Download measuring instruments

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ELECTRICAL
AND
ELECTRONICS INSTRUMENTS
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MEASURING INSTRUMENTS
instruments which is used for measurement
Necessary requirements of measurement
Circuit condition and the quantity to be measured should
not be altered, if measuring circuit is connected.
Power consumption by instrument for their operation
should be small
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Instruments used for measure
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Voltage – voltmeter
Current – ammeter
Power - wattmeter
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CLASSIFICATION OF MEASURING INSTRUMENTS
Indicating instruments
Recording instruments
Integrating instruments
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Indicating instruments:
Instruments used for indicating or pointing the unknown
quantity with the help of pointer or dial.
example: voltmeter, ammeter, etc.,
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Recording instruments:
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It gives a continuous record of electrical quantity to be
measured over a specific period.
Examples: XYXY- plotter
Readings are recorded by drawing graph.
Pointer of such instrument is provided with pen or pencil
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INTEGRATING INSTRUMENTS
to measure the total quantity of electricity delivered over a
period of time.
Example: energy meter
It consists of dials and pointers
Counting mechanism registers number of revolutions made by
the disc to measure the total quantity of electricity delivered
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Essential requirements of indicating instrument
Deflecting system should produce deflecting torque; Td
Controlling system should produce controlling torque; Tc
Damping system should produce damping torque;
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TYPES OF INSTRUMENTS
Permanent magnet moving coil (PMMC)
Moving iron
Electro – dynamometer
Hot wire
Thermo couple
Induction
Electro static
Rectifier
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PMMC
Most accurate type for dc measurements
Principle:
When a current carrying conductor is placed in the
magnetic field produced by the permanent magnet , coil
experiences a force and moves.
As the coil is moving and the magnet is permanent, this
instrument is called as permanent magnet moving coil
instrument.
This principle is called as D’ Arsonal principle
force experienced is proportional to the current flowing in
coil.
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PMMC construction
It consists of
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Moving coil
Magnet systems
Control
damping
Pointer and scale
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PMMC INSTRUMENT
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MOVING COIL:
Many number of turns of fine wire
either rectangular or circular in shape
Moves freely in the field of permanent magnet
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Magnet systems
Control:
Controlling torque is provided by two phosphor
bronze hair springs
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Damping
Damping torque is produced by the aluminium former
moving in the magnetic field of the permanent magnet.
Pointer and scale:
Carried by spindle over a graduated linear scale
Light eight so that it deflect rapidly
Mirror is placed below the pointer to reduce to get accurate
reading to remove parallax error.
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TORQUE EQUATION
Let B = flux density in air gap
N = no of turns in the coil
L= active length of coil in meters
d = average width of coil in meters
I= coil current
F = force in newton
force acting on one side of coil =F
ie F = BILN newton
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Deflecting Torque Td = Force X radius X 2
= BILN d/2 X 2 taking
account force on both sides
= BI . L N d Nm
put L X d = area A
then Td -= NBAI Nm
Control torque Tc = some constant X 0.
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the constant depends on qualities of the spring.
under steady – state condition Tc = Td
Therefore
is proportional to I (BAN are Constants)
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ADVANTAGES OF PMMC INSTRUMENTS
linear scale
Low power consumption
High accuracy
Torque to weight ratio is high
Single instrument may be used for different ranges
Errors due to stray magnetic fields are small
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DISADVANTAGES OF PMMC INSTRUMENTS
Instruments are used only for dc
Higher cost compared moving iron instruments
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Errors in PMMC instruments
Basic sources of errors in instruments are
Weakening of permanent magnets due to ageing and
temperature effects
Weakening of springs due to ageing and temperature
effects
Change of resistance of the moving coil with temperature.
