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.
Change AC and DC current and voltage: Resistance - joined in series a)rheostat - large current consumption b)potentiometer – small current consumption Transformer – more economical than resistance used transformer – Autotransformer Change AC current and voltage:
Used method: Deflection method - voltage is measured by Ammeters to show the value of the measured current deflection or figure. Involvement of the measuring device: Ammeters are connected in series to the measured circuit 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 Properties of the meter:
Magnification range ammeter: Shunt constant device k A = n x range/number of scale intervals Measuring transformer current– MTP - only AC nominal secondary voltage MTP I 2 = 5 A (1A – for energy - billing metering) transfer MTP p P = I 1 / I 2 = I 1 / 5 Constant device with MTP k p = k A p P
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 R P 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 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. Properties of the measuring device:
Magnification range of the voltmeter: Series resistance - series a)connection via terminals - one series b)more series c)by using the ranges Constant device k U = n x range/ number of scale intervals Measuring transformer voltage– MTN - only AC nominal secondary voltage MTN U 2 = 100 V transfer MTU p N = U 1 / U 2 = U 1 / 100 Constant device with MTN k p = k V p N
Magnification range of the voltmeter: Calculation of series Current in the circuit R P - R V is the same Substituting according to Ohm's law or Expression of R P 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 ◦ Derive equations for a ballast device for increasing the extent of current meter ◦
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- konkurenceschopnosthttp://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.