Anglicky v odborn ch p edm tech support of teaching technical subjects in english
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Anglicky v odborných předmětech "Support of teaching technical subjects in English “. Tutorial : Mechanic - electrician Topic : Basics of electrical engineering the 2nd. y ear Measurement of resistance Prepared by: Ing. Jiří Smílek.

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Anglicky v odborn ch p edm tech support of teaching technical subjects in english

Anglicky v odborných předmětech"Support ofteachingtechnicalsubjects in English“

Tutorial: Mechanic - electrician

Topic: Basicsofelectricalengineering

the 2nd. year

Measurementofresistance

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.


Measurement of resistance

Measurement 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 10-6to 1 Ω

  • middle - from 1 to 106 Ω

  • large - from 106 to 1014Ω

  • Definition of resistance:


Measurement of resistances

Measurement of resistances

  • 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:


Measurement of resistances1

Measurement of resistances

  • The direct method - measuring the evaluative ohmmeter . is based on the magneto electrical ratio system

  • Used method:

  • Direct method - measuring bridge method - Wheatstone bridge

  • in a balanced state there does not flow a measuring diagonal current, then


Measurement of resistances2

Measurement of resistances

  • 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 – RXshort-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


Measurement of resistances3

Measurement of resistances

  • 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


Measurement of resistances4

Measurement of resistances

  • 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:


Measurement of resistances5

Measurement of resistances

  • 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

  • circuit for measuring small resistances

  • b) circuit for measuring large resistances

  • Used method:


Measurement of earth resistance

Measurement of earth resistance

  • 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

  • Used method:

  • Principlemethods

  • 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:


Anglicky v odborn ch p edm tech support of teaching technical subjects in english

Activities for students – questions

  • 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 -


Activities for students questions

Activities for students – questions

  • 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


Bibliography

Bibliography

  • 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-pro-konkurenceschopnost

  • 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.


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