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MOVING IRON INSTUMENTS
Use at power frequencies
Used to measure current and voltage accurately
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There are two types of moving iron instruments
Attraction type
Repulsion type
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Attraction type moving iron instrument
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Construction of moving iron attraction type instruments
It consists of
Moving iron
Scale
Piston
Coil winding
Air damping chamber
Pointer
Balance weight
Control weight
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Attraction type moving iron instrument
Torque equation:
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Force pulling the soft iron piece inward depends on
H, magnetic field strength produced by the coil.
m , magnetic pole strength of the soft iron piece
however m depends on H, magnetic field strength of the
coil
So force , F α mH α H2
And deflection torque Td α F α H2
Since H depends on current ,
Td α I2
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When spring control is used,
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Control torque α angular displacement
Thus Td α θ
At steady state deflection ,Td = Tc
i.e.., I2 α θ
Or θ α I2
Deflection α (rms value)2
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REPLUSION TYPE MI
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Comparison between attraction type and repulsion type:
Attraction type
Repulsion type
Lower inductance
Higher inductance
Accurate over wide
range of frequency
Greater possibility of
using shunt
less Accurate over wide
range of frequency
Economical in nature,
lesser possibility of using
shunt
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ADVANTAGES OF MOVING IRON
INSTRUMENTS
It can be used for d.c and a.c(universal use)
Less friction errors
Low cost
High deflecting torque
Accuracy
Simple in construction
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DISADVANTAGES OF MOVING IRON
INSTRUMENTS
Scales are not uniform
Power consumption is high at low voltage
measurements
Stiffness of spring decreases with rise in temperature
Errors are introduced due to Hysteresis and magnetic
fields
Changes in supply frequency cause serious errors in
reading
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ERRORS IN MOVING IRON INSTRUMENTS
The causes of errors in MI may be divided as
(I) Errors when used in a.c only
(ii) Errors when used in both a.c and dc
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Errors in a.c only
1 Change in impedance of coil
2. Change in eddy current magnitudes
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ELECTRO DYNAMOMETER INSTRUMENT
Used as voltmeters and ammeters at power frequencies
Capable of service as transfer instrument device
Principle:
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CONSTRUCTION OF ELECTRO
DYNAMOMETER INSTRUMENT
Fixed coils
Moving coil
Control
Moving system
Damping
Shielding
Cases and scales
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ELECTRO DYNAMOMETER INSTRUMENT
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Torque equation of electro dynamometer instruments
Let
I1= instantaneous value of current in the fixed coils ;A
I2 = instantaneous value of current in the moving coils;
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Force or torque on a moving system α flux x current
Air gap flux is directly related to I1 since fixed coil is air
cored.
Then deflecting torque Td α I1 I2
in voltmeter I1 = I2 = I say
θ α I2
In ammeter, θ α I1 I2
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Advantages of electro dynamometer instruments
Free from hysterisis and eddy current errors
Accuracy
Used for both a.c and d.c
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Disadvantages of electro dynamometer instruments
Low torque/weight ratio
Low sensitivity
Increased frictional losses
More expensive than PMMC
Sensitive to overloads and mechanical impact
Non uniform scale
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INSTRUMENT TRANSFORMERS
transformers used in conjunction with the measuring
instrument for measurement purpose are called instrument
transformers.
Transformers used for measuring current are called current
transformers.
Transformers used for measuring voltage are called potential
transformers.
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CURRENT AND POTENTIAL TRANSFORMER
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Advantages
The readings do not depend upon the values of (R,L,C) and
also number of instruments connected in the circuit.
Very cheap moderate rating can be used for measurement of
high voltages and current.
Replacement of these transformers are very easy
Measuring circuit is isolated from the power circuit
Low power consumption
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Ratios of current transformers
Transformation ratio:
It is the ratio of the magnitude of the primary phasor to
the secondary phasor.
R = primary phasor/secondary phasor
For C.T , R = primary winding current/secondary winding
current
For P.T, R = primary winding voltage/secondary winding
voltage
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Nominal ratio
It is the ratio of the rated primary winding current (or voltage)
to the rated secondary winding current (or voltage).
For C.T ,
Nominal ratio Kn = rated primary winding current / rated
secondary winding current
For P.T,
Nominal ratio Kn = rated primary winding voltage/ rated
secondary winding voltage
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Turns ratio
Turns ratio for C.T, n = number of turns of secondary
winding/number of turns of primary winding.
Turns ratio for P.T, n = number of turns of winding primary
/number of turns of secondary winding.
Ratio correction factor:
The ratio of correction factor is the transformation
ratio divided by nominal ratio.
transformation ratio = ratio correction factor x nominal ratio
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Current transformers
Connected in series with line current to be measured.
Primary winding consists of very few turns and no
appreciable voltage drop.
Secondary winding has large no. of turns
Ammeter or wattmeter directly connected across the secondary
winding.
