Unit 03 Construction & Operation of Watt meter & Energy meter

UNIT- 03
Construction & Operation of
Watt meter & Energy meter

Wattmeter:
A wattmeter, measures Electric Power of the given an
Electric Apparatus or Circuit.
• Types of wattmeter:
1. Electric-Dynamometer wattmeter - for both D.C and A.C
power
2. Induction wattmeter - for A.C power only
3. Electrostatic type wattmeter - for small amount of A.C
power and low power factor

Single phase electro-dynamometer
(electro-dynamic ) type wattmeter

Principle:
A electro-dynamometer wattmeter is used for the
measurement for D.C as well as A.C power.
“It works on the dynamometer principle i.e.,
mechanical force exists between two current
carrying conductor or coils”.

Construction:
• When a dynamometer instrument is used as a wattmeter, the fixed
coils are connected in series with the load and carry the load
current(Il), while the moving coil is connected across the load through
a series multiplier R and carries a current(I2) proportional to the load
voltage as shown in figure.
• The fixed coil is called the current coil(CC)and the movable coil is
known as potential coil(PC). The controlling torque is provided by two
spiral springs which also serve the additional purpose of leading
current into and out of the moving coil. Air friction damping is
provided in such instruments. A pointer is attached to the movable
coil.

Operation or working:
• When the wattmeter is connected in the circuit to measure
the power, the current coil carries load current and potential
coil carries current proportional to the load voltage, due to
currents in the coils, mechanical force exists between them.
The result is that movable coil moves the pointer over the
scale. The pointer comes to reset at a position where
deflecting torque is equal to the controlling torque (reversals
of current reverses current’s in both the fixed coils and the
movable coil so that the direction of deflecting torque remains
unchanged).
• Deflecting torque; Td  load power
 VICos φ ------ For A.C Supply
 VI1 ---------------For D.C Supply

Errors in watt meters:
1. Error due to pressure coil inductance.
Due to self inductance, the pressure coil creates
phase-shift in the current flowing through it. So the
true angle between voltage and current slightly
changed, which leads to an error in the readings.
The wattmeter gives a higher reading for lagging
load and lesser reading for leading pf load. A low
value capacitor across a part of the multiplier will
reduce the effect of pressure coil inductance

2. Error due to pressure coil capacitance:
The effect of pressure coil capacitance is opposite to that of
inductance. The pressure coil current leads the supply voltage
and causes the errors in reading.
3. Eddy current errors:
In metal parts of the instrument the eddy current get
produced. The eddy current interact with instrument current,
to cause change in the deflecting torque to cause error, hence
metal parts should be kept as minimum as possible. Also the
resistivity of the metal parts used must be high to reduce the
eddy current.

4. Error due method of connection:
There are two ways of connecting wattmeter in a given circuit
Fig.a Fig.b
• There are two possible connections possible based on the nature of load
current, either the main and common can be shorted (called as MC
short) or the load and common terminals can be shorted(called as LC
short).
• For high load current it advised to use the LC short combination as
shown in figure.
• In this case the current measured by the current coil is the load current
plus the current through the voltage coil. The current taken by the
voltage coil is a small percent of the total load current and hence the
error introduced is small.

5. Strong magnetic field error:
Similar to moving coil instrument the operating field in
electrodynamometer instrument is very weak. Hence,
external magnetic can interact with operating field to cause
change in deflection, causing the error. To reduce the effect
of strong magnetic field, the shields must be used for
instrument.
6. Temperature error:
The temperature errors are caused due to the self heating of
the coil, which causes change in the resistance of the coil.
Thus temperature compensating resistors can be used in the
precise instrument to eliminate the temperature errors.

7. Frequency errors:
The changes in the frequency causes to change self
inductance of moving coil and fixed coil. This causes errors in
the readings. The frequency error can be reduced by having
equal time constants for both moving and fixed coil circuits.
8. Friction errors:
As operating force is small, the friction in moving parts also
can causes errors. This can be reduced by using light weight
moving system with minimum friction.

Merits and de-merits of dynamometer type wattmeter
• Merits:
1. They can be used for the measurement of A.C as well as
D.C power.
2. They have uniform scale.
3. By careful design, high accuracy can be obtained.
4. Free from hysteresis errors.
5. Low power consumption.
6. Light in weight.

• Demerits:
1. At low power factor(p.f), the impedance of potential coil
causes serious errors.
2. The reading of the instrument may be affected by strong
magnetic fields(in order to prevent it, the instrument is
shielded from the external magnetic field by enclosing it
in a soft-iron case).
3. They are more expensive than other type of instruments.
4. These instruments are sensitive to overloads and
mechanical impacts. Therefore care must be taken while
handling them.
5. The operating current of these instruments is large due
to the fact that they have weak magnetic field.

