MODULE 3 MEASUREMENT OF RESISTANCE, POWER, POWER FACTOR AND ENERGY

Transcription

1 MODULE 3 MEASUREMENT OF RESISTANCE, POWER, POWER FACTOR AND ENERGY 1

2 Measurement of resistance Measurement of low resistance (upto 1 ohm) Measurement of medium resistance (1Ω to 0.1M Ω) Measurement of high resistance (greater than 0.1M Ω) Measurement of earth resistance 2

3 Measurement of low resistance Ammeter Voltmeter method Measurement of low resistance Potentiometer method Kelvin Double Bridge method Series type Ohm meter method Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 3

4 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Series type Ohm meter method Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 4

5 Ammeter Voltmeter method Resistance = Voltage Current So to find resistance R = V I Measure potential across the resistance using voltmeter Measure current through the resistance using Ammeter 5

6 Ammeter Voltmeter method 6

7 Ammeter Voltmeter method Connection (a) Voltmeter resistance is infinite 7

8 Ammeter Voltmeter method Connection (a) If ammeter resistance is zero, then measured value is the actual value of unknown resistance If R x >>>R A Ammeter effect becomes negligible 8

9 Ammeter Voltmeter method Connection(b) Voltmeter reading is the true voltage across resistance Ammeter reads total current which is sum of current through resistance and voltmeter 9

10 Ammeter Voltmeter method 10

11 Ammeter Voltmeter method 11

12 Ammeter Voltmeter method Actual value of resistance is measured if (R m /R V )=0 Which means voltmeter resistance is infinite If R x <<<R V then R m /R V becomes negligibly small hence actual value of resistance can be measured 12

13 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Series type Ohm meter method Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 13

14 Potentiometer method 14

15 Potentiometer method R-rheostat B- battery S- standard resistance X- unknown resistance 15

16 Potentiometer method Circuit is connected as shown in figure Rheostat R- regulate current Voltage drop across standard resistance and unknown resistance is measured with the help of potentiometer (V s and V x ) X S = Potentiometer reading across X Potentiometer reading across S = V X V S 16

17 Potentiometer method Advantage High accuracy 17

18 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Series type Ohm meter method Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 18

19 Kelvin Double Bridge method 19

20 Kelvin Double Bridge(contd ) Why it is called double bridge?? it is because it incorporates the second set of ratio arms as shown 20

21 Kelvin Double Bridge(contd ) In this the ratio arms p and q are used to connect the galvanometer at the correct point between j and k to remove the effect of connecting lead of electrical resistance t. Under balance condition voltage drop between a and b (i.e. E) is equal to F (voltage drop between a and c) 21

22 Kelvin Double Bridge(contd ) 22

23 Kelvin Double Bridge(contd ) 23

24 Kelvin Double Bridge(contd ) 24

25 Kelvin Double Bridge(contd ) It is used to measure resistances as low as Ω 25

26 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Ohm meter method Series type Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 26

27 Ohm meter method An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or in the case of a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is ohms (Ω). 27

28 Ohm meter method Instead of measuring current and voltage, if one quantity is kept constant, then resistance is proportional to other quantity Principle of Ohmmter If current is kept constant, then resistance is proportional to voltmeter reading connected across the resistance 28

29 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Ohm meter method Series type Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 29

30 Series type Ohm meter 30

31 Series type Ohm meter 31

32 Series type Ohm meter Current flowing through the meter depends on unknown resistance Meter deflection is proportional to value of resistance 32

33 Series type Ohm meter To mark zero on the scale A-B is shorted R2 is adjusted so that current (I fsd ) through the meter gives full scale deflection This position is marked as zero 33

34 Series type Ohm meter To mark infinity A-B is opened No current flows throgh the circuit So pointer does not deflect This point is marked as infinity Rh is half scale resistance marking 34

35 Series type Ohm meter For half scale deflection, Since total resistance presented to battery is 2R h 35

36 Series type Ohm meter To produce full scale deflection, current required is 36

37 Series type Ohm meter 37

38 Series type Ohm meter 38

39 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Ohm meter method Series type Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 39

40 Shunt type Ohmmeter 40

41 Shunt type Ohmmeter Consists of battery in series with adjustable resistance R1 and meter Switch- provided to disconnect the battery when instrument is not in use Unknown resistance is connected in parallel with meter hence the name shunt type Ohmmeter 41

42 Shunt type Ohmmeter A-B shorted Entire current passes though short, hence meter reads zero Pointer position is marked as zero A-B opened Entire current passes through the meter and the deflection of the pointer is marked as infinity. A-B some unknown resistance Meter will show intermediate readings 42

43 Shunt type Ohmmeter AB is opened Current flowing through meter is: I m = V R 1 + R m 43

44 Shunt type Ohmmeter If AB is shorted Current flowing through the meter: I m = 0 Amperes 44

45 Shunt type Ohmmeter If unknown resistance R x is connected across AB At node 1 I = I m + I R I = R 1 + V R mr x R m + R x I = (R m + R x )V (R m + R x )R 1 +R m R x 45

46 Shunt type Ohmmeter Current through meter is I m = IR x R m + R x I m = (R m +R x )V R 1 (R m +R x )+R m R x * R x R m +R x 46

47 Shunt type Ohmmeter For full scale deflection, R x = 0 I m = 0 Amperes 47

48 Shunt type Ohmmeter For half scale deflection, R m = R x Current through meter is: I m = V 2R 1 +R M 48

49 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Ohm meter method Series type Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 49

50 Crossed coil Ohmmeters These instruments are also called ratiometers It consists of two rigidly fixed coils at an angular seperation of 90 0 Indication depends on ratio of currents through two coils 50

51 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Series type Ohm meter method Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 51

52 Fixed Magnet Moving Coil Ohmmeter 52

53 Fixed Magnet Moving Coil Ohmmeter The two moving coils moves in permanent magnetic field. 53

54 Fixed Magnet Moving Coil Ohmmeter Torque produced is: 54

55 Fixed Magnet Moving Coil Ohmmeter The two torques produced acts in opposite directions 55

56 Fixed Magnet Moving Coil Ohmmeter At equilibrium, 56

57 Fixed Magnet Moving Coil Ohmmeter Current I 1 is proportional to voltage drop across unknown resistance X I 1 = E X 57

58 Fixed Magnet Moving Coil Ohmmeter That means deflection tan θ = K 1E XI 2 That means deflection depends on value of X 58

