Measuring instrument task

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Measuring instrument task

  1. 1. Measuring Instrument Task MEASURING INSTRUMENT TASK DISUSUN OLEH KELOMPOK RAHMAWATI TH. DIAMANTI PERNI KRISTA U. KARUNDENG UNIVERSITAS NEGERI MANADO FAKULTAS MATEMATIKA DAN ILMU PENGETAHUAN ALAM JURUSAN FISIKA 2010
  2. 2. Measuring Instrument Task TO THE EFFECT LEARNING For over know electricity and its component BENEFIT can apply concept to electrify at in various shooting problem and also in day-to-day life PREFACE In to electrify will result still electricity that aroused by menggosokkan as erect as glass, look on it as contraption of object then there are many theory which grows and present cognitive it is accepted and is called ‖ electron theory ‖ one arises around year 1900. At century final to eighteen while electric source first times found by Voltaic Volta so maybe to be studied effect to electrify it ruled by jurisdictional given so maybe to be accounted its effect. Electric current can be equalled by liquid in one pipe if is jointed one introduction goes to pole pole current source. Meaning electric current current of electric one flows to pass through introduction at one particular enclosed series. Current electricing to evoke effect in introductory. Instrument to electrify Herein there is three instrumental types, namely 1. Moving is instrument's coil 2. Moving iron instrument 3. Moving is instrument's magnet More explanation is as follows a ) Moving instrument's coil Moving instrument’s coil is length square that resident at one particular punk with bolster so gets pivot on among magnetic poles, indicator needle is pasted on punk and if no voltage to indicator needle instrument lies on course 0 (zero) because of roll spiral spring (spring's coil). Current of positive pole goes to moving coil via bottom rolled spiral spring. Resulting magnet field around moving coil concerning in style magnet field between magnetic poles so causes moving coil moves. Instrument as it a lot of is utilized on vehicle tools testing. Moving is instrument's coil one for voltmeter, resistor provedes with that instrument that linked quits its prisoner one is accounted deep its relationship with moving's prisoner coil. b ) Moving Iron Instrument Moving iron instrument has coil that magnet field effect it to one vane of soft iron, vane that was placed on needle punk and is pulled little farther if wax current, irregular scale because its magnet situation. A part first of scale with short division distance, this instrument match for current DC and AC.
  3. 3. Measuring Instrument Task c ) Moving instrument's Magnets One vane of soft iron to be pasted on needle punk and is placed magnetic pole betwixt nail rides on horseback. That armature's position determined by field of that magnet style and which that magnet field resultant by current that passes through koil. If current is adrift pass through koil vane that will revolve and deviates current. That instrument is utilized one for amperemeter on electric system, it points out charge (fill) or not charge but that instrument not precision. I. ELECTRODYNAMOMETER The electrodynamometer can be used as a wattmeter, a VARmeter, a power-factor meter, or a frequency meter. The electrodynamometer movement may also serve as a transfer instrument, because it can be calibrated on dc and then used directly on ac, establishing a direct means of equating ac and dc measurements of voltage and current. Where the d'Arsonval movement uses a permanent magnet to provide the magnetic field in which the movable coil rotates, the electrodynamometer uses the current under measurement to produce the necessary field flux. Figure 5-1 shows a schematic arrangement of the parts of this movement. A fixed coil, split into two equal halves, provides the magnetic field in which the movable coil rotates. The two coil halves are connected in series with the moving coil and are fed by the current under measurement. The fixed coils are spaced far enough apart to allow passage of the shaft of the movable coil. The movable coil carries a pointer, which is balanced by counterweights. Its rotation is controlled by springs, similar to the d'Arsonval movement construction. The complete assembly is surrounded by a laminated shield to protect the instrument from, stray magnetic fields which may affect its operation. Damping is provided by aluminum air vanes. moving in sector-shaped charribers. The entire movement is very solid and rigidly constructed in order to keep its mechanical dimensions stable and its calibration intact. A cutaway view of the electrodynamometer is shown in Fig. 5-2.
