Elect measuring

'PARSHVA CLASSES', [FOR ENGG. - DIPLOMA & DEGREE] [[ 1 ]]5/18
ELECTRICAL TECHNOLOGY
MEASURING INSTRUMENTS S.Y.
Mech.
 O
Mob.
FATEHGUNJ BRANCH :
SB-17, EMPEROR COMPLEX,
BESIDE " GOODIES "
FATEHGUNJ,
VADODARA.
SHREE PARSHVAJAY AMBE PRAKASH K. BHAVSAR'S
PARSHVA CLASSES
[ FOR ENGINEERING ]
DEGREE & DIPLOMA MSU & GTU
A NEW NAME OF 'GURU CLASSES'
HEAD OFFICE :
PANKIL CHAMBERS,
KHATRI POLE,
B/H. JUBILEE BAUG,
RAOPURA, VADODARA.
WAGHODIA BRANCH :
3-GAJANAND SOC.,
ABOVE INDIAN OVERSEAS
BANK, UMA CHAR RASTA,
WAGHODIA ROAD, VADODARA.
 Absolute and Secondary Instruments :
 The various electrical instruments is divided into (i) absolute instruments
and (ii) secondary instruments.
 Absolute instruments are those which give the value of the quantity to be
measured, in terms of the constants of the instrument and their deflection
only. No previous calibration or comparision is necessary in their case.
For e.g. tangent galvanometer, which gives the value of current, in terms of
the tangent of deflection produced by the current, the radius and number of
turns of wire used and the horizontal component of earth’s field.
 Secondary instruments are those, in which the value of electrical quantity to
be measured can be determined from the deflection of the instruments, only
when they have been precalibrated by comparison with an absolute instrument.
Without calibration, the deflection of such instruments is meaningless.
 ESSENTIAL OF INDICATING INSTRUMENTS :
Q. Explain essential of indicating instruments.
 An indicating instrument consists essentially of a pointer which moves over
a calibrated scale and which is attached to a moving system, pivoted in
jewelled bearings (occasionally suspended).
 This moving system is subjected to following three torques (forces) :
(1) Deflecting Torque (2) Controlling Torque (3) Damping Torque
[1]Deflecting Torque : The deflecting or operating torque (Td) is required for
moving the pointer from its zero position. The system producing deflecting
torque (force) is called 'Deflecting system' or 'Moving system'.
 The deflecting torque can be produced by effects such as magnetic effect,
electro-magnetic effect, electro-dynamic effect, electro-static effect, thermal
effect, chemical effect.
 Thus, the deflecting system of an instrument converts the electric current or
potential into a mechanical force called deflecting torque (force). Thus, the
the deflecting system acts as the prime-mover responsible for deflection of
the pointer.
[2]Controlling Torque (Restoring Torque) :
 This torque is required in an indicating instrument in order that the currents
produce deflection of the pointer proportional to their magnitudes. The system
producing a controlling force (torque) is called a 'Controlling System'.
 The function of the controlling system is to produce a force equal & opposite
to the deflecting torque at final steady position of pointer in order to make
the deflection of the pointer for a particular magnitude of current. In absence