59 Fixed Magnet Moving Coil Ohmmeter If X is not in circuit then pointer will set parallel to voltage coil which means X= infinity ohms If X is shorted, maximum current flows through current coil, which will set the pointer parallel to current coil, which means X= zero ohms 59

60 Measurement of low resistance Measurement of low resistance Ammeter Voltmeter method Potentiometer method Kelvin Double Bridge method Series type Ohm meter method Shunt type Crossed coil Ohm meters Fixed magnet moving coil Ohmmeter Crossed coil moving magnet Ohmmeter 60

61 Crossed coil Moving magnet type Ohm meter 61

62 Crossed coil Moving magnet type Ohm meter Consists of : Two fixed coils Pivoted magnetic needle attached with a pointer 62

63 Crossed coil Moving magnet type Case 1) Terminal ab is opened No current flows through current coil Bottom portion of the magnetic needle moves in the direction of pressure coil Pointer shows resistance is zero ohms Ohm meter 63

64 Crossed coil Moving magnet type Case 2) Terminal ab is shorted Current flowing through current coil is maximum Bottom portion of magnetic needle move towards current coil Resistance shown by the magnetic needle is zero Ohm meter 64

65 Crossed coil Moving magnet type Case 3) If unknown resistance X is connected across ab Deflection of the magnetic needle will be proportional to ratio of currents through pressure coil and current coil Ohm meter 65

66 Measurement of medium resistance Ammeter Voltmeter method Substitution method Measurement of medium resistance Wheatstone bridge method Carey Foster Bridge method Ohmmeter method 66

67 Measurement of medium resistance Ammeter Voltmeter method Substitution method Measurement of medium resistance Wheatstone bridge method Carey Foster Bridge method Ohmmeter method 67

68 Ammeter Voltmeter method Same as measuring low resistance 68

69 Measurement of medium resistance Ammeter Voltmeter method Substitution method Measurement of medium resistance Wheatstone bridge method Carey Foster Bridge method Ohmmeter method 69

70 Substitution method Method 1 R is variable resistance X is unknown resistance First X is put in the circuit, value of current is noted Then X is removed and value of current is noted 70

71 Substitution method Method 2 Initially switch is at position 1, current is measured Then switch put to position 2, current is measured 71

72 Measurement of medium resistance Ammeter Voltmeter method Substitution method Measurement of medium resistance Wheatstone bridge method Carey Foster Bridge method Ohmmeter method 72

73 Wheatstone s Bridge 73

74 Wheatstone s Bridge(contd ) For measuring accurately any electrical resistance Wheatstone bridge is widely used. There are two known resistors, one variable resistor and one unknown resistor connected in bridge form as shown below. By adjusting the variable resistor the current through the Galvanometer is made zero. When the electric current through the galvanometer becomes zero, the ratio of two known resistors is exactly equal to the ratio of adjusted value of variable resistance and the value of unknown resistance. In this way the value of unknown electrical resistance can easily be measured by using a Wheatstone Bridge. 74

75 Wheatstone s Bridge(contd ) It is a four arms bridge circuit where arm AB, BC, CD and AD are consisting of electrical resistances P, Q, S and R respectively. Among these resistances P and Q are known fixed electrical resistances and these two arms are referred as ratio arms. An accurate and sensitive Galvanometer is connected between the terminals B and D through a switch S 2 75

76 Wheatstone s Bridge(contd ) The voltage source of this Wheatstone bridge is connected to the terminals A and C via a switch S 1 as shown. A variable resistor S is connected between point C and D. The potential at point D can be varied by adjusting the value of variable resistor S. 76

77 Wheatstone s Bridge(contd ) Suppose current I 1 and current I 2 are flowing through the paths ABC and ADC respectively. If we vary the electrical resistance value of arm CD the value of current I 2 will also be varied as the voltage across A and C is fixed. 77

78 Wheatstone s Bridge(contd ) If we continue to adjust the variable resistance one situation may comes when voltage drop across the resistor S that is I 2.S is becomes exactly equal to voltage drop across resistor Q that is I 1.Q. Thus the potential at point B becomes equal to the potential at point D hence potential difference between these two points is zero hence current through galvanometer is nil. Then the deflection in the galvanometer is nil when the switch S 2 is closed. 78

79 Wheatstone s Bridge(contd ) 79

80 Wheatstone s Bridge(contd ) Now potential of point B in respect of point C is nothing but the voltage drop across the resistor Q. 80

81 Wheatstone s Bridge(contd ) Again potential of point D in respect of point C is nothing but the voltage drop across the resistor S. 81

82 Wheatstone s Bridge(contd ) Equating, equations (i) and (ii) we get, 82

83 Wheatstone s Bridge(contd ) The electrical resistances P and Q of the Wheatstone bridge are made of definite ratio such as 1:1; 10:1 or 100:1 known as ratio arms and S the rheostat arm is made continuously variable from 1 to 1,000 Ω or from 1 to 10,000 Ω Wheat stone s bridge can be used for measurement of resistances upto ohms 83

84 Wheatstone s bridge mthod Limitations While measuring low resistance, resistance of leads and contacts become significant resulting in error While measuring high resistances, galvanometer becomes insensitive to imbalance Change in resistance of bridge arms due to heating effect 84

85 Measurement of medium resistance Ammeter Voltmeter method Substitution method Measurement of medium resistance Wheatstone bridge method Carey Foster Bridge method Ohmmeter method 85

86 Carey Foster Bridge 86

87 Carey Foster Bridge In the diagram to the right, X and Y are resistances to be compared. P and Q are nearly equal resistances, forming the other half of the bridge. The bridge wire EF has a jockey contact D placed along it and is slid until the galvanometer G measures zero. The thick-bordered areas are thick copper busbars of almost zero resistance. 87

88 Carey Foster Bridge Place a known resistance in position Y. Place the unknown resistance in position X. Adjust the contact D along the bridge wire EF so as to null the galvanometer. This position (as a percentage of distance from E to F) is l 1. 88

89 Carey Foster Bridge Swap X and Y. Adjust D to the new null point. This position is l 2. If the resistance of the wire per percentage is σ, then the resistance difference is the resistance of the length of bridge wire between l 1 and l 2 : 89

90 Carey Foster Bridge 90

91 Carey Foster Bridge Two resistances to be compared, X and Y, are connected in series with the bridge wire. Thus, considered as a Wheatstone bridge, the two resistances are X plus a length of bridge wire, and Y plus the remaining bridge wire. The two remaining arms are the nearly equal resistances P and Q, connected in the inner gaps of the bridge. 91

