Download wattmeter

Active Learning Assignment
Sub: Electrical Measurement And Measuring Instruments
Topic : Three phase wattmeter and Errors in Wattmeter
Made by:
Clipwala.Azba [130800109013]
Dalwadi.Margesh [130800109014]
Der.Yogesh [130800109015]
Desai.Helly [130800109016]
Guided by:
Asst. Prof. Nishant.Gandhi
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A Dynamometer type three phase
wattmeter consists of two separate
wattmeters mounted together in
which two moving coils are
mounted on the same spindle. The
arrangement is as shown in figure.
In this there are two current coils
and two pressure coils .A current
coil together with it's pressure coil
is known as an element.
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Thus three phase wattmeter has two elements.
The connection of two elements of a three phase wattmeter is as
that for two wattmeter method using single phase wattmeter.
The torque of each element is proportional to the power being
measured by it. The total deflecting torque of the moving system
is the sum of the deflecting torques of the two elements .
So, the deflecting torque of element 1 Td is proportional to P1
,where P1 is the power consumed by wattmeter 1 and deflecting
torque of element two is Td proportional to P2, P2 is the power
consumed by wattmeter 2.
So the total deflecting torque Td is proportional to (p1+p2).
Hence the total deflecting torque of the moving system is
proportional to the total power P.
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In order that the three phase wattmeter read correctly, there
should not be any mutual interference between the two
elements.
The laminated iron shield may be placed between the two
elements to eliminate the mutual effects.
This can be done by using Wheatstone's bridge. The resistance
can be adjusted to compensate the errors caused by mutual
interference.
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Inductance of Pressure Coil
Eddy Currents
Power Loss in Pressure Coil or Current Coil.
Error Due To Friction
Error Due To Heating
Stray Fields
Error due to pressure coil capacitance.
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Inductance of pressure coil may cause an error
in the wattmeter reading. The instrument gives
high reading on lagging pf and low reading on
leading power factor and true power is given by
the expression.
True Power = (cos F)/(cosT (F-T)) × actual
reading of wattmeter
Where cos F is the power factor of the load and
R being the resistance of pleasure coil,
inductance of pressure coil and resistance
connected in series with the pressure coil
respectively.
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The pressure coil circuit may have capacitance also in
addition to inductance. Generally both inductance and
capacitance are present in the pressure coil circuit and,
therefore, cancel the effect of each other.
If the capacitive reactance = inductive reactance in the
pressure coil circuit then the error will be completely
eliminated.
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The alternating magnetic fields of current coil induce eddy currents in
the solid metal parts nearby the current coil. These eddy currents set
up their own magnetic field and thus alter the magnitude and phase
of the magnetic field causing deflection. Thus the error is introduced
in the instrument readings.
Since the error due to eddy currents cannot be determined and may
be serious if no care is taken to minimize it, therefore, solid metal
parts are removed as far away from the current coil as possible. If the
wattmeter current coil is designed for heavy currents, it should
consist of stranded conductors in order to minimize the eddy currents
flowing in the conductors of the current coil itself.
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There are two methods of connecting wattmeter in the circuit for
measurement of power. In one method the pressure coil of the
wattmeter is connected on the supply side of current coil whereas in
second method the pressure coil is connected on load side of current
coil.
In the former method the wattmeter reads the small copper loss in
addition to the load power. To overcome the error because of current
coil carrying the pressure coil current in addition to load current the
standard watt-meters are provided with compensating winding,
connected in series with the potential coil but placed so that it produces
a field in the opposite direction to that of current coil.
Thus magnetic compensation is affected for the amount of power taken
by the potential coil circuit.
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The frictional error becomes relatively important in such an
instrument due to availability of small deflecting torque.
This error is mainly due to pressure coil circuit, which carries
a current proportional to the pd across its terminals and
inversely proportional to its impedance. Such an error is
usually very small.
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External fields may cause serious errors in the ordinary types
of astatic dynamometer wattmeter unless shielded, either by
means of an iron case or by a laminated shield.