of controlling systen, the pointer will shoot (swing) beyond the final steady
position for any magnitude of current & thus deflection will be indefinite.
 The second function of controlling system is to bring the moving system
back to zero when the force causing the instrument to deflect is removed.
 The controlling torque can be produced by a weight or springs.
[3] Damping Torque :
 A damping torque (force) is one which acts on the moving system of
instrument only when it is moving and always opposes its motion.
 However the pointer of the instrument before coming to rest, oscillate about
its final position. This because of combination of inertia of the moving system
and the controlling torque which give moving system a natural frequency
oscillations. If no means are provided to prevent these oscillations then these
oscillations die slowly, under steady circuit conditions.
 Ammeters and voltmeters :
[1] Moving-iron type (both for D.C./A.C.) : (a) attraction type, (b) repulsion type
[2] Moving-coil type : (a) permanent-magnet type (for D.C. only)
(b) electrodynamic or dynamometer type (for D.C./A.C.)
[3] Hot-wire type (both for D.C./A.C.)
[4] Induction type (for A.C. only) : (a) Split-phase type, (b) Shaded-pole type
[5] Electrostatic type-for voltmeters only (for D.C./A.C.)
 Wattmeter : [1] Dynamometer type (both for D.C./A.C.),
[2] Induction type (for A.C. only) [3] Electrostatic type (for D.C. only)
 Energy Meters : [1] Electrolytic type (for D.C. only)
[2] Motor Meters : (i) Mercury Motor Meter. For d.c. work only. Can be used
as amp-hour or watt-hour meter.
(ii) Commutator Motor Meter. Used on D.C./A.C. Can be used as Ah or Wh meter.
(iii) Induction type. For A.C. only.
 Clock meters (as Wh-meters).
 PERMANENT MAGNET MOVING COIL INSTRUMENT : [PMMC]
Q. Explain construction, working principle, advantage and disadvantage
of permanent magnet moving coil instrument.
 Principle :
 When a current carrying conductor is placed in a
magnetic field, it experiences a mechanical force which
tends to move it to one side & out of the field.
 Construction :
 Figure indicates construction of PMMC
type instrument. It consists of horse shoe
permanent magnet and the rectangular
coil. This coil is placed between two poles
of permanent magnet in vertical position.
The coil is attached with the spindle & the
spindle is attached with pointer. The two
ends of a spindle are pivoted in jewelled
bearings. There are two springs attached
to spondle for controlling the detlecting
torque. The moving coil is wound with
many no of turns and placed concentrically
between two poles of permanent magnet.

 Working : When PMMC instrument is placed in the circuit, current passes
through the coil & magnetic field (magnetic lines of force) is produced. On
account of interaction between this field & the field of permanent magnet
the spindle and hence pointer gets deflection.
Here, magnitude of permanent magnet is high so eddy current damping is
used. The controlling torque is provided by spring control method.
 Deflecting Torque :
 When currents is passed through the coil, force acts upon its both sides
which produce a deflecting torque as shown in figure.
Let, B = flux density in Wb/m2
l = length or depth of the coil in metre
b = breadth of coil in metre
N = number of turns in the coil
If ‘I’ amp is the current passing through the coil,
then magnitude of the force experienced by each of its sides is = B I l Newton
 For N turns, the force on each side of the coil is = N B I l Newton
Deflecting torque, Td = Force x Perpendicular distance
= N B I l x b = N B I (l x b) = N B I A N-m
Where A is the face area of the coil.
 If B is constant, then Td  I current passing through the coil
Such instruments are invariably spring controlled so that Tc   (deflection).
Since at the final deflected position, Td = Tc    I
 Hence such instruments have uniform scale. Damping is electromagnetic i.e.
by eddy currents induced in the metal frame over which the coil is wound.
Since the frame moves in an intense magnetic field, the induced eddy currents
are large and damping is very effective.
 Advantages :
(1) Their scales are uniform & can be designed to extend over an arc of 170° or so.
(2) They have very effective and efficient eddy current damping.
(3) The power consumption of the instrument is low.
(4) There is no hystersis loss. (because it is never connected to A.C.supply)
(5) The working magnetic field of this type of instrument is very strong. Hence
such instrument are not affected by stray magnetic field.
(6) It requires very small operating current in the range of 15 mA to 50 mA.
(7) It has higher accuracy & less % error.
(8) They possess high torque to weight ratio.
(9) They can be modified with the help of shunts and resistances to over a wide
range of currents and voltages.
(10) Sensitivity is quite high.
 Dis-advantages :
(1) It is very costly instrument.
(2) It never measures the A.C. quantity.
(3) Errors may be introduced due to temperature rise, friction and ageing effects.
Note : Such instruments are mainly used for d.c. work only, but they have been
sometimes used in conjuction with rectifiers or thermo-junctions for a.c.
measurements over a wide range of frequencies.