92 Carey Foster Bridge Let l 1 be the null point D on the bridge wire EF in percent. α is the unknown leftside extra resistance EX β is the unknown rightside extra resistance FY σ is the resistance per percent length of the bridge wire: 92

93 Carey Foster Bridge 93

94 Carey Foster Bridge Equations 1 and 2 have the same left-hand side and the same numerator on the righthand side, meaning the denominator on the right-hand side must also be equal 94

95 Carey Foster Bridge 95

96 Carey Foster Bridge method Advantages high accuracy, unaffected by thermoelectric emfs Limitations unsuitable for measuring low resistances 96

97 Measurement of medium resistance Ammeter Voltmeter method Substitution method Measurement of medium resistance Wheatstone bridge method Carey Foster Bridge method Ohmmeter method 97

98 Ohm meter method Same as measuring low resistance using ohm meter 98

99 Measurement of high resistance Direct deflection method Measurement of high resistance Megger method Loss of charge method Mega Ohm bridge method 99

100 Problems associated with measurement of high resistances 1) Leakage that occurs over and around the component or specimen under test, or over the binding posts by which the component is attached to the instrument or within the instrument - If not controlled resistance measured will be a combined effect - Effect of leakage paths on measurements can be removed by Guard circuit 100

101 Problems associated with measurement of high resistances 2) Electrostatic effect stray charges appear in the measuring circuit- so errors 3) Absorption by insulating materials- current falls fairly and steeply in the beginning and gradually thereafter 4) Resistance of insulating materials- falls rapidly with increasing temperature 5) Galvanometer employed should be highly sensitive 6) Voltage supply of 100V or more should be applied depending on breakdown voltage of the component 7) One point of the circuit should be effectively grounded obtaining definite ratios in the potential distribution 101

102 Guard circuit 102

103 Guard circuit Case (a) without guard circuit High resistance is measured by Ammeter- Voltmeter method Micro ammeter carries sum of leakage current and resistance current (I R +I L ) So reading in the ammeter will not be accurate due to error caused by leakage current. 103

104 Guard circuit Case (b)- With guard Guard terminal surrounds the resistance entirely and is connected to battery side of micro ammeter So current through micro ammeter is I R only and hence resistance can be measured accurately 104

105 Simple guard circuit 105

106 Guarded wheat stones bridge 106

107 Measurement of high resistance Direct deflection method Measurement of high resistance Megger method Loss of charge method Mega Ohm bridge method 107

108 Direct deflection method Measurement of insulation resistance of a cable 108

109 Direct deflection method Case (a) cables with metal sheath Galvanometer G measures the current between core and metal sheath Leakage currents over the surface of insulating material are carried by the guard wire wound on the insulation and does not flow through the insulation The ration of voltage applied between the core and metal sheath and current flowing between them(galvanometer deflection) gives insulation resistance of the cable 109

110 Direct deflection method Case (b) Cable is immersed in water for at least 24hrs, so that it enters pores of the cable Initially galvanometer should be shunted, if possible it should be connected in series with a high resistance(megaohms) Leakage current flows through guard wire Ration of voltmeter reading to galvanometer deflection gives the value of insulation resistance 110

111 Direct deflection method Limitations Galvanometer should be highly sensitive Galvanometer should be prevented from initial inrush of currents Battery should be at least 500V and its emf should remain constant 111

112 Measurement of high resistance Direct deflection method Measurement of high resistance Megger method Loss of charge method Mega Ohm bridge method 112

113 MEGGER METHOD 113

114 MEGGER METHOD Working principle: Electromagnetic induction When a current carrying conductor is placed in a magnetic field it experiences force whose magnitude depends on strength of current and magnetic field 114

115 MEGGER METHOD It consists of three coils: Current coil or deflection coil Pressure or control coil Compensating coil Coils are mounted on a central shaft and is free to rotate along a C shaped structure 115

116 MEGGER METHOD It also consists of Permanent magnet to Ohmmeter and dc generator 116

117 MEGGER METHOD The coils are connected to the circuit with flexible leads called ligaments Current coil is connected in series with resistance R1 connected to T2 and one generator terminal In the event of short, R1 protects the current coil 117

118 MEGGER METHOD Potential coil One end connected to compensating coil in series with protective resistance R2 Other end is connected to generator Compensating coil is used for better scale operations 118

119 MEGGER METHOD When the test terminals T1 T2 is open, no current flows through current coil Some current flows through voltage coil, so the pointer move towards infinity ohm reading 119

120 MEGGER METHOD If test terminals are shorted, high current passes through current coil and the scale reading will be zero ohms 120

121 MEGGER METHOD When a high resistance is connected between T1 and T2 deflection of pointer is proportional to ratio of currents through pressure coil and current coil Guard terminal is provided to eliminate leakage current 121

122 MEGGER METHOD APPLICATIONS Measurement of high resistance Measurement of insulation resistance Used for testing continuity, pointer shows full deflection if continuity is there between two points in a circuit 122

123 Measurement of high resistance Direct deflection method Measurement of high resistance Megger method Loss of charge method Mega Ohm bridge method 123

124 Loss of charge method 124

125 Loss of charge method Step 1: Capacitor C is charged by battery- by keeping switch in position 1 Step 2: Capacitor C is discharged via R x and R leak 125

126 Loss of charge method Step 3: Time (t) taken for the potential difference to fall from V 1 to V 2 is noted during discharge. R eff = R XR leak R x +R leak 126

127 Loss of charge method At the time of discharge: i = dq dt i = C dv dt i = v R eff 127

128 Loss of charge method C dv dt = v R eff dv v = dt C R eff 128

129 Loss of charge method v 2 dv v 1 t = dt v 0 CR eff ln v 2 v 1 = t CR eff v 2 v 1 = e ( t CR eff) From the above expression R eff can be determined 129

130 Loss of charge method The test is then repeated with R leak only. So value of R leak is found and from the expression of resistance R X that is unknown value of resistance is found. 130

131 Measurement of high resistance Direct deflection method Measurement of high resistance Megger method Loss of charge method Mega Ohm bridge method 131

132 Mega Ohm Bridge Method 132

133 Mega Ohm Bridge Method It consits of: Power supply Bridge members Amplifier Indicating instrument 133

134 Mega Ohm Bridge Method Sensitivity for high resistance is obtained by : 1) using high voltages of 500V or 1000V 2) Use of sensitive null indicating arrangement such as high gain amplifier with CRO or electronic voltmeter 134

135 Mega Ohm Bridge Method Dial R 2 is calibrated as MΩ Dial R is in logarithmic scale Unknown resistance is R 3 = R 1R 4 R 2 Junction of arms R 1 and R 2 is brought out on the main panel and designated as Guard Terminal 135

