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Transcript

Tutorial: Mechanic - electrician Topic: Basics of electrical engineering the 2nd. year Measurement of resistance Prepared by: Ing. Jiří Smílek Projekt Anglicky v odborných předmětech, CZ.1.07/1.3.09/04.0002 je spolufinancován Evropským sociálním fondem a státním rozpočtem České republiky. Definition of resistance: Electrical resistance is a fundamental property of all passive and active elements : • appliances • circuits • Insulation – electrical equipment The basic unit- 1 Ω (ohm ) • 1 Ω is the resistance of a conductor which passes through a current of 1 A at the ends of the conductor is 1 V • Range of measured resistances in practice • small - from 10-6 to 1 Ω • middle - from 1 to 106 Ω • large - from 106 to 1014 Ω Used method: Direct method - resistance is measured by Ohmmeters showing the value measured of resistance deviation or a figure. • measurement is quick and easy • deflection apparatus corresponds to the size of an unknown resistance - reverse calibrated voltmeter • value Rx→∞ corresponds to the mechanical zero on the right end of the scale • electrical zero - the left end of the scale • deflection of the hands depends on the size of voltage sources - must be zero balance - adjusted by the device Used method: • The direct method - measuring the evaluative ohmmeter . is based on the magneto electrical ratio system • Direct method - measuring bridge method - Wheatstone bridge • in a balanced state there does not flow a measuring diagonal current, then Used method Comparative method- RX ≐ RV • Principle - Distribution of voltage across the resistor in series • we know the internal resistance of voltmeter RV • Procedure • first we measure the voltage source U1 – RX short-circuit switch • then the voltage U2 – in series with V meter is connected a resistor • • RX voltage is divided in proportion of resistances RV and RX The method is suitable for resistors approximately the same size as the resistance of V-meter Used method Comparative method - for small resistances RX ≪ RV • Principle - there is a voltage division across the resistor in series in proportion to their resistances • RN - normal resistance (specific resistance of known size) • Procedure • we measure the voltage source UX • we measure the voltage source UN • if we measure on the same scale then the following applies : Used method: Comparative method - for large resistors RX ≫RA • Principle - distribution of currents in parallel resistors in inverse proportion to their resistances • Procedure • we measure the current resistance RX • we measure the current resistance RN • if we measure on the same A- meter then the following applies: Used method: Indirect method - Ohm's method • using Ohm's law, resistance is found by measuring current and voltage drop over the resistor • The resistance is calculated from Ohm's law according to the formula a) circuit for measuring small resistances b) circuit for measuring large resistances Used method: Indirect method - Wenner method - measuring the voltage drop Used in practise: • great importance to work safety – the size of earth resistance depends on the emergence of a potential dangerous contact voltage with the structure of appliances • earth resistance is the resistance between the earthing and the ground • ground resistance is an earth resistance earthing including resistance of ground leads - entire ground distribution Principle methods • the voltage drop occurs only in the vicinity of earth electrodes • in the space between the ground electrode is a large cross-section there is no voltage drop • The resistance of earth electrodes is calculated: Describe what it is electrical resistance ◦ Write what methods can measure el.resistance ◦ direct – ◦ indirect – Describe what is different in the comparison methods for measuring of large and small resistances ◦ Large resistances ◦ Small resistances - Write the measurement procedure for resistance in a V- meter with comparative method for RV ≐ RX ◦ ◦ Describe what is the principle of Ohm’s method for measurement of resistance ◦ ◦ ◦ Explain why when there is a greater distance from the earthing there is no voltage drop, even when going through the earth is an el. current Mužík, J. Management ve vzdělávání dospělých. Praha: EUROLEX BOHEMIA, 2000. ISBN 80-7361-269-7. Operační program Vzdělávání pro konkurenceschopnost, ESF 2007 – 2013. Dostupné na: http://www.msmt.cz/eu/provadeci-dokument-k-op-vzdelavani-prokonkurenceschopnost Učebnice Elektrická měření – Ing. Pavel Vylegala, SŠE Ostrava, 2006 Učebnice Elektrické měření – SNTL, 1981 Elektrotechnika - Laboratorní cvičení pro bakalářské studium - Doc. Ing. Miloš Hammer, CSc.