EMI UNIT1.2 Electrostatic instruments Module

Electrostatic instruments electrostatic instruments. Extension of range of instruments, Torque equation
Types of Electrostatic instruments
Errors present in Electrostatic instruments
Advantahes and disadvantages of Electrostatic instruments

1. MATRUSRI ENGINEERING COLLEGE DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING TOPIC NAME: Electrostatic instrument FACULTY NAME: Dr.N.KALPANA MATRUSRI ENGINEERING COLLEGE 11/9/2024 Dr.N.Kalpana 1

2. MATRUSRI ENGINEERING COLLEGE Contents: Electrostatic instruments Construction Principle of operation Torque equation Types of Electrostatic Voltmeters Course Outcomes: At end of this course the student will able to CO1. Illustrate the types measurements and meters construction and working principles . MODULE-2

3. MATRUSRI ENGINEERING COLLEGE Electrostatic instrument Definition: The instrument whose working depends on the principle of attraction or repulsion of electrodes that carry electrical charges such type of instrument is known as the electrostatic instrument. In other words, the instrument which uses the static electric field for producing the deflecting torque is known as the electrostatic instrument. The electrostatic instrument is used for measuring the high and low voltage and also the power of the given circuit . the electrostatic type instrument use static electrical field to produce the deflecting torque. These types of instrument are generally used for the measurement of high voltages but in some cases they can be used in measuring the lower voltages and powers of a given circuit. 11/9/2024 Dr.N.Kalpana 3

4. MATRUSRI ENGINEERING COLLEGE The electrostatic instrument works on the principle of mechanical interaction of the electrodes that consists the opposite electrical charge. The quantity which is measured by the electrostatic instrument is converted into either AC or DC voltage. There are two ways of constructions of electrostatic instruments; When one of the plates is fixed and other plate is free to move, plates are oppositely charged in order to have attractive force between them. Now due this attractive force movable plate will move towards the stationary or fixed plate till the moving plate stored maximum electrostatic energy. In other arrangement there may be force of attraction or repulsion or both, due to some rotary of plate. Construction of Electrostatic Type Instruments 11/9/2024 Dr.N.Kalpana 4

5. MATRUSRI ENGINEERING COLLEGE Electrostatic Instruments Basically electrostatic instruments are all voltmeters, practically such instruments may be used tor measurement of current and power but both the types of measurements require measurement of voltage across a known impedance. The main advantage of such instruments is the measurement of high voltages in both a.c and d.c. circuits without any errors due to eddy current losses and hysteresis . Principle of Operation The operation of all the electrostatic instruments is based on the principle that there exists a force between the two plates with opposite charges. This force can be obtained using the principle that the mechanical work done is equal to the stored energy if there is a relative motion of plates , Consider two plates A and B where plate A İs fixed while B is movable. Two plates are oppositely charged and plate B is restrained by a spring connected to fixed point Let the force of attraction between the two plates be F newton. Let the capacitance between the two plates be C farad. The energy stored E is the given by, 11/9/2024 Dr.N.Kalpana 5

6. MATRUSRI ENGINEERING COLLEGE when applied voltage increases by dV , the current flowing through capacitance also changes and it is given by, (2) (3) Also due to change in applied voltage by value dV , the capacitance increases by dC Because plate B moves towards a fixed plate A which decreases the distance of Separation between two plates increasing net capacitance. Thus the net energy stored is given by,     The input energy is given by   11/9/2024 Dr.N.Kalpana 6

7. MATRUSRI ENGINEERING COLLEGE The change in stored energy is given by, (4) = +CV dV + + Neglecting higher order terms of small quantities such as dC and dV , we can write = (5)   11/9/2024 Dr.N.Kalpana 7

8. From the principal of the conservation of energy, we can write Input energy= Increment in stored energy + Mechanical work done.   From the above expression it is clear that the force of attraction is directly proportional to the square of the applied voltage V. The above theory can be extended to the rotational motion, with the angullar deflection θ in place of the linear displacement X. If the meter uses the spring control with torsional spring constant K then Tc =K θ   MATRUSRI ENGINEERING COLLEGE 11/9/2024 Dr.N.Kalpana 8

9. But Td=TC for steady state position. So from the above equation it is clear that deflection angle θ is proportional to the squareof the voltage to be measured V 2 so this instrument can be used for both a.c and d.c measurements It shows square law response hence scale is non uniform which is compressed at the lower end   MATRUSRI ENGINEERING COLLEGE 11/9/2024 Dr.N.Kalpana 9

