1. eCourseware@AIKTC

2. ELECTRICAL AND ELECTRONINCS MEASUREMENTS

3.  The advancement of Science & Technology is dependent upon a parallel progress in measurement techniques, the reason for this is obvious.  As Science & Technology move ahead, new phenomena and relationships are discovered and these advances make a new types of measurements imperative(crucial part or vital importance)  New discoveries are not of any practical utility unless the results are backed by actual measurements.  The measurements, no doubt, confirm the validity of a hypothesis but also add to its understanding.  This results in an unending chain which leads to new discoveries that require more, new and sophisticated measurement techniques. Hence modern Science & Technology are associated with sophisticated methods of measurement.  There are two major functions of all branches of engineering  i) Design of equipment and process  ii) Proper operation and maintenance of equipment  Both these require measurements. This is because proper and economical design, operation and maintenance require a feedback of information.  Measurement play a vital or significant role in achieving goals and objectives of Engineering because of the feedback information

4. MODULE 1 PRINCIPLES OF ANALOG INSTRUMENTS Errors in Measurement Difference between Indicating & Integrating Instruments Moving coil & Moving iron Ammeter & Voltmeter Instrument transformer Dynamometer type wattmeter Power factor meters DC Permanent magnet moving coil type galvanometer Ballistic Galvanometer AC Vibration Galvanometer

5. 1 Errors in Measurements  Measurements done in laboratory or at some other place always involve error. No measurement is free from error.  If the precision of the equipment is adequate, no matter what its accuracy is, a discrepancy will always be observed between two measured results.  Since errors are must in any measurements, it is imperative to interpret the results of a quantitave measurement in an intelligent manner.  An understanding and thorough evaluation of errors is essential.  No measurement that can be made with perfect accuracy but it is important to find out what accuracy actually is and how different errors have entered into the measurement.  A study of error is a first step in finding ways to reduce them.

6. Errors may arise from different sources and are usually classified as under GROSS ERRORS Human made errors Mistake normally lies with the experimenter Great care should be taken One should anticipate and correct them RANDOM ERRORS Errors due to a multitude of small factors Fluctuation of one measurment to another Disturbances which are unaware SYSTEMATIC ERRORS 1)Instrumental errors 2)Environmental errors 3)Observational errors

7. 1.1 GROSS ERRORS  This class of errors mainly covers human mistakes in reading instruments, recording and calculating measurement results.  The responsibility of mistakes mainly lies with the experimenter. The experimenter may grossly misread the scale. For eg:  He may due to oversight, read the temp as 31.5'C while the actual reading may be 21.5'C.He may transpose the reading while recording it, he made read 25.8'C and record 28.5'C instead.  Some gross errors are easily detected while some are difficult if their is a vast differences.  Gross errors may be of any amount and therefore their mathematical analysis is impossible.  However they can be avoided by adapting two means 1) Great care should be taken in reading and recording the data. 2) Two, three or even more readings should be taken for the quantity under measurement. These readings should be taken preferably by different experimenters and reading should be taken at a different reading point so as to avoid re-reading. This is advisable so that no gross error is committed.

8. 1.2 SYSTEMATIC ERRORS These types of errors are divided into 3 categories Instrumental Errors i)Inherent shortcomings of instruments ii)Misuse of the instruments iii)Loading effects of instruments Observational Errors Errors due to Parallax (Parallax is a displacement or difference in the apparent position Of an object viewed along two different lines of sight) Environmental Errors i)Errors are due to conditions external to the Measuring device including the atmospheric Conditions such as temperature,pressure,humidity Dust,vibrations&external magnetic fields

9. 1.2a INSTRUMENTAL ERRORS i) Inherent shortcomings of instruments  These errors are inherent in instruments because of their mechanical structure due to construction, calibration or operation of the instrument or measuring devices which may cause the instrument to read too low or to high.  Errors may be caused because of friction, hysteresis or even gear backlash. These errors can be reduced to a great extent by using the following methods i) Procedure of measurement must be carefully planned. Substitution methods or calibration against standards may be used. ii) Correction factors should be applied after determining the instrumental errors, iii) The instrument may be re-calibrated

