Tutorial: Mechanic - electrician Topic: Basics of electrical engineering the 2nd. year Measurement...
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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. 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.
Tutorial: Mechanic - electrician Topic: Basics of electrical engineering the 2nd. year Measurement of resistance Prepared by: Ing. Jiří Smílek Projekt
Tutorial: Mechanic - electrician Topic: Basics of electrical
engineering the 2nd. year Measurement of resistance Prepared by:
Ing. Ji Smlek Projekt Anglicky v odbornch pedmtech,
CZ.1.07/1.3.09/04.0002 je spolufinancovn Evropskm socilnm fondem a
sttnm rozpotem esk republiky.
Slide 3
Definition of resistance: Electrical resistance is a
fundamental property of all passive and active elements :
appliances circuits Insulation electrical equipment The basic unit-
1 (ohm ) 1 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 middle -
from 1 to 10 6 large - from 10 6 to 10 14
Slide 4
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 R x 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
Slide 5
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
Slide 6
Used method Comparative method- R X R V Principle -
Distribution of voltage across the resistor in series we know the
internal resistance of voltmeter R V Procedure first we measure the
voltage source U 1 R X short-circuit switch then the voltage U 2 in
series with V meter is connected a resistor R X voltage is divided
in proportion of resistances R V and R X The method is suitable for
resistors approximately the same size as the resistance of
V-meter
Slide 7
Used method Comparative method - for small resistances R X R V
Principle - there is a voltage division across the resistor in
series in proportion to their resistances R N - normal resistance
(specific resistance of known size) Procedure we measure the
voltage source U X we measure the voltage source U N if we measure
on the same scale then the following applies :
Slide 8
Used method: Comparative method - for large resistors R X R A
Principle - distribution of currents in parallel resistors in
inverse proportion to their resistances Procedure we measure the
current resistance R X we measure the current resistance R N if we
measure on the same A- meter then the following applies:
Slide 9
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
Slide 10
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:
Slide 11
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 -
Slide 12
Write the measurement procedure for resistance in a V- meter
with comparative method for RV RX Describe what is the principle of
Ohms 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
Slide 13
Muk, J. Management ve vzdlvn dosplch. Praha: EUROLEX BOHEMIA,
2000. ISBN 80-7361-269-7. Operan program Vzdlvn pro
konkurenceschopnost, ESF 2007 2013. Dostupn na:
http://www.msmt.cz/eu/provadeci-dokument-k-op-vzdelavani-pro-
konkurenceschopnosthttp://www.msmt.cz/eu/provadeci-dokument-k-op-vzdelavani-pro-
konkurenceschopnost Uebnice Elektrick men Ing. Pavel Vylegala, SE
Ostrava, 2006 Uebnice Elektrick men SNTL, 1981 Elektrotechnika -
Laboratorn cvien pro bakalsk studium - Doc. Ing. Milo Hammer,
CSc.