136 EARTHING AND MEASUREMENT OF EARTH RESISTANCE What is meant by earthing? How will you measure earth resistance? 136

137 EARTHING The connection of electrical machinery/equipment to a general mass of earth, with a conducting material of low resistance is called earthing or grounding The conducting material used is known as earth electrode 137

138 ADVANTAGES OF USING EARTH ELECTRODE All parts of the electrical equipment will be at zero potential. Leakage current flows through low resistance path provided by the earth electrode so human protection Voltage spikes/ current spikes due to lightning or short circuits or other faults will easily get dissipated to earth. 138

139 ADVANTAGES OF USING EARTH ELECTRODE In the case of three phase system, neutral is earthed which helps to maintain line voltage constant For telephone and traction work, earthing acts as return path. So cost of cable and cast of such cable is avoided. Earth electrode ensures low resistance path and hence able to carry leakage currents without deterioration. 139

140 MEASUREMENT OF EARTH RESISTANCE Measurement of earth resistance Fall of potential method Megger earth tester 140

141 MEASUREMENT OF EARTH RESISTANCE Measurement of earth resistance Fall of potential method Megger earth tester 141

142 FALL OF POTENTIAL METHOD 142

143 FALL OF POTENTIAL METHOD Potential E is applied Current I circulates through the E and Q Voltage between E and P is noted 143

144 FALL OF POTENTIAL METHOD Pattern of current flow through earth: 144

145 FALL OF POTENTIAL METHOD Current diverge from E Current converge at Q Current density is high near E and Q Near electrodes, voltmeter reads high, where as between the electrodes 145

146 FALL OF POTENTIAL METHOD The potential V rises near E and Q due to current density In the middle section V remains constant 146

147 FALL OF POTENTIAL METHOD Value of earth resistance is given by: R E = V EP I 147

148 FALL OF POTENTIAL METHOD The measurement of V EP is done at various points between E and Q The potential variation curve is shown in fig Resistance R E is determined when the potential curve is absolutely flat To get accurate reading, distance between P and Q should be large 148

149 FALL OF POTENTIAL METHOD Variation of resistance with distance is shown 149

150 MEASUREMENT OF EARTH RESISTANCE Measurement of earth resistance Fall of potential method Megger earth tester 150

151 MEGGER EARTH TESTER 151

152 MEGGER EARTH TESTER It consists of DC generator Current reverser Rectifier Current coil Potential coil Electrodes E,P and Q 152

153 MEGGER EARTH TESTER Current reverser and rectifier have L type commutators These are mounted on the shaft and rotated with handle Two brushes of commutator are arranged so that it they make contact alternately with each segment of the commutator 153

154 MEGGER EARTH TESTER Other two brushes of commutator are placed in such away that they always make contact with the commutator 154

155 MEGGER EARTH TESTER Earth tester consists of the terminals P1, C1, P2,C2 P1 andd C1 is shorted and connected to common point E P2 and C2 are connected to auxiliary electrodes P and Q respectively 155

156 MEGGER EARTH TESTER The ratio of voltage sensed by voltage coil and current passing through current coil, directly gives the value of earth resistance R E Deflection of the pointer gives R E It can be used for dc purposes only, but to measure ac reverser and rectifier is used 156

157 MEGGER EARTH TESTER AC current through soil prevents back emf in the soil due to electrolytic action 157

158 MEASUREMENT OF POWER Measurement of power is done by wattmeters Wattmeter is a combination of Ammeter and Voltmeter. So it contains current coil and voltage coil(pressure coil) 158

159 MEASUREMENT OF POWER Dynamometer type Wattmeters Induction type Electrostatic type 159

160 Wattmeter Current coil carries load current Pressure coil carries current proportional to voltage Inductance of pressure coil should be minimum to avoid phase lag between current and voltage 160

161 MEASUREMENT OF POWER Dynamometer type Wattmeters Induction type Electrostatic type 161

162 Dynamometer type wattmeter 162

163 Dynamometer type wattmeter Fixed coil current flowing is proportional to load current Moving coil- current flowing is proportional to load voltage 163

164 Dynamometer type wattmeter Strength of magnetic field- proportional to currents through two coils 164

165 Dynamometer type wattmeter V- supply voltage I load current R- resistance of moving coil circuit 165

166 Dynamometer type wattmeter Fixed coil current: i f = i Moving coil current: i m = v R Deflecting torque: T d α i f i m = iv R 166

167 Dynamometer type wattmeter For dc circuit deflecting torque is proportional to power For ac circuit deflecting torque is proportional to voltage, current and power factor P = VI cosϕ 167

168 Dynamometer type wattmeter i 1 current in fixed coil i 2 current in moving coil M- mutual inductance between two coils Instantaneous torque is given by: T i = i 1 i 2 dm dθ 168

169 Dynamometer type wattmeter Instantaneous voltage in pressure coil is: v = 2 V sinωt Instantaneous current through pressure coil is: i = 2 V sinωt R p = 2 i p sinωt R p - resistance of pressure coil 169

170 Dynamometer type wattmeter Current through current coil lags voltage by an angle ϕ i c = 2 I sin ωt ϕ 170

171 Dynamometer type wattmeter Instantaneous torque is given by: 171

172 Dynamometer type wattmeter Average deflecting torque is given by: 172

173 Dynamometer type wattmeter Control torque is K- spring constant T c = Kθ Θ- final steady state deflection At balance position T C = (T d ) av 173

174 Dynamometer type wattmeter At balance condition: 174

175 Shape of scale of dynamometer wattmeter 175

176 Shape of scale of dynamometer wattmeter Deflection θ is proportional to power measured and scale is uniform since dm is constant. dθ Wattmeters are designed such that dm dθ remains over 40 to 50 degree on each side of zero mutual inductance position. M varies linearly in this zone with respect to θ 176

177 Shape of scale of dynamometer wattmeter If zero mutual inducatnce position is kept in the middle then M varies linearly for deflection upto 80 to 100 degrees 177

