L-44(GDR)(ET) ((EE)NPTEL) - Module 10 Measuring Instruments...

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Module 10 Measuring Instruments Version 2 EE IIT, Kharagpur
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Lesson 44 Study of Single Phase Induction Type Energy Meter or Watt-hour Meter Version 2 EE IIT, Kharagpur
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Objectives To understand the basic construction and different components of a single phase induction type energy meter. Explain basic principle and development of torque expressions for energy meter. To study the errors involve in the energy meter. Use of instrument transformers for extension of Instrument range. Understanding the basic theory of shielded-pole shunt magnet. L.44.1 Introduction An instrument that is used to measure either quantity of electricity or energy, over a period of time is known as energy meter or watt-hour meter. In other words, energy is the total power delivered or consumed over an interval of time t may be expressed as: 0 () () t Wv t i t d t = If () vt is expressed in volts, it in amperes and t in seconds, the unit of energy is joule or watt second. The commercial unit of electrical energy is kilowatt hour (KWh). For measurement of energy in a.c. circuit, the meter used is based on “electro-magnetic induction” principle. They are known as induction type instruments. The measurement of energy is based on the induction principle is particularly suitable for industrial or domestic meters on the account of lightness and robustness of the rotating element. Moreover, because of smallness of the variations of voltage and frequency in supply voltage, the accuracy of the induction meter is unaffected by such variations. If the waveform of the supply is badly distorted, the accuracy, however, is affected. Basically, the induction energy meter may be derived from the induction watt-meter by substituting for the spring control and pointer an eddy current brake and a counting train, respectively. For the meter to read correctly, the speed of the moving system must be proportional to the power in the circuit in which the meter is connected. L.44.2 Construction of induction type energy meter Induction type energy meter essentially consists of following components Version 2 EE IIT, Kharagpur
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(a) Driving system (b) Moving system (c) Braking system and (d) Registering system. Driving system: The construction of the electro magnet system is shown in Fig. 44.1(a) and it consists of two electromagnets, called “shunt” magnet and “series” magnet, of laminated construction. A coil having large number of turns of fine wire is wound on the middle limb of the shunt magnet. This coil is known as “pressure or voltage” coil and is connected across the supply mains. This voltage coil has many turns and is arranged to be as highly inductive as possible. In other words, the voltage coil produces a high ratio of inductance to resistance.
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This note was uploaded on 08/03/2010 for the course ELECTRICAL EE212 taught by Professor Shetty during the Spring '10 term at International Institute of Information Technology.

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L-44(GDR)(ET) ((EE)NPTEL) - Module 10 Measuring Instruments...

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