10. ii) Misuse of the Instruments  Errors caused in measurements are to the fault of the operator than that of the instrument.  A good instrument used in ignorant(unintelligent) way may give error results  Improper practice may not cause a permanent damage to the instrument but all the same they cause errors,  Certain ill practices like using the instrument contrary to manufacturer's instructions and specifications which in addition to producing errors cause permanent damage to the instrument as a result of overloading & overheating that may ultimately result in failure of the instrument and sometimes system itself

11. iii) Loading effects of Instruments One of the most common errors committed by the beginners, is the improper use of an instrument for measurement work. For eg: A well calibrated voltmeter may give a misleading voltage reading when connected across a high resistance circuit. a)Errors caused by loading effects can be avoided by using instruments intelligently b)In planning any measurement, the loading effect of instrument should be considered and proper corrections for these effects should be made c)Suitable instruments should be used d)Preferably those methods should be used which is results in negligible or no loading effects.

12. 1.2b ENVIROMENTAL ERRORS Errors due to conditions external to the measuring device including the area of surrounding the instrument. Effects of temperature pressure,humidity,dust,vibrations or external magnetic or electrostatic fields. The corrective measures employed to eliminate these undesirable effects are:  Arrangements should be made to keep the conditions as nearly & as constant as possible.  Using equipment which is immune to these effects.  Employing techniques which eliminate the effects of these disturbances.  In case it is suspected that external magnetic or electrostatic fields effect the readings of the instruments, magnetic or electrostatic shields may be provided.  Applying computed corrections

13. 1.2c OBSERVATIONAL ERRORS  Many sources of observational errors exist as an, example the pointer of voltmeter rests slightly above the surface of the scale. Thus an error on account of Parallax will be incurred unless the line of vision of the observer is exactly above the pointer.  Modern electrical instruments have digital display of output which completely eliminates the errors on account of human observational.

14. 1.3 RANDOM ERRORS Errors due to a multitude of small factors which change or fluctuate from one measurement to another. The factors influencing the measurement and disturbances which we are unaware are lumped together are called 'Random errors or Residual errors' For eg: A spring balance might show variations in measurement due to fluctuations in temperature, condition of loading & unloading.  Mechanical vibrations: When the instrument is used in vibrating place the parts of the instrument start vibrating giving faulty readings.  Backlash in the movement: This is the error due to time lag between the application of the parameter and the instrument actually showing reading. Even though some value of the parameter changes, there is no indication.  Hysteresis of the elastic members: Over the period of time the elastic members tend to loose some elasticity leading to errors in the indicated value of the instrument.  Finite scale divisions: The scale marking can be made only up to certain limits and they not be hundred percent accurate.

15. Precision : Precision is defined by the degree of exactness for which an instrument is designed or intended to perform. Repeatability : It is the closeness of agreement among a number of consecutive measurements of the output for the same value of the input, under the same operating conditions. Reproducibility : It is the closeness of agreement among repeated measurements of the output for the same value of the input, made under the same operating conditions over a period of time. Drift : It is an undesired change or a gradual variation in output over a period of time that is unrelated to changes in input and operating conditions. CHARACTERSTICS OF MEASURING INSTRUMENTS

16. Span : If in a measuring instrument the highest point of calibration is y units and the lowest point x units. Then the instrument range y units The instrument span is given by Span=(x-y)units Sensitivity : Sensitivity can be defined as the ratio of a change in output to the change in input which causes it. Resolution : The smallest increment in input (the quantity being measured) which can be detected with certainty by an instrument is its resolution. Dead zone : Dead zone is the largest range of values of a measured variable to which the instrument does not respond.

