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

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

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
3

Measurement of low resistance
4
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Ammeter Voltmeter method
=
=
So to find resistance
Measure potential across the resistance using voltmeter
Measure current through the resistance using Ammeter
5

Ammeter Voltmeter method
6

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

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

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

Ammeter Voltmeter method
10

Ammeter Voltmeter method
11

Ammeter Voltmeter method
Actual value of resistance is measured if (Rm/RV)=0
Which means voltmeter resistance is infinite
If Rx

Measurement of low resistance
13
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Potentiometer method
14

Potentiometer method
Rrheostat
B battery
S standard resistance
X unknown resistance
15

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 (Vs and Vx )
=
=
16

Potentiometer method
Advantage
High accuracy
17

Measurement of low resistance
18
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Kelvin Double Bridge method
19

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

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
http://www.electrical4u.com/electricalresistanceandlawsofresistance/http://www.electrical4u.com/voltageorelectricpotentialdifference/

Kelvin Double Bridge(contd)
22

Kelvin Double Bridge(contd)
23

Kelvin Double Bridge(contd)
24

Kelvin Double Bridge(contd)
It is used to measure resistances as low as 0.00001
25

Measurement of low resistance
26
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Ohm meter method
An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current.
Microohmmeters (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 ().
27
http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Measuring_instrumenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Meggerhttp://en.wikipedia.org/wiki/%CE%A9

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

Measurement of low resistance
29
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Series type Ohm meter
30

Series type Ohm meter
31

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

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

Series type Ohm meter
To mark infinity
AB 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

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

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

Series type Ohm meter
37

Series type Ohm meter
38

Measurement of low resistance
39
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Shunt type Ohmmeter
40

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

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

Shunt type Ohmmeter
AB is opened
Current flowing through meter is:
=
1 +
43

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

Shunt type Ohmmeter
If unknown resistance is connected across AB
At node 1 = +
=
1 +
+
45
=( + )
( + )1+

Shunt type Ohmmeter
Current through meter is
=
+
46
=(+)
1(+)+ *
+

Shunt type Ohmmeter
For full scale deflection, = 0
= 0 Amperes
47

Shunt type Ohmmeter
For half scale deflection,
=
Current through meter is:
=
21+
48

Measurement of low resistance
49
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Crossed coil Ohmmeters
These instruments are also called ratiometers
It consists of two rigidly fixed coils at an angular seperation of 900
Indication depends on ratio of currents through two coils
50

Measurement of low resistance
51
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Fixed Magnet Moving Coil Ohmmeter
52

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

Fixed Magnet Moving Coil Ohmmeter
Torque produced is:
54

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

Fixed Magnet Moving Coil Ohmmeter
At equilibrium,
56

Fixed Magnet Moving Coil Ohmmeter
Current I1 is proportional to voltage drop across unknown resistance X
1 =
57

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

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

Measurement of low resistance
60
Me
asu
re
me
nt
of
low
r
esi
st
an
ce
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

Crossed coil Moving magnet type Ohm meter
61

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

Crossed coil Moving magnet type Ohm meter
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
63

Crossed coil Moving magnet type Ohm meter
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
64

Crossed coil Moving magnet type Ohm meter
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
65

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

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

Ammeter Voltmeter method
Same as measuring low resistance
68

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

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

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

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

Wheatstones Bridge
73

Wheatstones 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

Wheatstones 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 S2
75

Wheatstones Bridge(contd)
The voltage source of this Wheatstone bridge is connected to the terminals A and C via a switch S1 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
http://www.electrical4u.com/idealdependentindependentvoltagecurrentsource/

Wheatstones Bridge(contd)
Suppose current I1 and current I2are flowing through the paths ABC and ADC respectively. If we vary the electrical resistance value of arm CD the value of current I2 will also be varied as the voltage across A and C is fixed.
77

Wheatstones Bridge(contd)
If we continue to adjust the variable resistance one situation may comes when voltage drop across the resistor S that is I2.S is becomes exactly equal to voltage drop across resistor Q that is I1.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 S2 is closed.
78