  4. 4. Measuring Instrument Task The operation of the instrument may be understood by returning to the expression for the torque developed by a coil suspended in a magnetic field. We previously stated, Eq. (4-1). that T = B x A x I x JV indicating that the torque, which deflects the movable coil, is directly proportional to the coil constants (A and N), the strength of the magnetic field in which the coil moves (B), and the current through the coil (I). In the electrodynamometer the flux density (B) depends on the current through the fixed coil and is therefore directly proportional to the deflection current (I). since the coil dimensions and the number of turns on the coil frame are fixed quantities for any given meter, the developed torque becomes a function of the current squared (I2 ). If the electrodynamometer is exclusively designed for dc use, its square-law scale is easily noticed, with crowded scale markings at the very low current values, progressively spreading out at the higher current values. For ac use, the developed torque at any instant is proportional to the instantaneous current squared (I2 ). The instantaneous-value of i2 is always, positive and torque pulsations are therefore produced. The movement, however, cannot follow the rapid variations of the torque and takes up a position in which the average torque is balanced by the torque of the control springs. The meter deflection is therefore a function of the mean of the squared current. The scale of the electrodynamometer is usually calibrated in terms of the square root of the average current squared, and the meter therefore reads the rms or effective value of the ac. The transfer properties of the electrodynamometer become apparent when we compare the effective value of alternating current and direct current in terms of their heating effect or transfer of power. An alternating current that produces heat in a given
  5. 5. Measuring Instrument Task resistance at the same average rate as a direct current (I) has, by definition, a value of I amperes. The average rate of producing heat by a dc of I amperes in a resistance R is 11 R watts. The average rate of producing heat by an ac of i amperes during one cycle in the same resistance R is . By definition, therefore, I2 r = and I = = This current, I, is then called the root-mean-square (rms) or effective value of the alternating current and is often referred to as the equivalent dc value. If the electrodvnamometer is calibrated with a direct current of I-A and a mark is placed on the scale to indicate this I -A dc value, then that alternating current which causes the pointer to deflect to the same mark on the scale must have an rms value of I A. We can therefore "transfer" a reading made with dc to its corresponding ac value and have thereby established a direct connection between ac and dc. The electrodynamometer then becomes very useful as a calibration instrument and is often used for this purpose because of its inherent accuracy. The electrodynamometer, however, has certain disadvantages. One of these is its high power consumption, a direct result of its construction. The current under measurement must not only pass through the movable coil, but it must also provide the fiel d flux. To get a sufficiently strong magnetic field, a high mmf is required and the source must supply a high current and power. In spite of this high power consumption, the magnetic field is very much weaker than that of a comparable d'Arsonval movement because there is no iron in the circuit, i.e., the entire flux path consists of air. Some instruments have been designed using special laminated steel for part of the flux path, but the presence of metal introduces calibration problems caused by frequency and vaveform effects. Typical values of electrodynamometer flux density are in the range of approximately 60 gauss. This compares very unfavorably with the high flux densities (1,000-4.000 gauss) of a good d'Arsonval movement. The low flux density of the electrodynamometer immediately affects the developed torque and therefore the sensitivity of the instrument is typically very low. The addition of a series resistor converts the electrodynamometer into a voltmeter, which again can be used to measure dc and ac voltages. For reasons previously mentioned, the sensitivity of the electrodynamometer voltmeter is low, approximately 10 to 30 /V (compare this to the 20 k /V of a d'Arsonval meter). The reactance and resistance of the coils also increase with increasing frequency, limiting the application of the