17. Types of Instruments Absolute Instruments : Absolute instruments are those which give the value of the electrical quantity to be measured, in terms of the constant of the instruments and their deflection only, e.g. tangent galvanometer. Secondary Instruments : Secondary instruments are those which have been precalibrated by comparison with an absolute instrument. The value of the electrical quantity to be measured in these instruments can be determined from the deflection of the instrument. Without calibration of such an instrument, the deflection is meaningless. Different types of secondary instruments: 1)Indicating 2)Integrating 3)Recording

18. 1)Indicating:  Indicating instruments are those which indicate the instantaneous value of the electrical quantity being measured, at the time at which it is being measured.  Their indications are given by pointers moving over calibrated dials(scale), e.g. ammeters, voltmeters and watt-meters.

19. Integrating instruments are those which measure and register the total quantity of electricity (in ampere-hour) or the total amount of electrical energy(in watt-hours or kilowatt- hours) supplied to a circuit over a period of time, e.g. ampere- hour meters, energy meters. 2)Integrating: ODOMETER AMPERE HOUR METER ENERGY METER

20. 3)Recording:  Recording instruments are those which give a continuous record of variations of the electrical quantity over a selected period of time.  The moving system of the instrument carries an inked pen which rests tightly on a graph chart. E.g. recording voltmeters used in supply station.

21. Essentials of Indicating Instruments Deflecting Torque(Td): It is the torque which deflects the pointer on a calibrated scale according to the electrical quantity passing through the instrument. The deflecting torque causes the moving system and hence the pointer attached to it to move from zero position to indicate on a graduated scale the value of electrical quantity being measured. Controlling Torque(Tc): It is the torque which controls the movement of the pointer on a particular scale according to the quantity of electricity passing through it. If deflecting torque were acting alone, the pointer would continue to move indefinitely and would swing over to the maximum deflected position irrespective of the magnitude of current (or voltage or power) to be measured.  SPRING CONTROL  GRAVITY CONTROL

22. 1) Spring Control: In the spring control method, a hair-spring, usually of phosphor- bronze, attached to the moving system is used. With the deflection of the pointer, the spring is twisted in the opposite direction. This twist in the spring produces a restoring torque which is directly proportional to the angle of deflection of the moving system. The pointer comes to a position of rest (or equilibrium) when the deflecting torque (Td) and controlling torque (Tc) are equal. Tc ∞ θ To give a controlling torque which is directly proportional to the angle of deflection of the moving system, the number of turns of the spring should be fairly large so that the deformation per unit length is small. The stress in the spring must be limited to such a value that there is no permanent set. Springs are made of materials which are  Non magnetic  Not subject to much fatigue  Low in specific resistance  Have low temperature coefficient of resistance.

23. 2) Gravity Control: Gravity control is obtained by attaching a small weight to the moving system in such a way that it produces a restoring or controlling torque when the system is deflected. Tc ∞ Sinθ Thus, controlling torque in a gravity control system is proportional to the sine of the angle of deflection. The degree of control is adjusted by screwing the weight up or down on the carrying system.

24. Damping Torque: If the moving system is acted upon by deflecting and controlling torques alone, then pointer, due to inertia, will oscillate about its final deflected position for quite sometime before coming to rest. This is often undesirable because it makes difficult to obtain quick and accurate readings. In order to avoid these oscillations of the pointer and to bring it quickly to its final deflected position, a damping torque is provided in the indicating instruments. There are three types of damping:  Air friction damping: air friction damping uses either aluminium piston or vane, which is attached to or mounted on the moving system and moves in an air chamber at one end.  Fluid friction damping: In fluid friction damping, a light vane (attached to the moving system) is dipped into a pot of damping oil. The fluid produces the necessary opposing (or damping) force to the vane. The vane should be completely submerged in the oil. The disadvantage of this type of damping is that it can only be used in the vertical position.  Eddy Current Damping: Eddy-current damping uses a conducting material which moves in a magnetic field so as to cut through the lines of force, thus setting up eddy currents. Force always exists between the eddy current and magnetic field which is always opposite to the direction of motion. This is most efficient type of damping and is largely used in permanent magnet moving coil instruments.

