Download 2. electrostatic type voltmeter

ELECTRICAL MEASUREMENT &
MEASURING INSTRUMENT
Guided By: J.B. Patel
PREPARED BY:
Enrollment No.
Name
Three-phase Wattmeter
Electrodynamic Wattmeter
Rectifier Type Voltmeter
Electrostatic Type
Voltmeter
Electrostatic
Type Ammeter
Thermo -couple
PROGRESS CHART:
TOPICS:
1.
2.
3.
4.
5.
6.
Thermocouple
Electrostatic Type Ammeter
Electrostatic Type Voltmeter
Rectifier Type Voltmeter
Electrodynamic Wattmeter
Three-phase Wattmeter
1. THERMOCOUPLE
The thermocouple is widely used temperature sensor in industry.
 Whenever two different types of metals are connected together, a
thermoelectric potential (sometimes called thermoelectric EMF) is
generated across the two free ends the metals according to the
temperature of the joint. This is known as the thermoelectric effect.
 This thermoelectric effect was discovered by Thomas Johan
Seeback discovered in 1821. This thermoelectric EMF is generated
due to the combination of Peltier effect and Thomson effect.
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The EMF generated can be approximately expressed by the
relationship:
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The values of constants a1, a2, a3, etc. depend on the metals A
and B
A Typical Thermocouple
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In thermocouple temperature sensor, the temperature of hot junction is
measured in respect of cold junction of vise varsa.
The thermoelectric potential is generated in a thermocouple instrument
is in range of μV.
Hence, the voltmeter connected to measure the thermoelectric potential
is extremely sensitive and the temperature can be read directly from this
voltmeter, if it is calibrated properly in the scale of temperature.
Thermocouples are a very important class of device as they provide the
most commonly used method of measuring temperatures in industry.
The major reasons behind popularity of thermocouple temperature
measurement are;
1. They are very strong and readings are uniform,
2. They can measure wide range of temperatures,
3. Their characteristics are almost linear with a accuracy of ±0.05%.
Temperature Characteristics for Some Standard Thermocouple Materials
2. ELECTROSTATIC TYPE VOLTMETER
Electrostatic type instruments are almost
always used as voltmeters and that too more as
a laboratory rather than as industrial
instruments.
 The underlying principle of their operation is
the force of attraction between electric charges
on neighboring plates between which a p.d. is
maintained.
oThis force gives a rise to a deflecting torque.
oUnless the p.d. is sufficiently large, the force is
small.
oHence, such instruments are used for
measurement of very high voltages.
There are two general types of such instruments:
1. The quadrant type-used upto 20kV
2. The attracted disc type-used upto 500kV

1. QUADRANT TYPE VOLTMETERS
A light aluminium vane C is mounted on a spindle S and is
situated partially within a hollow metal quadrant B.
 When the vane and the quadrant are oppositely charged by the
voltage under measurement, the vane is further attracted
inwards into the quadrants thereby causing the spindle and
hence the pointer to rotate.