Measurement of Three Phase power by two wattmeter method:

• The best method of measuring power of three phase
circuit, Whether the circuit is star connected or delta
connected by using two single phase wattmeter which is
called two wattmeter method of measurement of three
phase power.
• The current coil of the two wattmeter are connected in
any two lines while the voltage coil of each wattmeter is
connected between its own wattmeter coil terminal and a
line without a current coil.
• It can be shown that when two wattmeter's are
connected in this way, the algebraic sum of the two
wattmeter readings gives the total power dissipated in
the three phase circuit.

If W1 and W2 are the wattmeter readings then total power
W= W1+W2= Three phase power= 3 VLILCosφ
Power factor calculation by two wattmeter method:
In case of balanced load, the power factor can be calculated
from W1 & W2 readings.
For balanced, lagging p.f load,
W1= VLILCos(30-φ) &
W2= VLILCos(30-φ)
W1+W2= 3VLILCosφ ----------------------- Eqn-1
W1-W2= VLIL[Cos(30-φ ) – Cos(30+φ )]
=VLIL[Cos30.Cosφ + Sin30.Sinφ - cos30.Cosφ + Sin30.Sinφ]
=VLIL[2 Sin30 Sinφ ]
= VLIL[2 ×
1
2
×Sinφ]
W1-W2= VLILSinφ ------------------------- Eqn-2

Taking ratio of Eqn-1 & Eqn-2
W1
−W2
W1
+W2
=
VLIL Sinφ
3 VLIL cosφ
W1
−W2
W1
+W2
=
tan Ø
3
3(W1
−W2
)
(W1
+W2
)
= tan Ø
Ø = tan-1
3(W1
−W2
)
W1
+W2
Power factor (p.f) = CosØ = Cos tan−1
3(W1
−W2
)
W1
+W2

Problems
1.A balanced Three Phase star connected load draws power from 440V supply the two
wattmeter connected indicate W1=5KW & W2 = 1.2KW, calculate power, power Factor &
current in the circuit.
Given data :
VL=440V , W1 = 5kW , W2 = 1.2kW
Power (P) = ? Power factor (p.f) = ? , Current (I) = ?
Solution :
Power (P) = W1 + W2
= 1.2kW + 5kW
P = 6.2 kW
CosØ = Cos tan−1
3(W1−W2)
W1+W2
= Cos tan−1 3 (5−1.2)
(5+1.2)
= Cos tan−1 (1.06157)
CosØ = 0.6856 Lagging
P = 3VLILCosφ
6.2×103 = 3 ×440 × IL × 0.6856
IL = 6.2×103
3 × 440 × 0.6856
IL = 11.866 Amps

2.If the reading of the two wattmeter connected across the load are 250W &
1.5KW, Determine P.f of the load.
Given : W1 =250, W2=1.5KW=1500W
P.f = CosØ = ?
Solution :
CosØ = Cos tan−1
3(W1
−W2
)
W1
+W2
CosØ = Cos tan−1
3(250−1500)
(250+1500)
CosØ = 0.6286 leading

Calibration of wattmeter by comparison method
using a standard meter:

• As shown in the above circuit diagram Calibration of
wattmeter by comparison method using a standard meter, in
this circuit diagram Wm is under Calibration meter and Ws is
the Standard meter, Standard meter shows the accurate
Power consumptions in the circuit and by comparing the both
the value of the meters , error of the meter can be find and
correct it.
• The % error of the meters can be calculate by following
formula.
%Error=
𝑊 𝑚
−𝑊𝑠
𝑊 𝑠
×100

Energy meter
• An instrument used to measure the energy consumed in a
given circuit is called an Energy meter.
• An electrical energy can also be expressed in the unit
watt-hour(wh) or Kilo watt-hour(Kwh).
• Thus one Kilo watt-hour energy means the expenditure of
1 Kw power over a time interval of 1 hour.
1 Kwh = 1 unit of energy.

Types of Energy meters:
1. Electrolytic energy meters
2. Motor meters
a) Mercury motor meters
b) Commutator meters
c) Induction energy meters
i. 1- energy meters
ii. 2- two element
iii. 3- three element
3. Clock meters
4. Electronic energy meters

• Single phase induction type energy meter:
1- induction watt-hour meters (Energy Meters) are
extensively used for the measurement of electrical energy in
A.C circuit.
An induction watt-hour meter is essentially an induction
wattmeter with control spring and pointer removed but brake
magnet and counting mechanism provided.

