type electrostatic instrument.

Plate A is positively charged and plate B is negatively charged. we have linear motion between the plates. The plate A is fixed and plate B is moveable. Let us assume there exists some force F between the two plates at equilibrium when electrostatic force becomes equal to spring force. At this point, the electrostatic energy stored in the plates is Now suppose we increase the applied voltage by an amount dV, due to this the plate B moves towards the plate A by a distance dx

The work done against the spring force due to displacement of the plate B be Fdx. The applied voltage is related to current as From this value of electric current the input energy can be calculated as From this we can calculate the change in the stored energy and that comes out to be By neglecting the higher order terms that appears in the expression.

Quadrant electrometer

The quadrant electrometer consists of a cylindrical metal box divided into quadrants which stands on insulating pillars. Opposite quadrants are connected electrically, and a light, thin metal vane of large area is suspended by a conducting torsion fiber inside the quadrants. In the space between them is suspended needle which consists of a light aluminium axis, to which are affixed a number of paddle-shaped aluminium blades.

 

Electrostatic Voltmeter

An instrument that measures electrostatic voltage without charge transfer is called an electrostatic voltmeter. A primary characteristic of an electrostatic voltmeter is that it accurately measures voltage on any kind of material without physical contact & therefore , no charge transfer. Electrostatic voltmeters are based on the principle that when 2 conductors are at different potentials they attract one another. This mechanical stress may be made the measure of the potential different between them if one of the conductors is fixed.

Advantages: These instrument may be used on ac or dc. They have no frequency and wave form errors as the deflection is proportional to square of voltage and there is no hysteresis. There are no errors caused by stray magnetic field as the instrument works on electrostatic principle. They are particularly suited for high voltages.

Disadvantages: The use of electrostatic instrument is limited to a certain special application in a.c. circuits of relatively high voltages. These instruments are expensive, large in size and are not robust in construction. There scale are not uniform. The operating force are small. This can be illustrated by comparing there operating forces in electromagnetic type instrumets

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