Download AVOP-ELEKTRO-SMI-009

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.
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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
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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
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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
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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
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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 :
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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:
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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
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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:
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Describe what it is electrical resistance
◦
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Write what methods can measure el.resistance
◦ direct –
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◦ indirect –
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Describe what is different in the comparison
methods for measuring of large and small
resistances
◦ Large resistances ◦ Small resistances -
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Write the measurement procedure for
resistance in a V- meter with comparative
method for RV ≐ RX
◦
◦
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Describe what is the principle of Ohm’s
method for measurement of resistance
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◦
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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
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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.