CHAPTER 3 - CHAPTER 3 AC METER 3.1 Introduction Figure 3.1:...

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CHAPTER 3 AC METER 3.1 Introduction Figure 3.1: Alternating current waveform 3.2 Calculation rms and average value a) Sine wave V avg = 0 V rms = 0.707V m Figure 3.2: Sine wave b) Full wave V avg = 0.636V m V rms = 0.707V m Figure 3.3: Full wave 34
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c) Half wave max V rms average time Output V avg = 0.318V m V rms = 0.5V m Figure 3.4: Half wave V avg = + T t t dt t V T 0 0 ) ( 1 = π ω 0 ) ( ) sin( 2 1 t d t V m + 0 = () 0 | ) cos( 2 t V m = ) 1 1 ( 2 + m V V avg = m V V avg = m V V rms = + T t t 2 0 0 dt ) t ( v T 1 = π ω ω π 0 2 m ) t ( d )] t sin( V [ 2 1 + 0 = π ω ω π 0 2 2 m ) t ( d ) t ( sin 2 V = π ω ω π 0 2 m ) t ( d ) t ( sin 2 1 V 2 x 2 cos 1 x Sin 2 = 35
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= ω π t 0 m 2 t 2 cos 1 2 1 V = 2 V p Five principal meter movements used in ac instrument: 1. Electrodynamometer 2. Iron Vane 3. Electrostatic 4. Thermocouple 5. D’Arsonval with rectifier Application of meter movements: Meter Movement DC Use AC Use Applications Electrodynamometer YES YES Standards meter, wattmeter, frequency meter Iron Vane YES YES “Indicator” applications such as in automobiles Electrostatic YES YES Measurement of high voltage when very little current can be supplied by the circuit being measured Thermocouple YES YES Measurement of radio frequency ac signal D’Arsonval YES YES with rectifier Most widely used meter movement for measuring direct current or voltage and resistance Table 3.1 36
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3.3 PMMC Instrument on AC The PMMC instrument is polarized, that is, its terminals are identified as + and -, and it must be connected correctly for positive (on scale) deflection to occur. When an alternating current with a very low frequency is passed through a PMMC instrument, the pointer tends to follow the instantaneous level of the ac. As the current grows positively, the pointer deflection increases to a maximum at the peak of the ac. Then as the instantaneous current level falls, the pointer deflection decreases toward zero. When the ac goes negative, the pointer deflected (off scale) to the left of zero. This kind of pointer movement can occur only with ac having a frequency of perhaps 0.1Hz or lower. With the normal frequency 50Hz or higher supply frequencies, the damping mechanism of the instrument and the inertia of the meter movement prevent the pointer from following the changing instantaneous levels. Instead the instrument pointer settles at the average value of the current following through the moving coil. The average value of purely sinusoidal ac is zero. Therefore, a PMMC instrument connected directly to measure 50Hz ac indicates zero. It is important to note that although a PMMC instrument connected to an ac supply may indicating zero, there can actually be very large rms current flowing in its coils. Two types of PMMC meter used in AC measurement:
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CHAPTER 3 - CHAPTER 3 AC METER 3.1 Introduction Figure 3.1:...

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