10. Types of Electrostatic Voltmeters MATRUSRI ENGINEERING COLLEGE the two types of electrostatic voltmeter , Quadrant type electrostatic voltmeter (which is used to measure voltages upto 10 kV to 20 kV .) ii) Attracted disc type electrostatic voltmeter (which is used to measure voltages above 20 kV .) Quadrant Type Electrostatic Voltmeter The instrument consists of four fixed metal double quadrants arranged such that there is a small air gap between the quadrants and the total assembly forms shallow circular box. Inside this box a double sectored needle is suspended by means of a phosphor bronze thread. The needle is suspended such that it is placed equidistant from above and below quadrant plates as shown in the Fig. 1.39. 11/9/2024 Dr.N.Kalpana 10

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12. MATRUSRI ENGINEERING COLLEGE As shown in the above Fig. 1-39 the fixed quadrants are connected together. The voltage to be measured either a.c . or d,c . is connected between the fixed quadrants and the moving needle. This needle rotates due to the electrostatic force set up due to the charge accumulation on the quadrant plates. Then the suspension exerts a controlling torque and the needle settles at the position where both the torques, controlling and deflection, are equal. There are two types of the electrical connections in the quadrant electrometer, 1) Heterostatic connection 2) Idiostatic connection 11/9/2024 Dr.N.Kalpana 12

13. MATRUSRI ENGINEERING COLLEGE Heterostatic Connection In this type of connection, a high voltage battery is used to charge the needle to a voltage considerably higher than the voltage to be measured. The connection diagram is as shown in the Fig. 1.40. 11/9/2024 Dr.N.Kalpana 13

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15. MATRUSRI ENGINEERING COLLEGE Theory of Heterostatic Connection 11/9/2024 Dr.N.Kalpana 15

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19. MATRUSRI ENGINEERING COLLEGE Thus scale is uniform 11/9/2024 Dr.N.Kalpana 19

20. MATRUSRI ENGINEERING COLLEGE Idiostatic Connection This connection is generally used in commercial instruments. In this type of connection, needle is connected to any one of the pairs of quadrant as shown in the Fig ., directly without external voltage, 11/9/2024 Dr.N.Kalpana 20

21. MATRUSRI ENGINEERING COLLEGE The moving needle is negatively charged, the left hand quadrant is negatively charged and the right hand quadrant is positively charged The force of attraction on needle due to top and bottom parts of right hand quadrant cancel each other, So there is no motion of needle due to right hand quadrant Similarly the force of repulsion on needle due to top and bottom parts of left hand quadrant also cancel each other. Thus the right hand positively charged quadrant attract the part of the needle near to left hand quadrant while the left hand negatively charged quadrant repels the part of the needle to right hand quadrant. This rotates the needle and hence the pointer. 11/9/2024 Dr.N.Kalpana 21

22. MATRUSRI ENGINEERING COLLEGE Theory of Idiostatic Connections For Idiostatic connection, the external voltage applied to the needle V 1 = 0 V. And the potential of quadrant is nothing but the voltage Vı which İs applied to the needle directly. Key Point  : As torque is proportional to square of the applied voltage, the scale is nonuniform for idiostatic connection.   Note that as deflecting torque is proportional to the square of the voltage to be measured, the idiostatic connection is used for a.c . measurements. A=L, B=M 11/9/2024 Dr.N.Kalpana 22

23. MATRUSRI ENGINEERING COLLEGE Kelvin Multicellular Voltmeter It is basically quadrant type electrostatic instrument with large number of needels . The moving mechanism is suspended with the help of Phospor -Bronze suspension. To protect the suspension against the vibration , the coach spring is used.   For the zero adjustment, warm wheel actuated torsion head is used. The pointer and scale are of edgewise type along with oil Immersed damping vane.   It is very essential to use a safety collar above the pointer to avoid movements of the system caused due to short circuit of quadrants. Two guard plates are fitted inside the case of the Instrument.   11/9/2024 Dr.N.Kalpana 23

24. MATRUSRI ENGINEERING COLLEGE These are electrically connected to the moving mechanism and metal case of the instrument. In such instruments, number of cells used increases as the voltage to be measured decreases.   For example to measure voltage of the order of 150 V, 10 to 15 cells a re required while for the measurement of 3000 V one cell may be sufficient. The deflection torque for n cells is given by 11/9/2024 Dr.N.Kalpana 24