178 Shape of scale of dynamometer wattmeter 178

179 Dynamometer type wattmeter Ranges: Current coil: 0.25A to 100A Pressure coil : 5V to 750V 179

180 Dynamometer type wattmeter Dynamometer type wattmeter Suspended coil torsion type Pivoted coil indicating type 180

181 Suspended coil torsion type dynamometer wattmeter 181

182 Suspended coil torsion type dynamometer wattmeter The moving, or voltage, coil is suspended from a torsion head by a metallic suspension which serves as a lead to the coil. This coil is situated entirely inside the current or fixed coils and the winding in such that the system is a static. Errors due to external magnetic fields are thus avoided. The torsion heads carries a scale, and when in use, the moving coil is bought back to the zero position by turning this head; the number of divisions turned through when multiplied by a constant for the instrument gives the power. Eddy currents are eliminated as far as possible by winding the current coils of standard wire and by using no metal parts within the region of the magnetic field of the instrument. 182

183 Suspended coil torsion type dynamometer wattmeter The mutual inductance errors are completely eliminated by making zero position of the coil such that the angle between the planes of moving coil and fixed coil is 90 degree. i.e. the mutual inductance between the fixed and moving coil is zero. The elimination of pivot friction makes possible the construction of extremely sensitive and accurate electrodynamic instruments of this pattern. 183

184 Pivoted coil indicating type dynamometer wattmeter 184

185 Pivoted coil indicating type dynamometer wattmeter In these instruments, the fixed coil is wound in two halves, which are placed in parallel to another at such a distance, that uniform field is obtained. The moving coil is wound of such a size and pivoted centrally so that it does not project outside the field coils at its maximum deflection position. The springs are pivoted for controlling the movement of the moving coil, which also serves as currents lead to the moving coil. 185

186 Pivoted coil indicating type dynamometer wattmeter The damping is provided by using the damping vane attached to the moving system and moving in a sector-shaped box. The reading is indicated directly by the pointer attached to the moving system and moving over the calibrated scale. The eddy current errors, within the region of the magnetic field of the instrument, are minimized by the use of non-metallic parts of high resistivity material. 186

187 Electrodynamometer type wattmeter Advantages: 1) In dynamometer type wattmeter, the scale of the instrument is uniform (because deflecting torque is proportional to the true power in both DC as well as AC and the instrument is spring controlled.) 2) High degree of accuracy can be obtained by careful design; hence these are used for calibration purposes. 187

188 Electrodynamometer type wattmeter Disadvantages: 1) The error due to the inductance of the pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect) 2) In dynamometer type wattmeter, stray field may affect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case. 188

189 Errors in dynamometer type wattmeter Due to pressure coil inductance Due to pressure coil capacitance Due power loss in pressure coil and current coil Errors Due to Eddy current Due to friction Due to temperature Due to stray fields 189

190 Errors in dynamometer type wattmeter Due to pressure coil inductance Due to pressure coil capacitance Errors Due power loss in pressure coil and current coil Due to Eddy current Due to friction Due to temperature Due to stray fields 190

191 Error due to pressure coil inductance Current through pressure coil will be in phase with voltage applied Due to inductance of pressure coil, current in pressure coil lags behind supply voltage So there will be error in reading 191

192 Error due to pressure coil inductance 192

193 Error due to pressure coil inductance Pressure coil current lagging behind voltage by an angle β 193

194 Error due to pressure coil inductance Let power factor be lagging 194

195 Error due to pressure coil inductance 195

196 Error due to pressure coil inductance 196

197 Error due to pressure coil inductance 197

198 Error due to pressure coil inductance If power factor is leading: 198

199 Error due to pressure coil inductance 199

200 Error due to pressure coil inductance 200

201 Compensation for Error due to pressure coil inductance 201

202 Compensation for Error due to pressure coil inductance 202

203 Compensation for Error due to pressure coil inductance 203

204 Errors in dynamometer type wattmeter Due to pressure coil inductance Errors Due to pressure coil capacitance Due power loss in pressure coil and current coil Due to Eddy current Due to friction Due to temperature Due to stray fields 204

205 Error due to pressure coil capacitance Pressure coil has capacitance as well as inductance Capacitance is due to inter-turn capacitance in the high values of series resistor. The effect produced is same as that of inductance circuit except that pressure coil currents leads applied voltage So wattmeter will read low on lagging power factors of load, by increasing angle between load and voltage coil currents 205

206 Error due to pressure coil capacitance Effect of frequency: vary angle between V and pressure coil current, the angle is increasing with frequency. If inductive reactance of pressure coil = capacitive reactance of pressure coil, there will be no error 206

207 Errors in dynamometer type wattmeter Due to pressure coil inductance Due to pressure coil capacitance Due power loss in pressure coil and current coil Errors Due to Eddy current Due to friction Due to temperature Due to stray fields 207

208 Error due to power loss in pressure coil and current coil Let Rp be the resistance of pressure coil Rc be resistance of current coil 208

209 Error due to power loss in pressure coil and current coil There are two methods of connecting wattmeter 209

210 Error due to power loss in pressure coil and current coil Voltage applied to the pressure coil = voltage across load + voltage drop across current coil So wattmeter measures power loss in current coil in addition to power consumed by load 210

211 Error due to power loss in pressure coil and current coil 211

212 Error due to power loss in pressure coil and current coil Current coil carries current to the load and current to pressure coil So wattmeter reads: power consumed by load + power loss in pressure coil 212

213 Error due to power loss in pressure coil and current coil 213

214 Error due to power loss in pressure coil and current coil Power loss in pressure coil is less comapared to current coil Connection (a) is preferred for small currents Where as vice versa for high currents 214

215 Compensation for Error due to power loss in pressure coil Compensating coil is used to eliminate error due to current coil carrying pressure coil current in addition to load current Compensating coil is coincident and identical to current coil If compensating coil is connected in series with current coil, current passed through two coils produces resultant magnetic field = zero 215

216 Compensation for Error due to power loss in pressure coil Compensating coil is connected in series with pressure coil so that magnetic field opposes that of current coil and neutralize pressure coil component of current in current coil So if no load current flows through instrument, deflection will be zero 216

217 Compensation for Error due to power loss in pressure coil 217

218 Errors in dynamometer type wattmeter Due to pressure coil inductance Due to pressure coil capacitance Errors Due power loss in pressure coil and current coil Due to Eddy current Due to friction Due to temperature Due to stray fields 218

219 Error due to eddy currents Eddy currents are induced in solid metal parts of instrument by alternating magnetic field The phase of induced emf will be 90 degree behind inducing flux Eddy currents are practically in phase with its emf and it sets up a magnetic field which is combined with that of current coil So a resultant magnetic field is produced which is less than current coil alone and which lags behind current coil by small angle 219

220 Error due to eddy currents Eddy current error cannot be easily calculated If metal parts are more in instrument it is significantly high 220

