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