Electrical And Electronic Measurments

March 25, 2018 | Author: Ashesh B Vignesh | Category: Inductor, Transformer, Electric Current, Alternating Current, Inductance


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NATIONAL INSTITUTE OF TECHNOLOGY, TIRUCHIRAPPALLIElectrical and Electronic Measurements Self – Study Notes Ashesh B Vignesh 110110013 ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 1 ELECTRI CAL AND ELECTRONI C MEASUREMENTS SELF – STUDY NOTES CONTENTS Chapter 1: Electro Mechanical Instruments........................................................................................... 2 Permanent Magnet Moving Coil Meters (PMMC Meters) .......................................................... 2 Moving Iron Meters .................................................................................................................................. 3 Electrodynamic Meters (Dynamometer Type) .............................................................................. 4 Clamp On Meters ....................................................................................................................................... 6 Analogue Multimeters ............................................................................................................................. 6 Measurement of High Frequency Signals Using Analogue Meters ......................................... 7 Thermocouple Meters ............................................................................................................................. 7 Current Transformer ............................................................................................................................... 8 Potential/Voltage Transformer ........................................................................................................... 9 ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 2 CHAPTER 1: ELECTRO MECHANICAL INSTRUMENTS PERMANENT MAGNET MOVING COIL METERS (PMMC METERS)  Commonly used form of analogue voltmeter o Due to its sensitivity, accuracy and linear scale  Responds only to DC input  Uses a pointer that moves over a calibrated scale to indicate a measured quantity  Three forces are operating in the electromechanical mechanism o A deflecting force o A controlling force o A damping force  The deflecting force cause the pointer to move from its zero position when a current flows o The deflecting force is magnetic  The controlling force is provided by springs o These springs retain the coil and point at zero position when no current is flowing  The pointer and coil tend to oscillate for some time before settling down at their final position  A damping force is required to minimize the oscillations o The damping force is providing by eddy currents Fig 1.1: Moving Coil Meter  Consists of a rectangular coil wound round a soft iron core that is suspended in the field of a permanent magnet  The signal being measured is applied to the coil and this produces a radial magnetic field  Interaction between this induced field and the field produced by the permanent magnet causes a torque, which results in rotation of the coil  The amount of rotation of the coil is measured by attaching a pointer to it that moves past a graduated scale ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 3  The theoretical torque produced is given by:  Where B is the flux density of the radial field o I is the current flowing in the coil o h is the height of the coil o w is the width of the coil o N is the number of turns in the coil  If the iron core is cylindrical and the air gap between the coil and pole faces of the permanent magnet is uniform, then the flux density B is constant o Equation can be rewritten as:  This implies that the torque is proportional to the coil current and the instrument scale is linear  Basic instrument operates only at low current levels of one milliamp or so o Suitable for measurement up to 2 volts  Measurement range can be increased by placing resistance in series with coil such that only a known proportion of applied voltage is measured by coil o Such resistance is called shunting resistor MOVING IRON METERS  As well as measuring DC signals, the moving-iron meter can also measure AC signals at frequencies up to 125 Hz  Cheapest form of meter available  The signal to be measured is applied to a stationary coil  The associated field produced is often amplified by the presence of an iron structure associated with the fixed coil  The moving element in the instrument consists of an iron vane that is suspended within the field of the fixed coil  When the fixed coil is excited, the iron vane turns in a direction that increases the flux through it  The majority of moving-iron instruments are either of the attraction type or of the repulsion type o A few instruments belong to a third combination type  Attraction and Repulsion types are shown in the figure below: ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 4 Fig 1.2: Moving Iron Meters: a) Attraction Type b) Repulsion Type  For an excitation current I, the torque produced that causes the vane to turn is:  Where o M is the mutual inductance o is the angular deflection  Rotation is opposed by a spring that produces a backwards torque given by:  At equilibrium, , and is therefore given by:  The instrument thus has a square-law response where the deflection is proportional to the square of the signal being measured o The output reading is a root-mean-squared (r.m.s.) quantity  The instrument can typically measure voltages in the range of 0 to 30 volts  It can be modified to measure higher voltages by placing a resistance in series with it, as in the case of moving coil meters  A series resistance is particularly beneficial in AC signal measurements o It compensates for the effect of coil inductance by reducing the total resistance/inductance ratio, and hence measurement accuracy is improved ELECTRODYNAMIC METERS (DYNAMOMETER TYPE)  Electrodynamic meters (or dynamometers) can measure both DC signals and AC signals up to a frequency of 2 kHz  The figure of a dynamometer is shown below: ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 5 Fig 1.3: Electrodynamic Meter  The instrument has a moving circular coil that is mounted in the