 Moving-iron Ammeters and Voltmeters :
 There are two basic forms of these instruments i.e. the attraction type and
the repulsion type.
 The operation of the attraction type depends on the attraction of a single piece
of soft iron into a magnetic field and that of repulsion type depends on the
repulsion of two adjacent pieces of iron magnetised by the same magnetic field.
 For both types of these instruments, the necessary magnetic field is produced
by the ampere-turns of a current-carrying coil.
 In case the instrument is to be used as an ammeter, the coil has comparatively
fewer turns of thick wire so that the ammeter has low resistance because it is
connected in series with the circuit. In case it is to be used as a voltmeter, the
coil has high impedance so as to draw as small a current as possible since it is
connected in parallel with the circuit. As the current through the coil is small, it
has large number of turns in order to produce sufficient ampere-turns.
 MOVING IRON-ATTRACTION TYPE AMMETER :
 Principle : When a coil carries current, magnetic field is produced which is
proportional to the current. If a piece of soft iron is pivoted near one end, it
is attracted inside. If pointer is attached to the spindle, it is deflected. The
deflection is proportional to the square of the current.
 Construction :
 Fig. shows constructional details of this type
of instrument.
 It consists of a coil of enamel insulated copper
conductor wound on an insulated bobbin. A
soft iron piece of oval shape is provided near
one end. This makes the deflecting system.
 Gravity control system is shown in which a
control weight is attached at the end of a rod.
Spring control may also be used instead of this.
 The damping system consists of a cylinder
and a piston. The piston rod is attached to
the spindle. The spindle is pivoted in the jewel bearings (not shown in the
fig.) A pointer with its balance weight is attached to the spindle. The pointer
can move on the graduated scale.
 Working : When coil carries current, magnetic field is produced. The soft iron
piece is attracted inside. The controlling system gives the controlling torque.
The air friction damping system damps out the oscillations produced due to
the inertia of the moving system and the system becomes stationary.
 The deflecting force is proportional to the product of the magnetization of
the iron piece and the component of the field strength H acting along the axis
of the piece.
 Since the magnetization  H, the deflecting force H2.
Since H I, the deflecting force or deflecting torque is proportional to the
square of the current passing through the coil. Td  I2
 MOVING IRON-REPULSION TYPE AMMETER :
 Principle : When a coil carries current, magnetic field is produced. If two soft
iron pieces are kept in the coil, the two pieces are magnetized similarly. They
are repelled. If one piece is fixed and the other is attached to the spindle, the
spindle deflects.

 Construction :
 It consists of a coil of enamel insulated copper wire wound on an insulated
bobbin. One soft iron piece of tongue shape is fixed inside the bobbin. A
small soft iron piece is attached on a spindle. A spiral spring is attached on
the spindle to produce the controllingd torque. The damping torque is obtained
by air friction damping system in which an aluminium vane is attached on the
spindle moves in a closed air chamber.
 The spindle is pivoted in jewal bearings. A pointer with balance weight is
attached on the spindle. Fig. shos the construction.
 Working :
 When current passes through the coil,
magnetic field is produced. The two soft iron
pieces are magnetized similarly. This produces
repulsion between the two iron pieces.
 As one piece is fixed, and the other is free to
deflect, the moving iron piece deflects. The
controlling torque is given by the spring. The
moving system tries to become stationary. Air
friction damping arrangement damps out the
oscillations produced due to the inertia of the
moving system.
 The two soft iron pieces are magnetized and
the repulsing force is proportional to the
product of the pole strengths is proportional
to the magnetizing force H, the deflecting force
or torque Id  H2 And the magnetizing force
is proportional to the current, i.e. H  I
 Id  I2
 MOVING IRON FREQUENCY METER :
 Principle : There is moving iron whose position depends on variation in
current distribution between two parallel circuits - one inductive and the
other non-inductive - when the frequecy changes.
 Construction :
 There are two sets of fixed coils A1, A2 and
B1, B2. Coils A1 and A2 are connected in
series. Coils B1 and B2 are also connected
in series. These two sets are kept such that
the fields produced by them are
perpendicular to each other.
 A long and thin soft iron piece is attached
to the spindle and it is kept such that the
piece remains in the magnetic field.
 Damping torque is obtained by air friction
system. There is no controlling device as the
iron piece is under the influence of two fields.
 A pointer with its balance weight is attached to the spindle.
 Set A1-A2 is connected in series with an inductor LA across a non-inductive
resistance RA and set B1-B2 is connected in series with a reistance RB across
an inductor LB. Inductor L is a harmonic filter whih prevents harmonic