221 Errors in dynamometer type wattmeter Due to pressure coil inductance Due to pressure coil capacitance Errors Due power loss in pressure coil and current coil Due to Eddy current Due to friction Due to temperature Due to stray fields 221

222 Error due to friction Deflecting torque is very less in the instrument So there is frictional error To reduce frictional error Weight of moving parts should be reduced Greater care must be taken on pivoting 222

223 Errors in dynamometer type wattmeter Due to pressure coil inductance Due to pressure coil capacitance Errors Due power loss in pressure coil and current coil Due to Eddy current Due to friction Due to temperature Due to stray fields 223

224 Error due to temperature As temperature changes Resistance of pressure coil changes Stiffness of springs changes The above effects are opposite in action So they neutralize If pressure coil composed of copper and of resistance alloy having negligible resistance, temperature coefficient is 1:

225 Errors in dynamometer type wattmeter Due to pressure coil inductance Due to pressure coil capacitance Errors Due power loss in pressure coil and current coil Due to Eddy current Due to friction Due to temperature Due to stray fields 225

226 Error due to stray fields Dynamometer wattmeter has weak operating field It is affected by stray magnetic fields resulting in errors So these instruments are shielded against effect of stray magnetic fields Lamination sheets are used in portable lab equipments, steel cases for switch board instruments Precision type is not shielded to keep down eddy current errors 226

227 Low power factor electrodynamometer wattmeter Ordinary electrodynamometer wattmeter are not suitable for measuring power at low power factors due to: 1) Small deflecting torque on the moving system even when pressure coil and current coil are excited. 2) Introduction of large error due to inductance of pressure coil 227

228 Low power factor electrodynamometer wattmeter So features incorporated in lpf meters are: 228

229 Low power factor electrodynamometer wattmeter 229

230 Low power factor electrodynamometer wattmeter 230

231 Low power factor electrodynamometer wattmeter 231

232 POWER FACTOR METER 232

233 POWER FACTOR METER But due to errors in meters used, the method mentioned is not accurate So a meter is necessary to measure power factor directly which is called power factor meter 233

234 POWER FACTOR METER Construction of power factor meter is similar to that of wattmeter Current circuit carries current or fraction of current whose power factor needs to be measured Voltage coil is split into two parts Inductive Non inductive 234

235 POWER FACTOR METER The currents in two paths of are proportional to voltage across the circuit Deflection depends on current through current circuit and currents in two branches of voltage circuit 235

236 TYPES OF POWER FACTOR METERS Power factor meters Electrodynamometer type Moving iron type Rotating field type Alternating field type 236

237 ELECTRODYNAMOMETER TYPE POWER FACTOR METER 237

238 ELECTRODYNAMOMETER TYPE POWER FACTOR METER Construction is same as that of wattmeter F1-F2 fixed coils connected in series A-B moving coils with their axes in quadrature A-B moves together and carries the pointer showing power factor 238

239 ELECTRODYNAMOMETER TYPE POWER FACTOR METER F1-F2 carries main current If current is high, then a portion of current is passed through it Magnetic field produced around F1-F2 is proportional to current 239

240 ELECTRODYNAMOMETER TYPE POWER FACTOR METER Coil A is connected in series with R Coil B is connected in series with L Values of R and L are adjusted so that coils A and B carries equal currents at normal frequency At normal frequency R = ωl 240

241 ELECTRODYNAMOMETER TYPE POWER FACTOR METER Current in coil A is in phase with supply voltage due to R Current in coil B is in quadrature with supply voltage due to L Current in coil B is frequency dependent due to L Current in coil A is frequency independent due to R 241

242 ELECTRODYNAMOMETER TYPE POWER FACTOR METER Current in coils A and B produces magnetic fields of equal strength displaced by 90 degrees 242

243 ELECTRODYNAMOMETER TYPE POWER FACTOR METER- working X-X uniform magnetic field produced by fixed coils Due to interaction of magnetic fileds produced by each coils, torque is developed Torque experienced by coil A and coil B are opposite to each other Pointer attains equilibrium when two torques are equal 243

244 ELECTRODYNAMOMETER TYPE POWER FACTOR METER- working 244

245 ELECTRODYNAMOMETER TYPE POWER FACTOR METER- working 245

246 ELECTRODYNAMOMETER TYPE POWER FACTOR METER- working So angular position taken up by moving coil is equal to system power factor angle Operation of meter is depended only on frequency and wave form 246

247 MOVING IRON POWER FACTOR METER Disadvantage: Accuracy is less 247

248 MOVING IRON POWER FACTOR METER TYPES Rotating field type Alternating field type 248

249 ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER 249

250 ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER Consists of three fixed coils F1,F2 and F3 displaced by These coils are fed from three CTs Coil F1- phase R Coil F2- phase Y Coil F3 phase B 250

251 ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER Coil Q- kept in middle of F1,F2 and F3 and is connected across any two lines of the supply through a series resistance 251

252 ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER Inside coil Q there is a short pivoted iron rod The rod carries two sector shaped vanes I1, I2 The rod also carries damping vanes and the pointer 252

253 ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER Coil Q and soft iro system produces alternating flux This flux interacts with fluxes produced by F1,F2 and F3 Due to R current through coil Q is in phase with supply So deflection of moving system is proportional to power factor angle 253

254 ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER Coils F1, F2, F3 displaced by 120 degrees produces rotating field by induction motor action It keeps the system rotating Due to high resistivity iron parts this motion is reduced by reducing induced currents 254

255 ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER This meter can be used for balanced loads It is also called Westinghouse power factor meter 255

256 ALTERNATING FIELD TYPE MOVING IRON POWER FACTOR METER 256

257 ALTERNATING FIELD TYPE MOVING IRON POWER FACTOR METER Spindle carries pointer, damping vanes and three moving irons The moving irons are sector shaped Moving iron have 120 degrees with respect to each other Q1,Q2,Q3- iron sectors 257

258 ALTERNATING FIELD TYPE MOVING IRON POWER FACTOR METER Iron sectors are magnetized by voltage coils P1,P2 and P3 Current coil is divided into two equal parts F1 and F2 Current coil carries one of the three line currents 258

259 ALTERNATING FIELD TYPE MOVING IRON POWER FACTOR METER When connected in circuit, moving system moves and attains a position in which mean torque in one of the iron pieces is neutralized by other two At this position deflection is proportional to phase angle between currents and voltages in three phase system 259

260 ALTERNATING FIELD TYPE MOVING IRON POWER FACTOR METER The voltage coils are at different levels hence resultant flux is alternating This instrument is also called Nalder-Lipman power factor meter 260