25. Types of indicating instruments  MOVING IRON INSTRUMENT Attraction type instrument Repulsion type instrument  MOVING COIL INSTRUMENT

26. PMMC PRINCIPLE: This type of instrument based on the principle that when a current carrying conductor is placed in a magnetic field, a force acts on the conductor, which tends to move it to one side and out of the field.

27. CONSTRUCTION OF PMMC It consists of a powerful U-shaped permanent magnet made of Alnico, and soft iron pole pieces bored out cylindrically. A soft iron core is fixed between the magnetic poles whose functions are i) to make the field uniform, and ii) to decrease the reluctance of the air path between the poles and hence increase the magnetic flux. Surrounding the core is a rectangular coil of many turns wound on a light aluminium or copper frame, supported by delicate bearings. A light pointer fixed to the frame moves on the calibrated scale according to the amount of electricity passed through the coil. The aluminium frame provides not only support for the coil but also a damping torque by the eddy currents induce in it. PMMC

28. WORKING: When the instrument is connected in the circuit to measure current or voltage, the operating current flows through the coil. Since the coil is carrying current and is placed in the magnetic field of the permanent magnet, a mechanical force acts on it. As a result, the pointer attached to the moving system moves in a clockwise direction over the graduated scale to indicate the value of current or voltage being measured. If the current in the coil is reversed, the deflecting torque also be reversed since the direction of the permanent magnet is same. Consequently, the pointer will try to deflect below zero. Deflection in this direction is prevented by a spring “stop”. Since the deflecting torque reverses with the reversal of current in the coil, such instrument can be used to measure direct current and voltage only.

29. ADVANTAGE: Low power consumption. Uniform scale extendable over an arc of 270º or so. High torque weight ratio. No hysteresis loss. Very effective and efficient eddy current damping. Not effected much by stray and magnetic fields due to strong operating field. DISADVANTAGE: Costlier compared to moving iron instruments, due to delicate construction and accurate machining and assembly of various parts. Some error arise due to the ageing of control springs and the permanent magnet. Use limited to d.c. only. Scale length of meter can be increased from 120º and 240º or even 270º or 300º. APPLICATION: PMMC instruments can be used as dc ammeter. And its range can be increased by using a large number of turns in parallel with the instrument. The range of this instrument, when used as a dc voltmeter, can be increased by using a high resistance in series with it.

30. Moving Iron instruments Moving Iron instruments depend for their action upon the magnetic effect of current, and are widely used as indicating instruments. In this type of instrument, the coil is stationery and the deflection is caused by a soft-iron piece moving in the field produced by the coil. There are two types of moving iron instruments: i) Attraction type ii) Repulsion type

31. Attraction type:  Construction: Fig shows the constructional details of an attraction type MI instrument. It consists of a cylindrical coil or solenoid which is kept fixed. An oval-shaped soft-iron piece. The controlling torque is provided by an aluminium vane, attached to the spindle, which moves in a closed air chamber.

32.  Working: When the instrument is connected in the circuit to measure current or voltage, the operating current flowing through the coil sets up a magnetic field. In other words, the coil behaves like a magnet and therefore it attracts the soft iron piece towards it. The result is that the pointer attached to the moving system moves from zero position. The pointer will come to rest at a position where deflecting torque is equal to the controlling torque. If current in the coil is reversed, the direction of magnetic field also reverses and so does the magnetism produce in the soft iron piece. Hence, the direction of the deflecting torque remains unchanged. For this reason, such instruments can be used for both d.c. and a.c. measurements.