Wheatstones Bridge(contd)
79

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

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

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

Wheatstones 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 or from
1 to 10,000
Wheat stones bridge can be used for measurement of resistances upto 100000ohms
83

Wheatstones 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

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

Carey Foster Bridge
86

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 thickbordered areas are thick copper busbars of almost zero resistance.
87
http://en.wikipedia.org/wiki/Busbar

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 l1.
88

Carey Foster Bridge
Swap X and Y. Adjust D to the new null point. This position is l2.
If the resistance of the wire per percentage is , then the resistance difference is the resistance of the length of bridge wire between l1 and l2:
89

Carey Foster Bridge
90

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

Carey Foster Bridge
Let l1 be the null point D on the bridge wire EF in percent.
is the unknown leftside extra resistance EX
is the unknown rightside extra resistance FY
is the resistance per percent length of the bridge wire:
92

Carey Foster Bridge
93

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

Carey Foster Bridge
95

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

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

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

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

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

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

Guard circuit
102

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 (IR+IL)
So reading in the ammeter will not be accurate due to error caused by leakage current.
103

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 IR only and hence resistance can be measured accurately
104

Simple guard circuit
105

Guarded wheat stones bridge
106

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

Direct deflection method
108
Measurement of insulation resistance of a cable

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

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

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

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

MEGGER METHOD
113

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

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

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

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

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

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

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

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

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

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

Loss of charge method
124

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 Rx and Rleak
125

Loss of charge method
Step 3: Time (t) taken for the potential difference to fall from V1 to V2 is noted during discharge.
=
+
126

Loss of charge method
At the time of discharge:
=
=
=
127

Loss of charge method
=
=
128

Loss of charge method
=
0
21
2
1=
2
1= ( )
From the above
expression Reff can be determined
129

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

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

Mega Ohm Bridge Method
132

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

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

Mega Ohm Bridge Method
Dial R2 is calibrated as
1 10 100 1000M
Dial R2 110 is in logarithmic scale
Unknown resistance is
3 =142
Junction of arms R1and R2 is brought out on the main panel and designated as Guard Terminal
135

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

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

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

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

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

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

FALL OF POTENTIAL METHOD
142

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

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

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

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

FALL OF POTENTIAL METHOD
Value of earth resistance is given by:
=
147

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

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

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

MEGGER EARTH TESTER
151

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

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

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

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

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

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

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

MEASUREMENT OF POWER
159
Wattmeters
Dynamometer type
Induction type
Electrostatic type

Wattmeter
160
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

MEASUREMENT OF POWER
161
Wattmeters
Dynamometer type
Induction type
Electrostatic type

Dynamometer type wattmeter
162

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

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

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

Dynamometer type wattmeter
Fixed coil current: =
Moving coil current:
=
Deflecting torque:
=
166

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
=
167

Dynamometer type wattmeter
1
2
M mutual inductance between
two coils
Instantaneous torque is given by:
= 12
168

Dynamometer type wattmeter
Instantaneous voltage in pressure coil is:
= 2
Instantaneous current through pressure coil is:
=2
= 2
 resistance of
pressure coil
169

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

Dynamometer type wattmeter
Instantaneous torque is given by:
171

Dynamometer type wattmeter
Average deflecting torque is given by:
172

Dynamometer type wattmeter
Control torque is =
K spring constant
 final steady state deflection
At balance position
= ()
173

Dynamometer type wattmeter
At balance condition:
174

Shape of scale of dynamometer wattmeter
175

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

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

Shape of scale of dynamometer wattmeter
178

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

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

Suspended coil torsion type dynamometer wattmeter
181

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

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

Pivoted coil indicating type dynamometer wattmeter
184

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

Pivoted coil indicating type dynamometer wattmeter
The damping is provided by using the damping vane attached to the moving system and moving in a sectorshaped 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 nonmetallic parts of high resistivity material.
186

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

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

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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
189

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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
190

