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Meter Is Expressed by a Number Which Is Compared with an Appropriate Scale for That Same Parameter

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Meter Is Expressed by a Number Which Is Compared with an Appropriate Scale for That Same Parameter Juliusz B. Gajewski Professor of Electrical Engineering 1 FACULTY OF MECHANICAL AND ELECTRIC POWER ENGINEERING Process Engineering and Equipment, Electrostatics and Tribology Research Group Wybrzeże S. Wyspiańskiego 27 50 - 370 Wrocław, POLAND Building A4 „Stara kotłownia”, Room 359 Tel.: +48 71 320 3201; Fax: +48 71 328 3218 E - mail: juliusz.b.gajewski@pwr. edu .pl Internet: www.itcmp.pwr.wroc.pl/elektra 2 Contents 1. Terms. Fundamental Definitions and Units. 2. Electrostatics. Electrostatic and Electric Fields. 3. Electrodynamics. DC Current. 4. Electromagnetism. Magnetic Field of DC Current. 5. Electric Circuit Elements. 6. Sinusoidal AC Voltage. 7. Complex Frequency Concept. 8. Electric Filters. 9. Electrical Measurements. 10. Three - Phase Circuits. 11. Electrical Signals. 12. Electric Switches. 3 Electrical Measurements 4 Electrical Measurements 5 Electrical Measurements Philosophy of Measurements „ Regardless of its character, information is usually encoded as a magnitude of a physical quantity, and measurement is necessary in order to determine it . In the measurement process the magnitude of the respective parameter is expressed by a number which is compared with an appropriate scale for that same parameter . Only two techniques for comparison are known . The first is an analog technique using a classical measuring instrument with a pointer . The position of the pointer is compared with a scale . The actual comparison is made by an observer . The second technique is based on an analog / digital (A/D) converter . The electric voltage, current or signal frequency is com - pared to a reference voltage, current or frequency by means of electronic digital tech - niques . The result of the comparison is a numerical value which is presented in a digital code . The great variety of physical parameters requires a great number of alternative techniques of comparison . If one is to measure such quantities, their magnitudes must be converted into magnitudes of a reference voltage, current or frequency, or into a magni - tude of the displacement of the pointer of an instrument . All these cases of conversion involve energy exchange . Therefore the magnitude of the respective physical quantity, the magnetic field, for instance, is converted into a certain kind of energy, a definite relationship existing between it and the basic parameter ” . *) *) Citation from „ Solid State Magnetic Sensors ” by Roumenin, C.S. , Elsevier 1994 6 Electrical Measurements Analog, Pointer Measuring Instruments Measurements of electrical quantities : voltage, current, power and work of current, etc . are performed with instruments in which the phenomena that follow or are a result of a current flow are used : t h e r m a l, c h e m i c a l and m a g n e t i c . The most significant phenomena from the practical point of view are the magnetic ones and forces that act in the magnetic field of current . In the steady - state conditions measurements are carried out with the use of p o i n t e r m e a s u r i n g i n s t r u m e n t s (meters) . In these meters a d r i v i n g t o r q u e acts on a moving organ ( part ) and causes a pointer to moves over the scale of a meter to show the value of the quantity measured . In the case of measurements of periodically varying currents and voltages the meter’s scale is cal ibrat ed in average or effective values . 7 Electrical Measurements Analog, Pointer Measuring Instruments The moving organ produces a restoring torque that balances the driving torque and to do so spiral springs are used . To set up a pointer in the zero position an eccentric cam is used . A counter - balance balances the moving organ in order that its centre of gravi - ty be on the axis of rotation . Under the driving torque the pointer deflects by an angle , at which the balance of the driving torque by the restoring torque . As a result of the moving part’s inertia the state of equilibrium can be achieved after some number of sways (oscillations) and not at once . To shorten the time of sways one uses a damper that pro - duces a damping torque within the movement of the moving organ . The air and eddy - current dampers are most frequently used . 