UNIT-1_Measurements, Introduction to Measuring Instruments

PMMC
MI
ELETROSTATIC VOLTMETERS
VOLTMETERS
AMMETERS

1. Measuring Instruments

2. Definition of Moving Iron Instrument The instrument which uses the soft iron core for measuring the current or voltage is known as the moving iron instrument. It works on the principle that the iron attracts towards the magnet. The magnetic field induces because of the electromagnet and the iron piece is placed between this field. The force of attraction acting on the soft iron core and the magnitude of the force depends on the strength of the magnetic field . There are two types of moving iron (MI) instruments. They are, attraction type and repulsion type moving iron instruments.

3. Attraction Type Moving Iron (MI) Instrument - Construction, Working & Torque Equation The working principle of attraction type moving iron instrument is based on magnetic attraction, which attracts an iron piece when placed near a magnet field. Here, the magnet field will be produced by an electromagnet.

4. Construction of Attraction Type Moving Iron Instrument: It consists of a fixed coil that is flat with a narrow opening in it. A moving iron that is made of soft iron is mounted on a spindle. The coils are wound with a number of turns that depend upon the range of the instrument. The pointer is mounted on a spindle which consists of a graduated scale for showing the deflection. The construction of attraction type moving iron is shown below.

5. The controlling torque is provided by the springs or if the instrument is vertically operated gravity control can also be employed. This instrument uses air friction damping to damp out oscillations which consist of a movable piston made of aluminum placed in an air chamber. Since the operating magnetic field produced by the coil winding is not much strong, the eddy current damping which uses permanent magnets can distort the main field. Thus eddy current damping cannot be used and fluid friction damping is not much preferred. The moving iron is made of sheet metal for obtaining a uniform scale.

6. Working of Attraction Type Moving Iron Instrument: Whenever coil winding is connected across the supply to be measured, it setups a magnetic field. The intensity of the magnetic field is higher inside the coil compared to the intensity of the outside, and hence low reluctance exists inside the coil. As the moving iron tries to occupy the low reluctance position, it is moved and gets attracted to the fixed coil. As the iron piece moves, the pointer also moves to show the deflection. The instrument attains the equilibrium position when controlling torque balances the deflecting torque.

7. Torque Equation of Moving Iron Instruments: Let , T d  = Deflection torque in N-m θ = Deflection in radians L = Inductance in Henry I = Initial current dI = Change in initial current dL = Change in inductance dθ = Change in deflection

8. Advantages of Attraction Type MI Instruments: The instruments can be used for measuring both dc and ac quantities. Simple in construction. Since the winding coil is kept stationary, these instruments are robust and reliable. As attraction type instruments have lower inductance, the measurement can be done over a wide range of frequencies. A shunt can be connected in parallel with the basic instrument in order to measure heavy currents.

9. Disadvantages of Attraction Type MI Instruments: These are not suitable for economical production in manufacturing. The power consumption is higher for a low voltage range. Accuracy in the readings cannot be obtained due to the non-uniform scale.

10. Applications of Attraction Type MI Instruments: Heavy current moving iron instruments. Moving iron voltmeters. Moving iron power factor meters. Moving iron synchro scope.

11. Repulsion Type Moving Iron (MI) Instrument - Construction, Working & Advantages Similar to attraction force there will be repulsion force when same magnetic poles are placed near each other. Based upon this repulsion force between two like poles the repulsion type moving iron instruments were developed. Repulsion type moving iron instruments are used both for ac and dc measurements. In these instruments, when current flows through the coil, the two vanes i.e., fixed vane and movable vane are magnetized and same polarities are induced in it which results in a force of repulsion between them.

12. Construction of Repulsion Type Moving Iron Instrument: Basically, a repulsion type instrument consists of a fixed field coil and two vanes present inside the coil, out of which one vane is fixed and the other vane is movable. The fixed vane is attached to the coil whereas the movable vane is mounted on the spindle of the instrument. The spindle carries the pointer which moves on a graduated scale. Depending on the design of construction, there are two types of repulsion type MI instruments. They are,

13. Radial vane type, and Co-axial or concentric vane type

14. Radial Vane Repulsion Type Instrument:

15. It consists of two iron strips (vanes) are placed radially, in which one is fixed and the other is movable. In this type of instrument, the deflection torque is directly proportional to the actual current in the coil, thus making the scale uniform and readings can be obtained directly. These are the most sensitive type of instruments.

16. Co-axial Vane Repulsion Type Instrument:

17. In this, the two vanes are co-axially placed inside the coil. In which one vane is fixed to the coil frame that remains stationary, while the other is movable which rotates at the central axis inside the stationary vane. But the deflecting torque on the pointer is proportional to the square of the actual current in the coil. Hence the scale cannot be uniform due to concentric vanes. Compared to radial type these instruments are less sensitive.