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

Error due to pressure coil inductance
192

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

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

Error due to pressure coil inductance
195

Error due to pressure coil inductance
196

Error due to pressure coil inductance
197

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

Error due to pressure coil inductance
199

Error due to pressure coil inductance
200

Compensation for Error due to pressure coil inductance
201

Compensation for Error due to pressure coil inductance
202

Compensation for Error due to pressure coil inductance
203

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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

Error due to pressure coil capacitance
Pressure coil has capacitance as well as inductance
Capacitance is due to interturn 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

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

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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
207

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

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

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

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

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

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

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

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

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

Compensation for Error due to power loss in pressure coil
217

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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
218

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

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

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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
221

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

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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
223

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: 10
224

Errors in dynamometer type wattmeter
Errors
Due to pressure coil inductance
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
225

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

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

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

Low power factor electrodynamometer wattmeter
229

Low power factor electrodynamometer wattmeter
230

Low power factor electrodynamometer wattmeter
231

POWER FACTOR METER
232

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

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

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

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

ELECTRODYNAMOMETER TYPE POWER FACTOR METER
237

ELECTRODYNAMOMETER TYPE POWER FACTOR METER
Construction is same as that of wattmeter
F1F2 fixed coils connected in series
AB moving coils with their axes in quadrature
AB moves together and carries the pointer showing power factor
238

ELECTRODYNAMOMETER TYPE POWER FACTOR METER
F1F2 carries main current
If current is high, then a portion of current is passed through it
Magnetic field produced around F1F2 is proportional to current
239

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 = L
240

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

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

ELECTRODYNAMOMETER TYPE POWER FACTOR METER working
XX 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

ELECTRODYNAMOMETER TYPE POWER FACTOR METER working
244

ELECTRODYNAMOMETER TYPE POWER FACTOR METER working
245

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

MOVING IRON POWER FACTOR METER
247
Disadvantage: Accuracy is less

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

ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER
249

ROTATING FIELD TYPE MOVING IRON POWER FACTOR METER
Consists of three fixed coils F1,F2 and F3 displaced by 1200
These coils are fed from three CTs
Coil F1 phase R
Coil F2 phase Y
Coil F3 phase B
250

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

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

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

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

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

ALTERNATING FIELD TYPE MOVING IRON POWER FACTOR METER
256

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

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

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

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 NalderLipman power factor meter
260

ENERGY
261

ENERGY
262

ENERGY METER TYPES
263
Energy meter
Single phase
Three phase

SINGLE PHASE ENERGYMETER CONSTRUCTION
264

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

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

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 900.
267

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

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

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

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 killowatthours (Kwh).
271

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

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

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

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

Working/operation of single phase energy meter
276

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

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

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

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

Working/operation of single phase energy meter
281

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

Derivation of Torque equation
Phasor Diagram
283

Derivation of Torque equation
284

Derivation of Torque equation
285

Derivation of Torque equation
286

Derivation of Torque equation
287

ENERGYMETER CONSTANT
288

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

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

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

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

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

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

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

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

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

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

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

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

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 noninductive load, the shielding loop must be moved towards the disc.
On the other hand, if the meter runs slow on noninductive load, the brake magnet must be moved towards the center of the disc.
301

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 inducedcurrent 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

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

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

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

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

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

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

ADVANTAGES OF INDUCTION TYPE ENERGYMETER
309

DISADVANTAGES OF INDUCTION TYPE ENERGYMETER
310

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

THREE PHASE THREE ELEMENT ENERGYMETER
312

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

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

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

THREE PHASE TWO ELEMENT ENERGYMETER
316

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

ELECTRONIC ENERGY METER
318

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

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

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 watthour
321

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

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

TRIVECTOR METER
324

TRIVECTOR METER
325

TRIVECTOR METER
326

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

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

TRIVECTOR METER VARIATION OF PERCENTAGE SPEED vs
PHASE ANGLE
329

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

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

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 offpeak hours
332

TOD METER
A special metering arrangement is done to measure energy consumption during different time zones of the day including onpaek 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 offpeak and onpeak hours
333

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

TOD METER
335