33. Repulsion type  Construction: Fig shows the constructional details of repulsion type moving-iron instrument. It consists of two soft-iron pieces or vanes surrounded by a fixed cylindrical hollow coil which carries the operating current. One of these vanes is fixed and the other is free to move. The movable vane is cylindrical shape and is mounted axially on a spindle to which a pointer is attached. The fixed vane, which is wedge-shaped and has a larger radius, is attached to the stationery coil. The controlling torque is provided by one spiral spring at the top of the instrument. It may be noted that in this instrument, springs do not provide the electrical connections. Damping is provided by air friction due to the motion of a piston in an air chamber.

34. Working: When current to be measured or current proportional to the voltage to be measured flows through the coil, a magnetic field is set up by the coil. This magnetic field magnetises the two vanes in the same direction i.e. similar polarities are developed at the same ends of the vanes. Since the adjacent edges of the vanes are of the same polarity, the two vanes repel each other. As the fixed vane cannot move, the movable vane deflects and causes the pointer to move from zero position. The pointer will come to rest at a position where deflecting torque is equal to controlling torque provided by the spring. If the current in the coil is reversed, the direction of deflection remains unchanged. It is because reversal of the field of the coil reverses the magnetisation of both iron vanes so that they repel each other regardless which way current flows through the coil. For this reason, such instruments can be used for both d.c. and a.c. applications.

35. o Advantages: i) Cheap, robust and give reliable service. ii) Usable in both a.c. and d.c. circuits. o Disadvantages: i) Have non-linear scale. ii) Cannot be calibrate with high degree of precision for d.c. on account of the affect of hysteresis in the iron vanes. iii) Deflection up to 240º only may be obtained with this instrument. iv) This instrument will always have to be put in the vertical position if it uses gravity control. o Errors with MI instruments: i) Due to hysteresis when used in a.c. and d.c. ii) Due to stray magnetic fields when used both in a.c. and d.c. iii) Due to frequency variation when used in a.c. iv) Due to waveforms effect when used in a.c.

36. Applications of MI instruments: As an ammeter: It may be constructed for full-scale deflection of 0.1 to 30A with out the use of shunts or current transformers. To obtain full-scale deflection with currents less than 0.1A, it requires a coil with a large number of fine wire turns, which results in an ammeter with a high impedance. As an voltmeter: The MI voltmeter is a fairly low impedance instrument, typically, 50Ω/V for a 100V instrument. The lowest full scale is of the order of 50V.The range of the instrument, when used as a voltmeter, can be extended by using a high non-inductive resistance R connected in series with it. This series resistance is known as ‘multiplier’.

37. INSRUMENT TRANSFORMER It is a transformer that is used in conjunction with any measuring instrument (i.e., Ammeter, Voltmeter, Wattmeter, Watt-hour-meter, …etc.)or protective equipment (i.e., Relays). It utilizes the current-transformation and voltage transformation properties to measure high ac current and voltage. Applications of Instrument Transformers: For measurement of high ac current, it is usual to use low range ac ammeter with suitable shunt. For measurement of high ac voltage, low range ac voltmeters are used with high resistances connected in series. For measurement of very high ac current and voltage, we cannot use these methods. Instead, we use specially constructed HV instrument transformers to insulate the high voltage circuit from the measuring circuit in order to protect the measuring instruments from burning.

38. CURRENT TRANSFORMER A current transformer is a transformer, which produces in its secondary winding low current, which is proportional to the high current flowing in its primary winding. The secondary current is usually much smaller in magnitude than the primary current. The design of CT depends on which type of instrument is connected to its secondary winding. Measuring instrument OR Protective instrument. -Measuring instrument CT is expected to give accurate results up to a maximum of 125% of its normal full-load rated current. -Protective instrument CT is expected to be accurate for up to 20 times of its normal full-load rated current. Based on the type of equipment for which the Ct is used for, its saturation point will vary. At the same time it is expected to be linear in the entire working range. Construction of C.T.: C.T. has a primary coil of one or more turns made of thick wire connected in series with the line whose current is to be measured. The secondary consists of a large number of turns made of fine wire and is connected across an ammeter or a relay’s terminals