18. Working of Repulsion Type Moving Iron Instrument: Initially, the current does not pass through the coil. Hence, the two vanes will touch each other and the pointer does not deflect (i.e., it will be at zero position). Whenever current flows through the coil, a magnetic field is set up in it and two vanes are magnetized with the same polarities i.e., north poles are produced at one end in both the vanes and south poles are produced on their other ends. Due to this, a repulsive force exists between two vanes and the movable vane tries to move away from the fixed vane as shown below.

19. Hence, the movable vane moves because of the repulsive force, and the pointer which is mounted on a spindle show deflection. The pointer stops deflecting when the controlling torque is equal to the deflecting torque. The amount of repulsion force depends upon the strength of the magnetization field produced by the coil. The magnetic field produced will depend upon the current supplied. In these instruments, controlling torque is provided by the spiral springs and damping torque is provided by air friction.

20. Advantages of Repulsion Type Moving Iron Instruments: The instruments can measure both ac and dc currents and voltages. In concentric or coaxial type, the deflection up to 250° can be produced. As the coil in the repulsion time instrument occupies more volume, it can allow overloaded currents for a longer duration of time, and hence it provides better cooling which eliminates the problem of overheating. These are suitable for economical production in manufacturing.

21. These are the most widely used instruments. The scale of these instruments is uniform (for radial vane type). The measuring range of the instrument can be extended. Due to stationary coil, the torque to weight ratio will be high thereby decreasing the frictional loss.

22. Disadvantages of Repulsion Type Moving Iron Instruments: Linearization of the scale is affected in coaxial vane type because of the shape of concentric vanes. Errors are caused because of hysteresis loss and also due to stray magnetic fields. Draws more power.

23. Errors in Moving Coil (PMMC) & Moving Iron Instruments Ageing of permanent magnets, Ageing of springs, and Temperature variations.

24. Errors due to Ageing of Permanent Magnets : Due to the use of permanent magnets, the ageing of magnets causes its weakening, thereby resulting in a reduced magnetic field. The decrease in the magnetic field reduces the deflection torque that has to be produced. Therefore , the pointer deflection decreases and the reading obtained will be lesser than the actual value.

25. Errors due to Ageing of Springs : Similar to the ageing of magnets, the ageing of springs causes an error in the readings obtained. Due to aging, the spring losses its ability to tolerate considerable bending or twisting. Due to which the control torque produced on the pointer decreases and the pointer deflects more than actual. Here we can see that the effects due to the ageing of magnets and springs are opposite to each other. Because the error introduced due to magnets ageing causes the reading obtained to be lesser than the actual value, whereas ageing of springs causes the reading obtained to be more than the actual value.

26. Errors due to Temperature Variations : The variations in surrounding temperature can also introduce errors in moving coil instruments. There are two effects due to temperature variation, it affects the stiffness of the spring and flux density in the air gap. But these effects are in such a way that they will cancel each other resulting in not much error in the readings obtained. Due to variations in temperature, the resistance of the conductor used for moving coil varies, thereby changing the amount of current flowing through the coil that effects the deflection of pointer.

27. Compensation of the Errors in PMMC Instruments : The permanent magnets are treated by heat and vibrations resulting in loss of some magnetic properties but the remaining magnetic properties are strongly held. So, the ageing effect will not appear in the future. The effects due to ageing of springs can be reduced by careful use of materials and extending the life span during manufacturing. By using a series resistance made of materials having a negligible coefficient of temperature (like manganin) with the moving coil voltmeter reduces the effects due to temperature changes. The resistance of the series resistor or swamping resistor should be relatively higher than that of the moving coil.

28. Errors in Moving Iron Instruments : Hysteresis error Temperature error Errors due to stray magnetic fields Errors due to change in frequency

29. Hysteresis Error : The main source of this error is the tendency of the iron parts in the instrument to store magnetism for short periods. Due to this error, for decaying values of currents, the flux appears to be more than its corresponding value. As the deflecting torque is directly proportional to the flux, the meter reads high. Methods adopted to reduce this error are, Reducing the size of the iron part in the instrument. Reducing the value of operating flux density. Replacing the active iron part with nickel-iron alloy as it has a narrow hysteresis loop. With this method, the hysteresis error can be reduced to a large extent.

30. Temperature Error : The main source of this error is the temperature coefficient of the springs (for spring controlled instruments only), moving coil, and series resistance. Due to an increase in temperature, the overall resistance of the meter increases and so the current through the meter gets reduced. Hence. the meter reads low. In order to reduce this error, the series resistance being employed is made with the material having a negligible temperature coefficient (like manganin, constantan, etc ). The value of series resistance should be comparatively much higher than that of moving coil and springs.