261 ENERGY 261

262 ENERGY 262

263 ENERGY METER TYPES Energy meter Single phase Three phase 263

264 SINGLE PHASE ENERGYMETER CONSTRUCTION 264

265 SINGLE PHASE ENERGYMETER CONSTRUCTION- PARTS Driving system Moving system Braking system and Registering system. 265

266 Driving system consists of two electromagnets, called shunt magnet and series magnet, of laminated construction. 266

267 Driving system A coil having large number of turns of fine wire is wound on the middle limb of the shunt magnet. This coil is known as pressure or voltage coil and is connected across the supply mains. This voltage coil has many turns and is arranged to be as highly inductive as possible. In other words, the voltage coil produces a high ratio of inductance to resistance. This causes the current, and therefore the flux, to lag the supply voltage by nearly

268 Driving system An adjustable copper shading rings are provided on the central limb of the shunt magnet to make the phase angle displacement between magnetic field set up by shunt magnet and supply voltage is approximately 90degrees. The copper shading bands are also called the power factor compensator or compensating loop 268

269 Driving system The series electromagnet is energized by a coil, known as current coil which is connected in series with the load so that it carry the load current. The flux produced by this magnet is proportional to, and in phase with the load current. 269

270 Moving system The moving system essentially consists of a light rotating aluminium disk mounted on a vertical spindle or shaft. The shaft that supports the aluminium disk is connected by a gear arrangement to the clock mechanism on the front of the meter to provide information that consumed energy by the load 270

271 Moving system The time varying (sinusoidal) fluxes produced by shunt and series magnet induce eddy currents in the aluminium disc. The interaction between these two magnetic fields and eddy currents set up a driving torque in the disc. The number of rotations of the disk is therefore proportional to the energy consumed by the load in a certain time interval and is commonly measured in killowatt-hours (Kwh). 271

272 Braking system Damping of the disk is provided by a small permanent magnet, located diametrically opposite to the a.c magnets. The disk passes between the magnet gaps. 272

273 Braking system The movement of rotating disc through the magnetic field crossing the air gap sets up eddy currents in the disc that reacts with the magnetic field and exerts a braking torque. By changing the position of the brake magnet or diverting some of the flux there form, the speed of the rotating disc can be controlled. 273

274 Registering or Counting system The registering or counting system essentially consists of gear train, driven either by worm or pinion gear on the disc shaft, which turns pointers that indicate on dials the number of times the disc has turned. 274

275 Registering or Counting system The energy meter thus determines and adds together or integrates all the instantaneous power values so that total energy used over a period is thus known. Therefore, this type of meter is also called an integrating meter 275

276 Working/operation of single phase energy meter 276

277 Working/operation of single phase energy meter Induction instruments operate in alternatingcurrent circuits and they are useful only when the frequency and the supply voltage are approximately constant The rotating element is an aluminium disc, and the torque is produced by the interaction of eddy currents generated in the disc with the imposed magnetic fields that are produced by the voltage and current coils of the energy meter. 277

278 Working/operation of single phase Let us consider a sinusoidal flux φ (t) is acting perpendicularly to the plane of the aluminium disc, the direction of eddy current (I e ) by Lenz s law is indicated in figure energy meter 278

279 Working/operation of single phase energy meter β=0 since reactance of Al dsc is zero 279

280 Working/operation of single phase energy meter So in all induction type meters, two eddy currents are there so that resultant torque will be there 280

281 Working/operation of single phase energy meter 281

282 Working/operation of single phase energy meter Current coil produces two fluxes in opposite directions So torques produced by the interaction of eddy current due to voltage and current coil is opposite at two points. Hence the disc will start to rotate 282

283 Derivation of Torque equation Phasor Diagram 283

284 Derivation of Torque equation 284

285 Derivation of Torque equation 285

286 Derivation of Torque equation 286

287 Derivation of Torque equation 287

288 ENERGYMETER CONSTANT 288

289 SOURCES OF ERRORS IN SINGLE PHASE ENERGYMETER Incorrect magnitude of fluxes due to abnormal voltages and load currents Incorrect phase relation of fluxes due to defective lagging, abnormal frequencies, changes in iron loss Unsymmetrical magnetic structure disc rotates when pressure coils alone is excited 289

290 SOURCES OF ERRORS IN SINGLE PHASE ENERGYMETER Changes in resistance of disc due to change in temperature Changes in strength of drag magnets due to temperature and ageing Phase angle errors due to lowering of power factor Abnormal deflection of moving parts Badly distorted waveform Changes in retarding torque of the disc 290

291 Errors ERRORS IN SINGLE PHASE ENERGY METER Phase error Frictional Error Creeping Speed Error Temperature error Overload compensation Voltage compensation 291

292 Phase error An error due to incorrect adjustment of the position of shading band results an incorrect phase displacement between the magnetic flux and the supply voltage (not in quadrature) This is tested with 0.5 p.f. load at the rated load condition 292

293 Compensation for phase error By adjusting the position of the copper shading band in the central limb of the shunt magnet this error can be eliminated. 293

294 Compensation for phase error A lag coil is placed as shown in figure so that phase angle between Voltage and flux is exactly 90 degrees This kind of compensation is called Power factor adjustment or Quadrature adjustment Or Inductive load adjustment 294

295 Frictional Error Frictional forces at bearings and registering mechanism give rise to unwanted braking torque on the disc So additional driving torque is required 295

296 Compensation for frictional error The two shading bands on the limbs are adjusted to create this extra torque. This adjustment is done at low load (at about 1/4th of full load at unity p.f.). 296

297 Creeping Error In some meters a slow but continuous rotation is seen when pressure coil is excited but with no load current flowing. This slow revolution records some energy. This is called the creep error. This slow motion may be due to (a) incorrect friction compensation, (b) stray magnetic field (c) for over voltage across the voltage coil. 297

298 Compensation for creeping error This can be eliminated by drilling two holes or slots in the disc on opposite side of the spindle. When one of the holes comes under the poles of shunt magnet, the rotation being thus limited to a maximum of 180 degrees 298

299 Compensation for creeping error In some cases, a small piece of iron tongue or vane is fitted to the edge of the disc. When the position of the vane is adjacent to the brake magnet, the attractive force between the iron tongue or vane and brake magnet is just sufficient to stop slow motion of the disc with full shunt excitation and under no load condition. 299

300 Speed Error Due to the incorrect position of the brake magnet, the braking torque is not correctly developed. This can be tested when meter runs at its full load current alternatively on loads of unity power factor and a low lagging power factor 300