39. Construction Types of Current Transformers Window-type Bar-type

40. FUNCTION OF CURRENT TRANSFORMER The principal function of a CT is to produce a proportional current at a level of magnitude, which is suitable for the operation of low-range measuring or protective devices such as indicating or recording instruments and relays. The primary and secondary currents are expressed as a ratio such as 100/5 or 1000/5. With a 100/5 ratio CT, 100A flowing in the primary winding will result in 5A flowing in the secondary winding, provided that the correct rated burden is connected to the secondary winding.

41. POTENTIAL TRANSFORMER A PT or sometimes called VT is a step-down transformer having many primary turns but few secondary turns. In a step-down transformer the voltage decreases and the current increases, thus voltage can be easily measured by using a low-range voltmeter instrument. The voltage is stepped-down in a known ratio called the voltage ratio. Construction: A potential transformer has many primary winding turns but few number of secondary winding turns that makes it a step-down transformer. A Voltmeter is connected to the secondary winding is usually a voltmeter of 150 V. Working (Measurement): Primary terminals are connected in parallel across the line to which the voltage is to be measured. The voltmeter reading gives the transformed value of the voltage across the secondary terminals. The deflection of the voltmeter when divided by the transformed ratio gives the actual voltage across the primary winding as: The Line voltage = deflection / transformation-Ratio Where transformation ratio = V2/V1

42. Electrodynamometer type instrument It works on dynamometer principle i.e. mechanical force exists between two current carrying conductors or coils. Similar to PMMC Portable, highest precision. Transfer instruments.

43. CONSTRUCTION Fixed Coils: The operating field is produced by the fixed coil which is divided into two sections to give a uniform field near the centre. The coil is wound with fine wire when used as a voltmeter and heavy wire when used as an ammeter and wattmeter. The wire is stranded, when necessary, to reduce eddy current losses in conductors. Moving Coil: The moving coil is wound either as a self sustaining coil or else on a non-metallic former. The use of metallic former is avoided in order to avoid the inducement of eddy currents in it. Both moving coils and fixed coils are air cored.

44. Moving System: The moving coil is supported by an aluminium spindle and jewel bearings and carries a pointer moving over a graduated scale. The entire movement is very solid and rigidly constructed in order to keep mechanical dimensions stable and its calibration in tact. Control system: The controlling torque is provided by two control springs, which also act as leads to the moving coil. Damping System: Air friction damping is used in these instruments and may be either piston type or vane type. Eddy current damping cannot be used in these instruments as introduction of a permanent magnet for the purpose would distort the working magnetic field of the instrument. Shielding: The operating magnetic field produced by the fixed coils in these instruments is somewhat weaker (0.005-0.006T) in comparison to that in instruments of other types. So it is essential to provide magnetic shielding to this arrangement. The complete assembly is surrounded by a laminated steel shield to protect the instrument from external magnetic field which may affect the operation of the instrument.

45. WORKING PRINCIPLE The operating principle of dynamometer type instruments is the interaction between the currents in the moving coil, mounted on a shaft, and the fixed coils. W hen two coils are energized, their magnetic fields interact and the resulting torque tends to rotate the moving coil. Since there is no iron, the field strength is proportional to the current in the fixed coil and, therefore, the deflecting torque is proportional to the product of the currents in the fixed coil and the moving coil. When used as a wattmeter and, the fixed coil is the current coil and the moving coil is the pressure coil. Thus the current in the latter is proportional to the voltage applied. Hence, the deflecting torque is proportional to the product of the voltage and current(that is power).