31. Errors due to Stray Magnetic Fields : The main source of this error is the external magnetic fields which are stronger than the operating field. If the stray magnetic field aids the operating field, then the meter reads high and when it opposes, the meter reads low. To reduce this error, magnetic shielding is provided by an iron case covering the instruments working parts.

32. Errors due to Change in Frequency : The main sources of this error are,Change in Reactance of the Meter Coil with Frequency - It is due to the dependence of the inductive reactance on the frequency of the supply i.e., the inductive reactance of the coil increases with an increase in frequency ( X L  = 2π fL ). Hence, overall impedance also increases and so the current through the meter reduces. Hence, the deflection will be less. In order to neutralize this error in the case of MI voltmeters, a properly designed capacitor is connected across the series resistor (i.e., multiplier).

33. Variations in the Eddy Currents with Frequency - The main source of this error is inducing eddy currents in the iron parts of instruments. The flux produced due to the flow of eddy currents opposes the main field (operating flux), according to Lenz's law. Hence, the meter reads low. The only remedy to minimize this error is to reduce the iron parts in the vicinity of operating flux.

34. Electrostatic Voltmeters An electrostatic instrument is basically a voltmeter that works on the principle of static electric field. They are used for voltage measurement (especially for high voltages) but can also be used for measuring current and power with the additional arrangement. The electrostatic voltmeters are of two types, Quadrant type electrostatic voltmeter (used up to 20kV) Attracted disc type electrostatic voltmeter (used up to 500kV)

35. Attracted Disc Type Electrostatic Voltmeter - Kelvin Absolute Electrometer An attracted disc type electrostatic instrument is basically known as a portable electrostatic instrument. This device contains two semicircular plates one being stationary and the other rotating. Both the plates are electrically insulated from each other . The unknown voltage to be measured is applied across the plates, which results in the development of an electrostatic field between the plates. The electrostatic field developed is attractive in nature. The below shows the portable electrostatic instrument.

36. Due to the electrostatic force of attraction, the moving plate gets attracted towards the stationary plate and gets deflected. The control spring fixed to the rotating plate provides the required controlling torque and the air friction provides the necessary damping.

37. Quadrant Type Electrostatic Voltmeter : The quadrant electrometer consists of four metallic quadrants and a double sector shaped moving vane or needle as shown in the below figure . Quadrants 1 and 2 lie above the needle and quadrants 3 and 4 below the needle. The needle is suspended in the air gap between the two sets of quadrants. Silver quartz or phosphor bronze thread is used for the suspension.

38. The needle carries a mirror that reflects the light focused on it on the scale. Sometimes, instead of this suspension, a spindle is placed on which the needle is mounted. Also, a spring is attached to the spindle, which provides the control torque. A resistance of high value should always be connected in series with these types of instruments in order to prevent a short circuit when the supply is given. When a potential V is applied across the fixed quadrants and moving vane, a static electric field is set up between them and a force of attraction exerts on the needle. Hence, the needle starts rotating due to the deflecting torque T d  and shows the deflection.

39. There are two types of connections in a quadrant electrometer. They are , Heterostatic connection, and Idiostatic connection.

40. Heterostatic Connection : In this type of connection voltage to be measured is applied across the fixed quadrants. A high tension (HT) battery is used to charge the moving needle more positively than the fixed quadrants (i.e., voltage to be measured). The heterostatic connection is shown below.

41. Here the fixed quadrants are charged in such a way that, the diagonal pair will be charged oppositely. The needle will be charged with positive polarity with help of an HT battery. Here the deflecting torque is produced due to the attraction force between the left quadrant and right moving sector and the repulsion force between the right quadrant and left moving sector.

42. Idiostatic Connection : In this type of connection, there is no additional external voltage. The moving needle is connected to any pair of quadrants directly as shown below.

43. Here, the moving needle is charged with negative polarity. Due to the negative polarity of the needle, there exists a force of attraction on the needle by the two positively charged quadrants i.e., on the right side, and these two forces will cancel each other. Similarly, there will be a repulsive force on the needle by the two quadrants on the left side, and these two forces will cancel each other. But, due to attraction force on the part of a needle lying on the left side, and a repulsive force on the part of a needle lying on the right side causes the production of deflecting torque and moves the needle. This movement causes to move the pointer attached to it. If the instrument is spring controlled and used for measuring low voltages, heterostatic connection is most suitable because, for idiostatic , the deflection obtained is proportional to the square of applied voltage (i.e., voltage to be measured) and for low voltages, the deflection will be small. Whereas in the case of heterostatic , deflection is proportional to the product of voltage applied and dc HT voltage, thus the voltage to the needle is very high and the deflection obtained will be considerably large even for low voltages.