25. MATRUSRI ENGINEERING COLLEGE An electrostatic voltmeter is controlled by a spring with a constant 4X10 -6 Nm/rad and has a full scale deflection of 90 when voltage of 1500V is applied to it. The capacitance at zero voltage is 10pF. Find its capacitance when the pointer indicates 1500V 11/9/2024 Dr.N.Kalpana 25

26. MATRUSRI ENGINEERING COLLEGE Attracted Disc Electrostatic Voltmeter The attracted disc type instruments are generally used for the measurement of voltages above 20 kV. The system consists of two plates such that one plate can move freely while other is fixed, Both the plates are perfectly insulated from each other The voltage to be measured is applied across the plates as a supply voltage as shown in the Fig .. Due to the supply voltage, electrostatic field gets produced which develops a force of attraction between the two plates. Due to the force of attraction, the movable plate gets deflected, In this mechanism the controlling torque is provided by a spring. 11/9/2024 Dr.N.Kalpana 26

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28. MATRUSRI ENGINEERING COLLEGE Theory of Attracted Disc Type Voltmeter As we have seen already that tho force of attraction F between the two parallel plates with potential difference V behveen them is given by, 11/9/2024 Dr.N.Kalpana 28

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30. MATRUSRI ENGINEERING COLLEGE Kelvin Absolute Electrometer This instrument is of attracted disc type electrostatic voltmeter. The basic structure of this Instrument is as shown In the Fig . . The moving disc is suspended exactly above the centre of fixed disc from a micrometer hcad with the help of a spring The moving disc is surrounded by a ring called guard ring, keeping some air gap between moving disc and ring, This guard ring is very useful in reducing ringing effects. it is connected electrically with the moving disc With the help of a device consisting lenses and finely pointed rod it is possible to determine zero setting of a disc consisting a fine cross-hair. in actual practice, the potential to be measured is supplied to the two disc Due to thc force of attraction, the moving disc is attracted downwards, with the help ofmicrometer head, the moving plate is brought back to zero position setting again. This movement required is observed accurately. For accurate measurement, it is very important to calibrate the instrument properly. During the calibration, first instrument is short circuited. Then the moving plate is set to its zero position. Then known weights are added to the disc. Then the calibration is done by observing the movements required to bring the moving disc back to the original zero position. Thus instrument measures the force of attraction produced by the potential difference between the two discs. Then the potential difference can be expressed interms of this force. 11/9/2024 Dr.N.Kalpana 30

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32. MATRUSRI ENGINEERING COLLEGE Both the AC and DC voltage can be measured by using the electrostatic instrument. The electrostatic type instrument consumes very less power. The high value of voltage can be measured by using the instrument. In the rotary type electrostatic instrument, in spite of linear displacement, the angular displacement occurs between the fixed and the moving plates. The instrument has less Waveform and frequency error. No error occurs because of the stray magnetic field. The instrument is designed for large voltage . Costly resistance wires are not used. The operating current is very small, hence does not effect the performance of other circuits connected to the same supply. Advantages of Electrostatic Instrument 11/9/2024 Dr.N.Kalpana 32

33. MATRUSRI ENGINEERING COLLEGE Disadvantages of Electrostatic type instrument The non-uniform scale is used in the instrument. The force of very small magnitude involves in the instrument. The instrument is quite costly as compared to the other instrument. The size of the instrument is also very large . They are not suitable for low voltages. 11/9/2024 Dr.N.Kalpana 33

34. MATRUSRI ENGINEERING COLLEGE Extension of Range of Electrostatic Instruments The range of various instruments can be extended using multipliers. Similarly the range of electrostatic instruments is also extended using multipliers. The multipliers used f or electrostatic instruments are of two types , 1 ). Resistance potential divider 2 ). Capacitance multipliers 11/9/2024 Dr.N.Kalpana 34

35. MATRUSRI ENGINEERING COLLEGE v = The voltage across an eleetro static voltmeter. C = The caparitance of an eleetro static voltmeter. r = The resistance whoso voltage drop is applied to an eleetrostatic voltmeter The resistance r and capacitance C forms a parallel circuit and the equivalent impedance is, Resistance Potential Divider The use of resistance potential divider for extending the range of an electrostatic instrument is shown in the Fig. 11/9/2024 Dr.N.Kalpana 35

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