Principle :
The basic principle of induction type energy meter
is electromagnetic induction.
When alternating current flow though two suitably
located coils produces rotating magnetic field which is cut by
the metallic disc suspended near to the coils thus EMF induce
in the disc which is circulates eddy current in it. By interaction
of rotating magnetic field and eddy currents, torque is
developed and cause the disc rotate.

Construction:
An induction type single phase energy meter, as show in fig has
following main parts of the operating mechanism.
• Driving system.
• Moving system.
• Breaking system.
• Recording mechanism.

• Driving system.
It consists of two electromagnets, called “shunt” magnet and “series” magnet,
Series magnet: it consists of a number of U-shaped
laminations of silicon steel together to form a core. A coil of
thick wire having a few turns is wounded in both legs of U-
shaped magnet. the coil is known is current coil which is
connected series with load. Produce the magnetic field
proportional and in phase with line current I.
Shunt magnet: it consists of number of M-shaped
laminations of silicon steel assembled together to form a
core. A coil of thin wire having large number of turn in
wound on central limb of the magnet. this coil is connected
across the load. thus it is excited by current proportional to
the supply voltage and known is potential coil.

• 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. 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 kilowatt-hours (kWh).

• Breaking 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. 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.
• Recording mechanism.
The function of recording or registering mechanism is to
record continuously a number on the dial which is
proportional to the revolutions made by the moving system.
the no of revolution on the disc is a measured the electrical
energy passing though the meter.

• Operation :
When the energy meter is connected in circuit, the current coil
carries the load current and pressure coil carries the current
proportional to the supply voltage. the magnetic field produced by
series magnetic in phase with the line current and magnetic field
produced by shunt magnet is in quadrature with the applied voltage.
thus, a phase difference exists between the fluxes produce by the
two coils. this setup rotating magnetic field which interacts with disc
and produce a driving torque and thus, disc starts rotating. the
number of revolutions made by the disc depend upon energy passing
though the meter. the spindle geared to the recording mechanism so
that energy consumed in the circuit is directly registered in kWh.
The speed of the disc is adjusted by adjusting the position of braking
magnet. example, if the energy meter registers less energy than the
energy actual consumed in the circuit. then the speed of disc has to
be increased which is obtained by sifting the braking magnet nearer
to the centre of the disc vice -versa.

When induction watt-hour meter is connected in the circuit to
measure energy, the shunt carries current proportional to the
supply voltage & the series magnet carries the load current.
Therefore,
Driving torque, Td  VCSin
 VI Sin (900 -)
 VI Cos 
 Power
The breaking torque is due to the eddy currents induced in the
aluminium disc. Since the magnitude of eddy current is
proportional to the disc speed, the braking torque will also be
proportional to the disc speed ‘n’ i.e.,

Braking torque, IB  n
For steady speed of rotation, Td = TB
Power  n
Multiplying both side by t,
The for which power is supplied,
Power × t  n × t
Energy  N
Where N(=n×t) is the total number of revolutions in time ‘t’.
The conducting mechanism is so arranged that the meter
indicates(Kwh) directly & not the revolutions.

• Meter constant,
N Energy
N= K Energy
Where, K is a constsnt called meter constant.
K=
N
Energy
=
No.of revolutions
Kwh
• Hence the number of revolutions made by the disc for 1 Kwh
of energy consumption is called meter constant.

Errors and adjustments in single phase
induction type energy meter
Errors:
1. Phase Error
2. Speed Error
3. Frictional Error
4. Creeping
5. Temperature Error
6. Frequency Error
7. Voltage Error

1. Phase Error:
The meter will read correctly only if the shunt magnet
flux lags behind the supply voltage by exactly 90. Since
the shunt magnet coil has some resistance and is not
completely reactive, the shunt magnet flux does not lag
the supply voltage by exactly 90. The result is that the
meter will not read correctly at all power factors.
Adjustment:
The flux in the shunt magnet can be made to lag behind
the supply voltage by exactly 90 by adjusting the
position of the shading coil placed around the lower part
of the central limb of the shunt magnet.

2. Speed Error:
Sometimes the speed of the disc of the meter is either
fast or slow, resulting in the wrong recording of energy
consumption.
Adjustment:
The speed of the energy meter can be adjusted to the desired
value by changing the position of the brake magnet.

3. Frictional Error:
Frictional forces at the rotor bearings & in the
counting mechanism cause noticeable Error
especially at light loads. At light loads, the torque
due to friction adds considerably to the braking
torque. Since friction torque is not proportional to
the speed but is roughly constant, it can cause
considerable error in meter reading.
Adjustment:
In order to compensate for this error, it is necessary
to provide a constant addition to the driving torque
that is equal & opposite to the friction torque.

