CHAPTER 2 - CHAPTER 2 DC CURRENT METER 2.1 Introduction A...

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CHAPTER 2 DC CURRENT METER 2.1 Introduction A meter is any device built to accurately detect and display an electrical quantity in a form readable by a human being. Usually this "readable form" is visual: motion of a pointer on a scale, a series of lights arranged to form a "bargraph," or some sort of display composed of numerical figures. The display mechanism of a meter is often referred to as a movement , borrowing from its mechanical nature to move a pointer along a scale so that a measured value may be read. Though modern digital meters have no moving parts, the term "movement" may be applied to the same basic device performing the display function. The design of digital "movements" is beyond the scope of this chapter, but mechanical meter movement designs are very understandable. Most mechanical movements are based on the principle of electromagnetism: that electric current through a conductor produces a magnetic field perpendicular to the axis of electron flow. The greater the electric current, the stronger the magnetic field produced. If the magnetic field formed by the conductor is allowed to interact with another magnetic field, a physical force will be generated between the two sources of fields. If one of these sources is free to move with respect to the other, it will do so as current is conducted through the wire, the motion (usually against the resistance of a spring) being proportional to strength of current. The essential requirements of a measuring instrument are: a) that its introduction into the circuit where measurements are to be made, should not alter the circuit. b) the power consumed by it be small. 15
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Types of instrument are mainly used as ammeter and voltmeters are: 1) PMMC (For DC measurement only) 2) Moving Iron 3) Electrodynamometer 4) Hot wire 5) Thermocouple 6) Induction Type 7) Electrostatic 8) Rectifier PMMC instrument is the most accurate type for DC measurement. 2.2 Construction and function of Permanent Magnet Moving coil (PMMC) meter movement The basic moving coil system generally referred to as a D’Arsonval meter movement or Permanent Magnet Moving Coil (PMMC) meter movement. Figure 2.1: The Construction of D’Arsonval meter movement 16
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Figure 2.1 shows a horseshoe shaped permanent magnet with soft iron pole pieces attached to it. Between the north-south pole pieces is a cylindrical-shaped soft iron core about which a coil of fine wire is wound. This fine wire is wound on a very light metal frame and mounted in a jewel setting so that it can rotate freely. A pointer attached to the moving coil deflects up scale as the moving coil rotates. Operation: Figure 2.2: The D’Arsonval meter movement Current from a circuit in which measurements are being made with the meter passes through the windings of the moving coil (refer to Figure 2.2). Current through the coil causes it to behave as an electromagnet with its own north and south poles. The poles of the electromagnet interact with the poles of the permanent magnet, causing the coil to
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This note was uploaded on 03/04/2011 for the course EET 307 taught by Professor Rahimrazak during the Spring '11 term at University of Malaya.

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CHAPTER 2 - CHAPTER 2 DC CURRENT METER 2.1 Introduction A...

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