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

Anglicky v odborných předmětech "Support of teaching technical subjects in English “

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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.

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

- 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 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 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 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 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 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

- 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

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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