Download AVOP-ELEKTRO-SMI-010

Tutorial: Mechanic - electrician
Topic:
Basics of electrical engineering
the 2nd. year
Measurement of current and voltage
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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Change AC and DC current and voltage:
Resistance - joined in series
a) rheostat - large current consumption
b) potentiometer – small current consumption
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Change AC current and voltage:
• Transformer – more economical than resistance
•
used transformer – Autotransformer
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Used method:
Deflection method - voltage is measured by Ammeters to show the
value of the measured current deflection or figure.
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Involvement of the measuring device:
Ammeters are connected in series
to the measured circuit
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Properties of the meter:
• Power consumption is negligible - Ammeter does not affect the
circuit,
• Internal impedance (resistance) - infinitely small, at least
significantly smaller than the impedance through which the
current is measured.
•
does not have a rise in voltage drop across the A- meter
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Magnification range ammeter:
• Shunt
• constant device
• kA = n x range/number of scale intervals
• Measuring transformer current– MTP - only AC
• nominal secondary voltage MTP
I2 = 5 A (1A – for energy - billing metering)
• transfer MTP
pP = I1/ I2= I1/ 5
• Constant device with MTP
kp = kA pP
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Magnification range of the ammeter:
Calculation of the shunt
• voltage on the A-meter - RA and shunt Rb is the same
• Substituting according to Ohm's law
• Expression RP
after adjustment
• We designate for the number, how many times we want to
extend the range of the ammeter, the letter n
• passes through the shunt a current (n - 1) times larger than
the ammeter and the shunt must have a resistance (n - 1)
times smaller than the resistance of the voltmeter.

Used method:
Deflection method - the voltage is measured by voltmeters showing
the value of the measured voltage deviation or figure.

Involvement of the measuring device:
Voltmeters are connected in
parallel to the measured circuit

Properties of the measuring device:
• Power consumption is negligible - a voltmeter does not have an
affect on the circuit being measured,,
• Internal impedance (resistance) - infinitely large - at least
substantially larger than the impedance at which the voltage is
measured.

Magnification range of the voltmeter:
• Series resistance - series
a) connection via terminals - one series
b) more series
c) by using the ranges
• Constant device
• kU = n x range/ number of scale intervals
• Measuring transformer voltage– MTN - only AC
• nominal secondary voltage MTN
U2 = 100 V
• transfer MTU
pN = U1/ U2= U1/ 100
• Constant device with MTN
kp = kV pN

Magnification range of the voltmeter:
Calculation of series
• Current in the circuit RP - RV is the same
• Substituting according to Ohm's law
or
• Expression of RP
after adjustment
• If we need the range of the voltmeter to increase n-times, it
must have a ballast resistor (n - 1) times larger,

Name the ways of changing the current and
voltage AC and DC circuit measurements
◦ current ◦ voltage -


Describe the method of measuring current
and voltage
Draw and write the name of the methods
involved in the measuring device for
measuring current and voltage
◦ current
◦ voltage

List and describe the characteristics of measuring devices
for measuring voltage and current
◦ voltage ◦ current -

Derive equations for a ballast device to enlarge the
measuring range of the device voltage
◦
◦
◦
◦
◦
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Derive equations for a ballast device for increasing the
extent of current meter
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◦
◦
◦
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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.