Chapter 3.ppt - 3.0 AC METERS Objectives List four...

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12/07/18 Chapter 1 : AC Meters 1 3.0 AC METERS Objectives: List four principle meter movements and application for each. Describe the purpose and operation of the diode in half- wave rectifier circuit and full-wave rectifier Describe the purpose of the second diode in a half-wave rectifier which is often used in rectifier circuits List five applications for electrodynamometer movement List the typical frequency range of the iron-vane meter movement

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2 3.1 Introduction Five principle meter movements used in ac instruments are: 1. Electrodynamometer - Standard meter, transfer instrument, wattmeter, frequency meter 2. Iron-vane - Indicator applications such as in automobiles 3. Electrostatic - Measurement of high voltage when very little current can be supplied by the circuit being measured
4. Thermocouple - Measurement of radio-frequency as signals 5. D’Arsonval (PMMC) - Most widely used meter movement for measuring direct current or voltage and resistance In this chapter we will discover that we can use the same d’Arsonval meter movement to measure alternating current and voltage. Even though it cannot directly measure alternative current with d’Arsonval. We must first rectify the alternating current by use of a diode rectifier to produce unidirectional current flow. Cont’d…

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12/07/18 Chapter 1 : AC Meters 4 Conversion AC to DC concept: Supply from Tenaga National Berhad (TNB) as shown in Fig. 3.1 is 50Hz, 240V rms root-mean-square (RMS) and 340V peak. • TNB sine-wave supply gives zero DC component! Cont’d… Fig.3.1: Alternating current (AC) signal waveform. V
12/07/18 Chapter 1 : AC Meters 5 • We can use simple half-wave rectifier as shown in Fig.3.2. A fixed DC voltage is now obtained. This is a simple power electronic (PE) system. Cont’d… Fig.3.2: Half-wave rectifier circuit. V s

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12/07/18 Chapter 1 : AC Meters 6 Cont’d…
12/07/18 Chapter 1 : AC Meters 7 3.2 D’Arsonval Meter Movement Used with half-wave rectifier Types of diodes are selected such as copper oxide , vacuum diode, semiconductor or crystal diode and etc. Figure 3.1 DC voltmeter meter circuit modified to measure ac voltage

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12/07/18 Chapter 1 : AC Meters 8 The circuit from Figure 3.1 produces an approximate half sine wave across the load resistor. The average value of this circuit is referred to as the dc voltage. The forward biased diode will have no effect on the operation of the circuit if we assume an ideal diode. Recall from chapter 2 that the sensitivity of DC voltmeter is Now we assume that a multiplier of ten times of this value means a 10Vdc input will cause exactly full-scale deflection of d’Arsonval instrument in Figure 3.1. Cont’d… V k V I S fs 1 ) / ( 1
12/07/18 Chapter 1 : AC Meters 9 When replace 10Vdc with 10Vrms, the voltage across the meter movement is just the positive half cycle of the sine wave because of the rectifying action of the diode. The peak value of the 10Vrms sine wave is: The arsonval instrument will respond to the average (DC) value of the ac sine wave where the average is : Cont’d… Vrms V p 10 2 V V V V V rms rms P ave 5 . 4 45 . 0 2

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• Fall '17
• puan nor azilah binti ahmad
• Rectifier, AC Meters

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