  6. 6. Measuring Instrument Task electrodynamometer voltmeter to the lower frequency ranges. It is, however, very accurate at the powerline frequencies and is therefore often used as a secondary standard. The electrodynamometer movement (even unshunted) may be regarded as an ammeter, but it becomes rather difficult to design a moving coil which can carry more than approximately 100 mA. Larger current would have to be carried to the moving coil through heavy lead-in wires, which would lose their flexibility. A shunt, when used, is usually placed across the movable coil only. The fixed coils are then made of heavy wire which can carry the large total current and it is feasible to build ammeters for currents up to 20 A. Larger values of ac currents are usually measured by using a current transformer and a standard 5-A ac ammeter (Sec. 5-11). II. ELECTRODYNAMOMETERS IN POWER MEASUREMENTS II.1 Single-phase Wattmeter The electrodynamometer movement is used extensively in measuring power. It may be used to indicate both dc and ac power for any waveform of voltage and current and it is not restricted to sinusoidal waveforms. As described in Sec. 5-2, the electrodynamometer used as a voltmeter or an ammeter has the fixed coils and the movable coil connected in series, thereby reacting to the effect of the current squared. When used as a single-Phase power meter, the coils are connected in a different arrangement (see Fig. 5-18). The fixed coils or field coils, shown here as two separate elements, are connected in series and carry the total line current (ic). The movable coil, located in the magnetic field of the fixed coils, is connected in series with a current-limiting resistor across the power line and carries a small current (ip). The instantaneous value of the current in the movable coil is ip = e/Rp, where e is the instantaneous voltage across the power line, and P, is the total resistance of the movable coil and its series resistor. The deflection of the movable coil is proportional to the product of these two currents, ic. and ip, and we can
  7. 7. Measuring Instrument Task write for the average deflection over one period: av = K c ip dt Where av = average angular deflection of the coil K = instrument constant ic = instantaneous current in the fiel d coils ip= instantaneous current in the potential coil Assuming for the moment that ic is equal to the load current, i (actually, ic = ip + i,) and using the value for ip = e/Rp, we see that Eq. (5-8) reduces to av = K dt = K2 dt By definition, the average power in a circuit is Pa v = dt which indicates that the electrodynamometer-movement, connected in the configuration of Fig. 5-18, has a deflection proportional to the average power. if e and i are sinusoidally varying quantities of the form e = Em sin t and i = im sin ( t ), Eq. (5-9) reduces to av = K3EI cos where E and I represent the rms values of the voltage and the current, and represents the phase angle between voltage and current. Equations (5-9) and (5-10) show that the electrodynamometer indicates the average power delivered to the load. Wattmeters have one voltage terminal and one current terminal marked ― .‖ When the marked current terminal is connected to the incoming line, and the marked voltage terminal is connected to the line side in which the current coil is connected, the meter will always read up-scale when power is connected to the load. If for any reason (as in the two- wattmeter method of measuring three-phase power), the meter should read backward, the current connections (not the voltage connections) should be reversed. The electrodynamometer wattmeter consumes some power for maintenance of its magnetic field, but this is usually so small, compared to the load power, that it may be neglected. If a correct reading of the load power is required, the current coil should carry exactly the load current, and the potential coil s hould be connected across the load terminals. With the potential coil connected-, to point A, as in Pig. 5-18, the load voltage is properly met- -d, but the current through the field coils is greater by the amount I The wattmeter therefore reads high by the amount of additional power loss in the potential circuit. If, however, the potential coil is connected to point B in Fig. 5-18, the field coils meter the correct load current, but the voltage across the potential coil is higher by the amount of the drop across the field coils. The wattmeter will again read high, but now by the amount of the 12 R losses in the field windings. Choice of the correct connection