componentns of the supply to enter the instrucment. Seris combination of
inductor L, resistance RA and inductor LB is connected across supply.
 Working :
 When frequency is normal, the two sets of coils carry equal currents, so the
fields produced by them are equal in magnitude. The resultant field is at 45°.
The soft iron piece positions its axis parallel to the field. The pointer shows
normal frequency - here 50 Hz.
  If frequency decreases, inductive
reactance of inductor LB decreases.
So voltage drop across LB decreases
and across RA increases. So current
through coils A1-A2 increases which
increases the field A produced by these coilds and the field B produced by
coils B1-B2 decreases. This shifts the resultant field to the left. The iron piece
also moves such that the pointer shows reduced frequency on the scale.
  When the frequency increases, inductive reactance of LB increases which
increase the voltage drop across this so the current through coils B1-B2
increases and hence B increases. Voltage drop across RA decreases and the
inductive reactance of LA increases. This decreases the current through A1-
A2 and hence the field A is reduced. Resultant field shift to the right, the
iron piece moves to the right, the pointer shows higher frequency.
 Moving iron type power factor meter :
 In this, there are three coils A1, A2 and A3 placed 120° apart. These are
supplied from three lines through current transformers. One coil B is placed
at the centre. It is connected across two lines through a series resistance.
 Inside the coil B, there are two sector shaped soft iron pieces I1 and I2
attached to the spindle. Damping vane is attached to the spindle. The spindle
also carries a pointer with balance weight. There is no controlling device.
 Due to the current flowing through the coils A1, A2 and A3, a rotating magnetic
field is produced. The coil B produces alternating flux. Due tod the combined
effect of these two fields the soft iron piece does not tend to rotate
continuously but sets to a definite position which depends upon the phase
angle between the voltage and current vectors.
 Dynamometer-type power factor meter :
 Dynamometer principle is also used for the measurement of power factor.
 A single phase type meter is shown in fig.

 Two fixed coils F1 and F2 are connected
in series and they carry current flowing
through the load. This produces flux .
There are two moving coils M1 and M2
kept 90° apart and are mounted on the
spindle. Coil M1 is connected across
supply through a series resistance R. Coil
M2 is also connected across supply but
through series inductor L.
 Suppose the load power factor is
unit. Current through fixed coil is in
phase with the voltage. Coil M1
carries current which is in phase
with the voltage. Thus, current of
M1 is in phase with the current carried by the fixed coils. The current carried by
coil M2 lags 90° behind the current through the fixed coils. Coil M1 positions
itself such that the flux produced by it is in line with that produced by the fixed
coils. It occupies the position as shown. The pointer indicates unit power factor.
 When power factor is zero lagging, current in coil M2 is in phase with the
main current while current in M1 is 90° out of phase. So, coil M2 will try to
keep its flux in line with that of the main flux. So, it will move clockwise such
that the pointer indicates zero lagging.
 When power factor is zero leading, the torque acting on coil M2 is in opposite
direction, so the pointer shows zero leading.
 VIBRATING-REED OR MECHANICAL RESONANCE TYPE FREQUENCY
METER :
 Principle :
 The meter depends for its
indication on the mechanical
resonance of thin flat steel
reeds arranged alongside and
close to an electromagnet as
shown in fig.
 Construction :
 The electromagnet has a laminated armature and its winding, in series with
a resistance, is connected across a.c. supply whose frequency is required.
In that respect, the external connection of this meter is the same as that of
a voltmeter.
 The metallic reeds (about 4 mm wide and 0.5 mm thick) are arranged in a
row and are mounted side by side on a common and slightly flexible base
which also carries the armature of the electromagnet. The upper free ends
of the reeds are bent over a right angles so as to serves as flags or targets

