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# Anglicky v odborných předmětech "Support of teaching technical subjects in English “ - PowerPoint PPT Presentation

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 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.

• 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:

• 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

• Used method:

• 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 – 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

• 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

• 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

• 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:

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

• 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.