magnetic field produced by two separately wound, series-connected, circular stator coils  The torque is dependent upon the mutual inductance between the coils and is given by:  Where o and are the currents flowing in the fixed and moving coils o M is the mutual inductance o represents the angular displacement between the coils  When used as an ammeter, the measured current is applied to both coils o The torque is thus proportional to  If the measured current is AC, the meter is unable to follow the alternating torque values and it displays instead the mean value of  By suitable drawing of the scale, the position of the pointer shows the squared root of this value, i.e. the r.m.s. current  Electrodynamic meters are typically expensive o Have the advantage of being more accurate than moving-coil and moving-iron instruments  Voltage, Current and Power can all be measured if the fixed and moving coils are connected appropriately  When used for voltage measurement, the instrument can typically measure voltages in the range of 0 to 30 volts  It can be modified to measure higher voltages by placing a resistance in series with it, as in the case of moving-coil and moving-iron meters  A series resistance is particularly beneficial in AC signal measurements as it compensates for the effect of coil inductance o Reduces the total resistance/inductance ratio, and hence measurement accuracy is improved ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 6 CLAMP ON METERS Fig 1.4: Clamp On meter  These are used for measuring circuit currents and voltages in a non-invasive manner that avoids having to break the circuit being measured  The meter clamps onto a current – carrying conductor, and the output reading is obtained by transformer action  From the figure above, it can be seen that the clamp – on jaws of the instrument act as a transformer core and the current-carrying conductor acts as a primary winding  Current induced in the secondary winding is rectified and applied to a moving- coil meter  Although it is a very convenient instrument to use the clamp-on meter has low sensitivity and the minimum current measurable o Usually about 1 amp ANALOGUE MULTIMETERS  The analogue multimeter is a multi-function instrument that can measure current and resistance as well as DC and AC voltage signals  The instrument consists of a moving-coil meter with a switchable bridge rectifier to allow it to measure AC signals  This is shown in the figure below: Fig 1.5: Analogue Multimeter ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 7  A set of rotary switches allows the selection of various series and shunt resistors o Makes the instrument capable of measuring both voltage and current over a number of ranges  An internal power source is also provided to allow it to measure resistances as well  This instrument is very useful for giving an indication of voltage levels o Compromises in its design that enable it to measure so many different quantities necessarily mean that its accuracy is not as good MEASUREMENT OF HIGH FREQUENCY SIGNALS USING ANALOGUE METERS  Major limitation in using analogue meters for AC voltage measurement is that the maximum frequency measurable directly is low o 2 kHz for the dynamometer voltmeter o 100 Hz for the moving-iron instrument  A partial solution to this limitation is to rectify the voltage signal and then apply it to a moving-coil meter, as shown in figure below: Fig 1.6: Measurement of High – Frequency voltage signals using rectifier circuit  This extends the upper measurable frequency limit to 20 kHz  The inclusion of the bridge rectifier makes the measurement system particularly sensitive to environmental temperature changes  Also, non – linearities significantly affect measurement accuracy for voltages that are small relative to the full-scale value THERMOCOUPLE METERS  The principle of operation of the thermocouple meter is shown in figure below: ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 8 Fig 1.7: Thermocouple meter  The measured AC voltage signal is applied to a small element  This heats up and the resulting temperature rise is measured by a thermocouple  The DC voltage generated in the thermocouple is applied to a moving-coil meter  The output meter reading is an r.m.s. quantity that varies in a non-linear fashion with the magnitude of the measured voltage  Very high-frequency voltage signals up to 50MHz can be measured by this method CURRENT TRANSFORMER  Current transformers provide an alternative method of measuring high- magnitude currents that avoids the difficulty of designing a suitable shunt  Different versions of these exist for transforming both DC and AC currents  A DC current transformer is shown in the figure below Fig 1.8: A DC Current Transformer  The central DC conductor in the instrument is threaded through two magnetic cores that carry two high impedance windings connected in series opposition  It can be shown that the current flowing in the windings when excited with an AC voltage is proportional to the DC current in the central conductor o This output current is commonly rectified and then measured by a moving-coil instrument ELECTRICAL AND ELECTRONIC MEASUREMENTS NOTES Ashesh B Vignesh Page 9  An AC current transformer typically has a primary winding consisting of only a few copper turns wound on a rectangular or ring-shaped core  The secondary winding on the other hand would normally have several hundred turns according to the current step-down ratio required  The output of the secondary winding is measured by any suitable current- measuring instrument  The design of current transformers is substantially different from that of voltage transformers  The rigidity of its mechanical construction has to be sufficient to withstand the large forces arising from short-circuit currents o Special attention has to be paid to the insulation between its windings for similar reasons  A low-loss core material is used and flux densities are kept as small as possible to reduce losses  In the case of very high currents, the primary winding often consists of a single copper bar that behaves as a single turn winding o The clamp-on meter, is a good example of this POTENTIAL/VOLTAGE TRANSFORMER
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