1. ATTRACTED-DISC TYPE VOLTMETERS
It consists of two discs or
plates C & D mounted parallel
to each other.
 Plate D is fixed and is earthed
while C is suspended by a
coach spring, the support for
which carries a micrometer
head for adjustment.
 When a p.d. is applied between
two plates, then C is attracted
towards D but may be returned
to its original position by the
micrometer head.
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3. RECTIFIER TYPE AMMETER
•To deal with current exceeding 5A, use a 5A(1A) meter in
combination with a C.T.
•A.C ammeter is used with a current transformer externally
mounted, and it is noted that primary current of a current
transformer and a full scale of the meter are identical when used.
•The full scale of ammeter are indicated from 5~10000A etc.
•If the circuit voltage exceeds 500V even when the current is less
then 5A use a 5A (1A) meter in combination with a C.T to ensure
insulation.
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The scale range of ammeter for motor
circuit is extended up to 150%~500%
of rated current, so that these meters
are protected from the impact of the
starting current and ensured of long
life.
AM expanded scale has the normal
maximum reading (upper limit of the
effective measurement range) at the
app. 65% point on the scale, and the
rest of it (65%~100%) is devoted to
extension reading (tolerance of
extension area : + 5%)
Angle of deflection: 250o
ANSI C39.1, C12.10
Impulse
voltage
withstand
2.5KV/1min(1.2/50us)
Readily accessible span and zero
adjustments
4. RECTIFIER TYPE VOLTMETER
The AC Volt meter is rectifier type and moving iron type.
 The types of operating principle of voltmeter are the rectifier type
and moving iron type.
 The voltmeter is used with a potential transformer externally
mounted in case of high voltage.
 For higher ranges than 300V, use potential transformer (P.T) with a
150V A.C voltmeter. But the voltmeter of 600V is able to supply
by order of customer.
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When lettering the voltage ratio on
voltmeter, the meters for 3-phase 4wire are the indicating for line-toline voltage by line-to-ground
voltage, and line-to-line voltage by
line-to-line voltage.
Full scale of voltmeter when use the
P.T, the standard calibration value of
voltmeters are 150V and/or 300V,
but the full scale of voltmeters are
indicated the 150V~69KV etc.
Angle of deflection: 250o
ANSI C39.1, C12.10
Impulse
voltage
withstand
2.5KV/1min(1.2/50us)
Readily accessible span and zero
adjustments.
5. ELECTRODYNAMOMETER
 Working
Principle:
“When any current carrying conductor is placed inside a
magnetic field, it experiences a mechanical force and due this
mechanical force deflection of conductor takes place".

There are Two types of coils present in the
electrodynamometer.
1. Moving Coil 2. Fixed Coil
Moving coil:
Moving coil moves the pointer with the help of spring control
instrument. A limited amount of current flows through the
moving coil so as to avoid heating.
 So in order to limit the current we have connect the high value
resistor in series with the moving coil. The moving is air cored
and is mounted on a pivoted spindle and can moves freely.
 In electrodynamometer type wattmeter, moving coil works as
pressure coil.
 Hence moving coil is connected across the voltage and thus the
current flowing through this coil is always proportional to the
voltage.

Fixed coil:
The fixed coil is divided into two equal parts and these are
connected in series with the load, therefore the load current
will flow through these coils.
 Now the reason is very obvious of using two fixed coils
instead of one, so that it can be constructed to carry
considerable amount of electric current.
 These coils are called the current coils of electrodynamometer
type wattmeter.
 Earlier these fixed coils are designed to carry the current of
about 100 amperes but now the modern wattmeter are
designed to carry current of about 20 amperes in order to save
power.
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Control system:
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1.
2.
3.
4.
Out of two controlling systems i.e.
Gravity control
Spring control: Only spring controlled systems are used in
these types of wattmeter. Gravity controlled system cannot
be employed because they will appreciable amount of errors.
Damping system: Air friction damping is used, as eddy
current damping will distort the weak operating magnetic
field and thus it may leads to error.
Scale: There is uniform scale is used in these types of
instrument as moving coil moves linearly over a range of 40
degrees to 50 degrees on either sides.
Advantages of Electrodynamometer Type Wattmeter:
Scale is uniform upto certain limit.
 They can be used for both to measure ac as well dc quantities as
scale is calibrated for both.
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Errors in Electrodynamometer Type Wattmeter:
Errors in the pressure coil inductance.
 Errors may be due to pressure coil capacitance.
 Errors may be due to mutual inductance effects.
 Errors may be due connections.(i.e. pressure coil is connected after
current coil)
 Error due to Eddy currents.
 Errors caused by vibration of moving system.
 Temperature error.
 Errors due to stray magnetic field.
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5. THREE-PHASE WATTMETER
A dynamometer type threephase wattmeter consists of two
separate wattmeter movements
mounted together in one case
with the two moving coils
mounted on the same spindle.
 There are two current coils and
two pressure coils.
 A current coil together with its
pressure coil is known as an
element. Therefore, a threephase wattmeter has two
elements.
 The torque of each element is
proportional to the power being
measured by it.
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The total deflecting torque on the moving system is proportional to
the total power.
 Compensation for mutual effects can be done by using Weston’s
method. Resistance R may be adjusted to compensate for errors
caused by mutual interference.
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