4. Creeping:
sometimes the disc of the meter makes slow but continuous
rotation i.e., when potential coil is excited but with no-
current flowing in the load this is called creeping. This error
may caused due to over compensation for friction, excessive
supply voltage, vibrations, strong magnetic field etc.
Adjustment:
In order to prevent this creeping two diametrically opposite
holes are filled in the disc, this causes sufficient distortion of
the field. The result is that the disc tends to remain stationary
when one of holes comes under one of the poles of the shunt
magnet.

5. Temperature Error:
Since watt-hour meters are frequency required to operate on
outdoor installations and are subjected to extreme
temperatures, the effects of temperature & their
compensation are very important. The resistance of the disc,
of the potential coil & characteristics of magnetic circuit &
the strength of brake magnet are affected by the changes in
temperature, therefore great case is exercised in the design
of the meter to eliminate the errors due to temperature
variations.

6. Frequency variations:
The meter is designed to give minimum error at a particular
frequency (generally 50 Hz). If the supply frequency changes,
the reactance of the coils also changes, resulting in a small
error. Fortunately, this is not of much signification because
commercial torque are held within close limits.
7. Voltage variations:
The shunt magnet flux will increase with an increase in
voltage. The driving torque is proportional to the first power
of flux where as braking torque is proportional to the square
if the flux.
The small error due to voltage variations can be eliminated by
the proper design of the magnet circuit of the shunt magnet.

Calibration of single phase energy meter

• Before the energy meter is used for measurement of
energy in any circuit, it has to be calibrated, so that the
indicated energy , is approximately equal to the actual
consumed energy, within reasonable limits of error
+5%.
• Connections are made as shown in above figure & the
applying load, note down the energy meter reading in-
terms of revolutions & converted into Kwh (Ei) by
multiplying the energy meter constant & note down
the wattmeter reading & multiplying the wattmeter
constant & time to the wattmeter readings, this will be
in terms of Kwh(Ea). & find The error using formula-
% Error =
𝐸𝑖−𝐸𝑎
𝐸 𝑎
× 100

Problems on errors in energy meter
1. The meter constant of 230V,10A energy meter is 1000rev/Kwh . The meter is tested at
half load and rated voltage at unity p.f. and found to make 40rev in 65sec.Determine
meter error at half load.
Given data :
V=230V , I = 10A
Energy meter Constant (K) = 1000rev/Kwh
Time (t) taken for 40 Revolution = 65 Seconds
Error for half load = ?
Solution:
Half Load (Current)=
10
2
= 5A
Actual Energy consumed (Ea) = V × I × cosØ × t
= 230 × 5 × 1 × 65
= 74750 Watt-seconds
=
𝑤𝑎𝑡𝑡−𝑠𝑒𝑐𝑜𝑛𝑑𝑠
60×60 𝐾𝑤
=
74750
3600 ×103 = 0.04408 Kwh
Energy consumed Showing by meter(Es) =
No.of Revolutions
K
=
40
1000
= 0.04 Kwh
% Error =
𝐸𝑠 −𝐸𝑎
𝐸𝑎
=
0.040 −0.044
0.044
= 𝟎. 𝟎𝟗𝟐𝟓%

2. A 50A , 230V energy meter on full load test makes 61
rev/37 seconds. If the meter constant is 520 rev/Kwh ,
What is the %age error ?
Answer : 0.8 %
3.The meter constant of a 230 V, 10A energy meter is 1800
rev/kwh. The meter is tested at half load & rated voltage &
unity p.f The meter is found to make 80 rev/138 sec.
Determine the meter error at half load.
Answer : 0.817%

Model Questions
• Cognitive Level: UNDERSTAND
1. Explain the construction and operation of a dynamometer type
wattmeter a with neat diagram.
2. Explain the construction and operation of single phase induction
type energy meter with a neat diagram.
3. Describe the calibration of wattmeter with a neat circuit diagram.
4. List the any 3 errors in watt meters.
• Cognitive Level: APPLICATION
1. If the reading s of the two watt meters connected across the load
are 250W and 1.5KW,determinep.f. of the load.
2. The meter constant of 230V,10A energy meter is 1000rev/Kwh.The
meter is tested at half load and rated voltage at unity p.f.and
found to make 40rev in 65sec.Determine meter error at half load.
3. Describe the calibration of energy meter with a neat circuit
diagram.

Unit 03 Construction & Operation of Watt meter & Energy meter

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Unit 03 Construction & Operation of Watt meter & Energy meter