301 Compensation for speed error The speed can be adjusted to the correct value by varying the position of the braking magnet towards the centre of the disc or away from the centre and the shielding loop. If the meter runs fast on inductive load and correctly on non-inductive load, the shielding loop must be moved towards the disc. On the other hand, if the meter runs slow on non-inductive load, the brake magnet must be moved towards the center of the disc. 301

302 Temperature Error Energy meters are almost inherently free from errors due to temperature variations. Temperature affects both driving and braking torques equally (with the increase in temperature the resistance of the induced-current path in the disc is also increases) and so produces negligible error. A flux level in the brake magnet decreases with increase in temperature and introduces a small error in the meter readings 302

303 Compensation for temperature error This error is frequently taken as negligible, but in modern energy meters compensation is adopted in the form of flux divider on the brake magnet 303

304 Overload compensation When the disc rotates in the field of series magnetic field under load conditions, it cuts series flux and dynamically induced emfs is produced on the disc This produces eddy currents on the disc Due to interaction of eddy current flux and series magnet flux, braking torque is produced This is proportional to square of current 304

305 Overload compensation This braking torque is called self braking torque and if load is high it causes serious errors in the instrument To minimize this braking torque, full load speed of the disc is limited to 40rpm The current coil series flux is kept minimum with respect to shunt coil flux 305

306 Overload compensation Practically an overload compensating device in the form of saturable magnet shunt is used At high loads, shunt saturates and diverts some series magnetic flux. This compensates for self braking torque 306

307 Voltage compensation When supply voltage varies, energymeter causes errors This is due to: Non linear magnetic characteristics of shunt magnet core Braking torque is proportional to square of supply voltage 307

308 Voltage compensation Voltage compensation is provided by saturable magnetic shunt It diverts a large proportion of flux into the active path when supply voltage increases This is done by increasing side limb reluctance and providing holes in side limbs 308

309 ADVANTAGES OF INDUCTION TYPE ENERGYMETER 309

310 DISADVANTAGES OF INDUCTION TYPE ENERGYMETER 310

311 THREE PHASE ENERGYMETER Three phase system Four wire Three wire Three element energy meter Two element energy meter 311

312 THREE PHASE THREE ELEMENT ENERGYMETER 312

313 THREE PHASE THREE ELEMENT ENERGYMETER It consists of three elements Each element is similar to that of single phase energy meter Pressure coils are P1,P2 and P3 Current coils are C1, C2 and C3 All elements are mounted in a vertical line in common case and have a common spindle, gearing and recording mechanism 313

314 THREE PHASE THREE ELEMENT ENERGYMETER The coils are connected in such a manner that the net torque produced is equal to sum of torques produced by each element These are employed for three phase four wire system, where fourth wire is the neutral wire Current coils are connected in series with the lines where as pressure coils are connected in parallel across line and neutral 314

315 THREE PHASE THREE ELEMENT ENERGYMETER One unit of three phase energy meter is cheaper than three individual units Due to interaction of eddy currents between all elements, errors are produced which are reduced by suitable adjustments 315

316 THREE PHASE TWO ELEMENT ENERGYMETER 316

317 THREE PHASE TWO ELEMENT ENERGYMETER It is provided with two discs for an element Shunt magnet is carrying pressure coil Series magnet is carrying current coil Pressure coils are connected in parallel Current coils are connected in series Torque is produced in same manner as that of single phase energy meter The total torque on registering mechanism is sum of torques on two discs 317

318 ELECTRONIC ENERGY METER 318

319 ELECTRONIC ENERGY METER Average power = mean product of instantaneous voltage across the load and instantaneous current through load Potential divider- for making voltage to required level Voltage is scaled in the required range using voltage scaling device Current scaling device scales load voltage which is proportional to load current 319

320 ELECTRONIC ENERGY METER Both scaled voltages are connected to voltage and current multiplier unit Voltage and current multiplier unit outputs current as a result of product of ac voltage and current The current is proportional to instantaneous power applied to voltage controlled oscillator VCO works on the principle of constant current charging capacitor 320

321 ELECTRONIC ENERGY METER VCO basically voltage to frequency converter Output of VCO is square wave The frequency of square wave is proportional to output current of VCO So power dependent current and frequency dependent current decides the value of consumed energy ADC (analog to digital converter) converts analog signal to digital signal Display unit displays energy in watt-hour 321

322 ADVANTAGES OF ELECTRONIC High sensitivity No frictional losses Less loading effect ENERGYMETER Low load, full load, creeping adjustments are not required High frequency range High accuracy of ±1% 322

323 MAXIMUM DEMAND METER ASSIGNMENT What is maximum demand meter? Explain the following meters with neat figures 1) Merz price maximum demand indicator 2) Thermal type maximum demand indicator 3) Digital Maximum demand indicator 323

324 TRIVECTOR METER 324

325 TRIVECTOR METER 325

326 TRIVECTOR METER 326

327 TRIVECTOR METER Ratchet coupling is linked to main common register shaft to which final drive from each gear system is connected Shaft is always driven by direct drive which has the maximum speed At that time all other slower shafts are idle on ratchets. 327

328 TRIVECTOR METER As power factor changes, other gear drive drives the shaft at higher speed and drive shifts to another ratchet For a given V-I product, speed of kwh meter varies as power factor varies 328

329 TRIVECTOR METER VARIATION OF PERCENTAGE SPEED vs PHASE ANGLE 329

330 TOD METER Electric company supplies electricity to various loads such as domestic, industrial and commercial purposes These loads varies over various time periods For some time load is maximum and for sometime load is minimum The hours in which load is maximum is called peak load hours 330

331 TOD METER The hours in which load is minimum is called off-peak hours During on-peak hours company has to generate more power to supply for the demand. This causes difficulties 331

332 TOD METER Time of delay rate is special service offered by electric company that allows consumer to take advantage of lower electricity price during a certain time period Consumer can save money being on TOD rate To lessen the load during a particular time of a day, company offers special rate to the consumers who are willing shift the load or portion of the load to off-peak hours 332

333 TOD METER A special metering arrangement is done to measure energy consumption during different time zones of the day including on-paek and offpeak hours Trivector meter itself is provided with capability required It is provided with a time of delay registser (TOD register) which is capable of being progarammed during off-peak and on-peak hours 333

334 TOD METER TOD meters are time- of delay meter which is suitable of recording and indicating consumption during specific time periods of the day Trivector meter with such an arrangement is called TOD meter 334

335 TOD METER 335