8 Electrical Measurements Analog, Pointer Measuring Instruments In the air damper the movement is attenuated with the drag of air in a chamber in which a wing of the damper moves . The eddy - current dampers work is based on eddy currents induced in a plate moving in the field of a magnet . The fundamental part of each meter is its measuring structure, that is that part of a meter in which any electrical quantity is measured and is converted into the deflection of a pointer . With regard to the construction and principle of operation, the magnetic field - based instruments are as follows : m a g n e t o e l e c t r i c, e l e c t r o m a g n e t i c, e l e c t r o d y n a m i c, i n d u c t i o n . 9 Electrical Measurements Analog, Pointer Measuring Instruments Pointer, counterbalance and restoring spring Air damper and eddy - current damper 10 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters Types: permanent - magnet moving - coil instrument — moving coil and stationary permanent magnet moving - magnet instrument 1. Permanent magnet. 2. Coil at the aluminum frame. 3. Soft - iron armature core — core of the soft magnetic material. 4. Two spiral restoring springs. 5. Pole pieces. 11 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters M o v i n g - c o i l i n s t r u m e n t — A measuring instrument in which current or voltage is determined by the couple on a small coil pivoted between the poles of a magnet with curved poles, giving a radial magnetic field . When a current flows through the coil it turns against a return spring . If the angle through which it turns is , the current I is given by I k / BAN , where B is the magnetic flux density, A is the area of the coil, and N is its number of turns ; k is a constant depending on the strength of the return spring . The instrument is suitable for measuring DC but can be converted for AC by means of a rectifier network . It is usually made as a galvanometer and converted to an ammeter or voltmeter by means of a shunt or a multiplier . 12 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters Driving torque M n M d 2 rFN 2 rBIlN BANI k 1 I Restoring torque M z M r k 2 Pointer’s deflection angle k 1 I kI k 2 13 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters The direction of the torque generated in a moving - coil instrument is dependent on the instantaneous direction of the current through the coil , that is t he direction of the deflection depends on the polarity, so that unmodified indicators are usable only on DC and unidirectional pulsat - ing currents , and may have a centre zero if required . Thus an alternating current produce s no steady - state deflection and the pointer indicates zero . Moving - coil instruments are provided with an A C response by the use of a full - wave bridge rectifier . The bridge rectifier converts the AC signal into a unidirectional signal through the moving - coil instrument which then responds to the average DC current through it . Such instru - ments measure the mean absolute value of the waveform and are cali - brated to indicate the RMS value of the wave on the assumption that it is a sinusoid . 14 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters Magnetoelectric moving - coil instruments can be used as : g a l v a n o m e t e r s, v o l t m e t e r s, a m m e t e r s . Coils are made of thin wires and in general are to be used for the low currents (up to several dozen miliampers), and because of its low resistance the range of the volatges measured is also narrow . To extend the measuring range of the instruments special resistors are connected in parallel with ammeters — s h u n t s — or in series with voltmeters — m u l t i p l i e r s . 15 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters Shunt A R a I a I I a I R s I a R a R a I a I I a R s R s R a I I a n 1 16 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters Multiplier U v U U v I V R R v m U U U v U v R v I and U U v R m I R m R v n 1 R v U 17 Electrical Measurements Analog, Pointer Measuring Instruments Magnetoelectric Meters 18 Electrical Measurements Analog, Pointer Measuring Instruments Electromagnetic Meters Types: one - core (solenoid - or attraction - type) meter two - core (repulsion - type) meter 1. Core (vane, disc) of the soft magnetic material. 2. Fixed foil. 3. Movable vane. 4. Unmovable fixed coil. 5. Fixed (unmovable) vane. 19 Electrical Measurements Analog, Pointer Measuring Instruments Electromagnetic Meters 1 W LI 2 2 d W I 2 d L M M n d d 2 d d L kI 2 d d L const d 20 Electrical Measurements Analog, Pointer Measuring Instruments Electromagnetic Meters In the one - core, attraction - type instruments current passes through a coil of wire (solenoid) wound on a hollow tube of some non - magnetic material .
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