  8. 8. Measuring Instrument Task depends on the situation. Generally, connection of the potential coil at point A is prefer red for high-current, low-voltage loads. connection at B is preferred for low-current, high-voltage "Loads. The difficulty in placing the connection of the potential coil is overcome in the compensated wattmeter, shown schematically in Fig. 5-19. The current coil consists of two windings, each winding having the same number of turns. One winding uses heavy wire that carries the load current plus the current for the potential coil. The other winding uses thin wire and carries only the current to the voltage coil. This current, however, is in a direction opposite to the current in the heavy winding, causing a flux that opposes the main flux. The effect of ip is therefore canceled out, and the wattmeter indicates the correct power. II.2 Polyphase Wattmeter Power measurements in a polyphase system require the use of two or more wattmeters. The total real power is then found by algebraically adding the readings of the individual wattmeters. Blondel's theorem states that real power can be measured by one less wattmeter element than the number of wires in any polyphase system, provided that one wire can be made common to all the potential circuits. Figure 5-20(a) shows the connection of two wattmeters to measure the power consumption of a balanced three-wire delta-connected three-phase load. The current coil of wattmeter I is connected in line A, and its voltage coil is connected between line A and line C. The current coil of wattmeter 2 is connected in line B, and its voltage coil is connected between line B and line C. The total power, consumed by the balanced three-phase load, equals the algebraic sum of the two wattmeter readings. The phasor diagram of Fig. 5-20(b) shows the three phase voltages VAC, VCB, and VBA and the three phase currents 1AC, 1CB, and 1BA. The delta-connected load is assumed to be inductive, and the phase currents lag the phase voltages by an angle 0. The current coil of wattmeter I carries the line current , 1A1 A, which is the vector sum of the phase currents IAC and
  9. 9. Measuring Instrument Task IAB. The potential coil of wattmeter I is connected across the line voltage VAC. Similarly, the current coil of wattmeter 2 carries the line current IB1 B, which is the vector sum of the phase currents IBA, and IBC, while the voltage across its potential coil is the line voltage VBC. Since the load is balanced, the phase voltages and phase currents are equal in magnitude and we can write VAC = VBC = V and IAC = ICB = IBA = I The power, represented by the currents and voltages of each wattmeter is W1 = VAC1A1 A cos (30o – ) = VI cos (30o – ) W2 = VBC IB1 B cos (30o + ) = VI cos (30o – ) and W1 + W2 = VI cos (30o – ) + VI cos (30o + ) = ( cos 30o cos + sin 30o sin + cos 30o cos - sin 30o sin ) = VI cos Equation (5-14) is the expression for the total power in a three-phase circuit, and the two wattmeters of Fig. 5-20(a) therefore correctly measure this total power. It may be shown that the algebraic sum of the readings of the two wattmeters will give
  10. 10. Measuring Instrument Task the correct value for power under any condition of unbalance, power factor, or waveform. If the neutral wire of the three-phase system is also present, as in the case of a four-wire star-connected load—according to Blondel's theorem—three wattmeters would be needed to make the total real power measurement. In Prob. 12 the reader is asked to prove that three wattmeters measure total power in a four-wire system. Problem : one alternating current voltmeter utilize series 4a, where is power d.' arsonval (PMMC) having prisoner in 50 and need direct currents as big as 1mA for deflection heaving full. if diode was looked on by ideal (zero forward prisoner and prisoner turns back not get until) and tension as big as 10rms linked to entry terminals, prisoners appreciative determinative Rs who result deflection heaving full. Answer : for full wave rectifier Edc = Em = Erms = 0,9 Erms Edc = 0,9 x 10 V = 9 V serieses totaled prisoner neglectfully diode prisoner in tenor forward : RT = Rs + Rm = = 9 K So, Rs = RT – Rm = 9000 - 50 = 8950
  11. 11. Measuring Instrument Task GLOSARIUM Calibrate : a comparison between measurements - one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device Damping : any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator Density : defined as its mass per unit volume D’arsonval : A commonly used sensing mechanism used in DC ammeters, voltmeters, and ohm meters is a current-sensing device Equivalent : a unit of amount of substance used in chemistry and the biological sciences a measuring tool that will show the potential difference in a series Quantities : a kind of property which exists as magnitude or multitude Voltmeter : a measuring tool that will show the potential difference in a series Wattmeter : Measure of Electric Power
  12. 12. Measuring Instrument Task LITERATURE www.google.com http://alifis.wordpress.com/2010/01/18/bocoran-soal-uas-instrumentasi-elektronika/#more-1191 http://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_mov ement.htm

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