46. ELECTRODYNAMOMETER

47. BALASSITIC GALVANOMETER Principle. When a current is passed through a coil, suspended freely in a magnetic field, it experiences a forces in a direction given by Fleming’s left hand rule. Construction. It consists of a rectangular coil of thin copper wire wound on a non- metallic frame of ivory. It is suspended by means of a phosphor bronze wire between the poles of a powerful horse-shoe magnet. A small circular mirror is attached to the suspension wire. Lower end of the coil is connected to a hair-spring. The upper end of the suspension wire and the lower end of the spring are connected to terminals T1 and T2. A cylindrical soft iron core (C) is place symmetrically inside the coil between the magnetic poles which are also made cylindrical in shape. This iron core concentrates the magnetic field and helps in producing radial field. The B.G. is used to measure electric charge. The charge has to pass through the coil as quickly as possible and before the coil stars moving. The coil thus gets an impulse and a throw is registered. To achieve this result, a coil of high moment of inertia is used so that the period of oscillation of the coil is fairly large. The oscillations of the coil are practically undamped.

48. BALASSITIC GALVANOMETER

49. VIBRATION GALVANOMETER A vibration galvanometer is a type of mirror galvanometer, usually with a coil suspended in the gap of a magnet or with a permanent magnet suspended in the field of an electromagnet. The natural oscillation frequency of the moving parts is carefully tuned to a specific frequency; commonly 50 or 60 Hz. Higher frequencies up to 1 kHz are possible. Since the frequency depends on the mass of the moving elements, high frequency vibration galvanometers are very small with light coils and mirrors. The tuning of the vibration galvanometer is done by adjusting the tension of the suspension spring. The vibration galvanometer is used for detecting alternating currents in the frequency of its natural resonance. Most common application is as a null indicating instrument in ACbridge circuits and current comparators. The sharp resonance of the vibration galvanometer makes it very sensitive to changes in the measured current frequency and it can be used as an accurate tuning device

50. VIBRATION GALVANOMETER

51. POWER FACTOR METER Power factor in an a.c. circuit just by dividing the power with product of current and voltage as these readings can be easily obtained from wattmeter, ammeter and voltmeter. Obviously there various limitations of using this method as it may not provide high accuracy, also chances of increment of error is very high. Therefore this method is not adopted in industrial world. Measurement of power factor accurately is very essential everywhere. In power transmission system and distribution system we measure power factor at every station and electrical substation using these power factor meters. Power factor measurement provides us the knowledge of type of loads that we are using, helps in calculation of losses happening during the power transmission system and distribution. Hence we need a separate device for calculating the power factor accurately and more precisely. General construction of any power factor meter circuit include two coils pressure coil and current coil. Pressure coil is connected across the circuit while current coil is connected such it can carry circuit current or a definite fraction of current, by measuring the phase difference between the voltage and current the electrical power factor can be calculated on suitable calibrated scale. Usually the pressure coil is splits into two parts namely inductive and non-inductive part or pure resistive part. There is no requirement of controlling system because at equilibrium there exist two opposite forces which balance the movement of pointer without any requirement of controlling force

52. Electrodynamometer Type Power Factor Meter In electrodynamometer type power factor meter there are further two types on the basis of supply voltage Single phase Three phase. The general circuit diagram of single phase electrodynamometer power factor meter is given below. Now the pressure coil is spitted into two parts one is purely inductive another is purely resistive as shown in the diagram by resistor and inductor. At present the reference plane is making an angle A with coil 1. And the angle between both the coils 1 and 2 is 90°. Thus the coil 2 is making an angle (90° + A) with the reference plane. Scale of the meter is properly calibrated shown the value values of cosine of angle A. Let us mark the electrical resistance connected to coil 1 be R and inductor connected to coil 2 be L. Now during measurement of power factor the values of R and L are adjusted such that R=wL so that both coils carry equal magnitude of current. Therefore the current passing through the coil 2 is lags by 90° with reference to current in coil 1 as coil 2 path is highly inductively in nature.