Synchronization
switch
Incoming
Machine
Moving
Coil
Synchroscope
Three limb
transformer
Busbar
V1
R Y B
Lamp
Pointer
R L I2
V2
R
Y
B
I1
Fixed cool
C
and enamelled white for better visibility. The successive reeds arenot exactly
similar, their natural frequencies of vibration differing by half cycle. The reeds
are arranged in ascending order of natural frequency.
 Working :
 When the electromagnet is connected across the supply whose frequency is
to be measured, its magnetism alternates with the same frequency. Hence
the electromagnet exerts attracting force on each reed once every half cycle.
 All reeds tend to vibrate but only that whose natural frequency is exactly
double the supply frequency vibrates with maximum amplitude due to
mechanical resonance [fig- a].
 The supply frequency is read directly by noting the scale mark opposite the
white painted flag which is vibrating the most (f = 50 Hz). The vibrations of
other reeds would be so small as to be almost unobservable.
 For a frequency exactly midway between the natural frequencies of the
two reeds (f = 49.75 Hz), both will vibrate with amplitudes which are equal
but much less than when the supply frequency exactly coincides with that
of the reeds.
 Advantage : Its indications are independent of the waveform of the applied
voltage and of the magnitude of the voltage, except that the voltage should
be high enough to provide sufficient amplitude for reed vibration so as to
make its readings reliable.
 Disadvantages : (i) it cannot read closer than half the frequency difference
between adjacent reeds. (ii) its error is dependent upon the accuracy with
which reeds can be turned to a given frequency.
 SYNCHROSCOPE :
 For two electrical systems
to be synchronized, both
systems must operate at
the same frequency, and the
phase angle between the
systems must be zero (and
two polyphase systems
must have the same phase
sequence).
 Synchroscopes measure and display the frequency difference and phase angle
between two power systems. Only when these two quantities are zero is it
safe to connect the two systems together. Connecting two unsynchronized
AC power systems together is likely to cause high currents to flow, which will
severely damage any equipment not protected by fuses or circuit breakers.
 Static Part, it consists of three limbed transformer. One of the uter limbs is
excited by busbar voltage V1 while other outer limb is excited by the incoming
machine voltage V2. The central limb carries the lamp. The fluxes produced
by the wo outer limbas are forced through the central limb. The phasor sum
of these two fluxes is the net flux in the central limb. This is responsible to
I2
I1
90°
V1, V2
In phase
90°
I2I1
V1
V2


induced an e.m.f. in the central limb which operates the lamp.
 The outer limb winding are so arranged that if the two voltage V1 and V2 are
in phase, two flux in central limb help each other and maximum e.m.f. gets
induced in central limb. This makes lamp glow with maximum brightness. If
the two voltage V1 and V2 are 180° out of phase, two fluxes in central limb
oppose each other and resultant flux in central limb is zero. Thus no e.m.f. is
induced in it and lamp remains dark.
 Dynamic Part this consist of an electrodynamometer type synchroscope it
consist of fixed coil divided into two parts while the moving coil consists of
a pointer. The fixed coil is connected to busbar with resistor and inductor in
series with it the moving coil is connected to the terminals incoming machine
with a capacitor in series. The inductor and capacitor are used in fixed and
moving coil circuit respectively because when the two voltages are in phase
then due to L and C, the two currents are in exact quadratuer (90°) to
each otherthus no ttorque will act, on the pointer the current I1 lags V1
while I2 leads V2 such hat I1 and I2 are in quadrature. If V1 and V2 are 180
out of phase, still the currents I1 and I2 will be is quadrature and pointer
remain stationary.
 Dynamometer Wattmeter :
 An electrodynamic instrument is a moving-coil instrument in which the
operating field is produced, not by a permanent magnet but by another
fixed coil. This instrument can be used either as an ammeter or a voltmeter
but is generally used as a wattmeter.
 As shown in Fig., the fixed coil is usually arranged in two equal sections F
and F placed close together and parallel to each other. The two fixed coils
are air-cored to avoid hysteresis effects,when used on a.c. circuits. This has
the effect of making the magnetic field in which moves the moving coil M,
more uniform. The moving coil is spring-controlled and has a pointer attached
to it as shown.
 The connections of a dynamometer type wattmeter are shown in Fig. The
fixed circular coil which carries the main circuit current I1 is wound in two
halves positioned parallel to each other. The distance between the two halves

can be adjusted to give a uniform magnetic field. The moving coil which is
pivoted centrally carries a current I2 which is proportional to the voltage V.
Current I2 is led into the moving coil by two springs which also supply the
necessary controlling torque. The equivalent diagrammatic view is shown in fig.
 Advantages :
By careful design, such instruments can be built to give a very high degree of
accuracy. Hence they are used as a standard for calibration purposes. They
are equally accurate on d.c. as well as a.c. circuits.
 Disadvantages :
At low power factors, the inductance of the voltage coil causes serious error
unless special precautions are taken to reduce this effect.

Elect measuring

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