One of the secondary terminals earthed for protection of
equipment and insulation breakdown in the auto transformer.
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Construction of current
transformers
Classification
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Wound type
Bar type
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Effect of secondary open circuit
If secondary is open circuited,
Large voltage is induced in the secondary winding
It is dangerous to the transformer insulation and the
person who has opened
Eddy current and hysterisis losses are more due to that
transformer overheated and damaged. If not core
become permanently magnetized and this
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Will give appreciable ratio and phase angle errors.
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Potential transformers
Primary winding is connected across the line carrying voltage
to be measured.
Voltage circuit is connected across the secondary winding
Normal secondary voltage rating is 120 V.
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Difference between C.T and P.T
POTENTIAL
TRANSFORMER
CURRENT
TRANSFORMER
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Parallel transformer, operating
under nearly open circuit
condition
Series transformer, operating
under
Virtual short circuit condition
Primary winding current in C.T
is independent of secondary
circuit condition.
Exciting current varies in
restricted range.
Primary winding current in P.T
is dependent of secondary
circuit burden
Exciting current varies over
wide limit range under norma
operation.
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CONSTRUCTION OF P.T
It consists of
Core
Windings
Insulation
Bushings
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High voltage P.T
Recently Potential transformer are designed which has resulted
In considerable reduction and cost of transformers.
Two designs developed to eliminate high voltage lead in
bushings .
Bushing elimination leads reduction in size and cost of
transformers.
These designs are intended to measure line to ground voltages
in three phase system. These desgns employ:
Insulated casing
Moulded rubber potential transformer
Cascaded transformer
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Measurement of frequency and phase
Types of frequency meters:
Mechanical or resonant type
Electrical or resonant type
Electrodynamometer type
Weston type
Ratio meter type
Saturable core type
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Mechanical resonant type frequency meter
(vibrating reed type)
Consists of number of thin metal strips called reeds.
When the frequency meter is connected across the supply,
whose frequency is to be measured, the coil of electromagnet
carries a current I which alternate at the supply frequency.
The force attraction between the reeds and the electromagnet
is proportional to i2 and therefore this force varies at twice the
supply frequency.
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Advantage of vibrating reed type meter
Indication is independent of waveform of supply voltage.
Indication is independent of magnitude of applied voltage.
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Disadvantage of vibrating reed type meter
Cannot be used for precision instruments
Reliability of readings depend upon the accuracy
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Electrical resonance type frequency meters
Types :
Ferro dynamic frequency meter
Electro dynamometer type frequency meter
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MAGNETIC MEASUREMENTS
Requirements in magnetic measurements are:
Measurement of magnetic field strength in air
Determination of BB-H curve and hysterisis loop for soft
ferro magnetic materials.
Determination of eddy current and hysterisis losses for soft
ferro magnetic materials subjected to alternating magnetic
fields.
Testing of permanent magnets
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Types of tests
Ballistic tests
A.C testing
Steady state tests
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Ballistic test
Used for
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Measurement of flux density
Determination of BB-H curves
Plotting of hysterisis loop
Measurement of flux density:
Can be done by winding a search coil over the specimen. this search
coil is known as B coil.
Coil is connected to a galvanometer or flux meter.
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Measurement of value of magnetizing force
It can be measured by using ballistic galvanometer and a
search coil.
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Magnetic potentiometer
Device for measurement of potential difference
Basis of magnetic potentiometer
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Line integral of magnetising force H produced by a coil of N
concentrated turns carrying current I isaround any losed path of the
linking coil.
Hdl = NI
This is called circuital law.
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Magneto potentiometer consists of
1m long flat and uniform coil(2 or 3 layers)
Wound on a strip of nonmagnetic material
Coil ends are brought out at middle of strip
This is connected to ballistic galvanometer
Let
A = are of the strip
n = number of turns of the strip
H1 = tangential component of the magnetising force
R = resistance of the galvanometer circuit
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Determination of BB-H curve
Methods to find:
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Method of reversal
Step by step method
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Method of reversal
Thin tape is put on the ring and search coil insulated by Para
finned wax is wound over the tape.
Another layer of tape is put on the search coil and magnetizing
winding.
Procedure:
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Set the magnetizing current value to lowest test value
Close galvanometer key K, the iron specimen is brought into
reproducible cyclic magnetic state by throwing the switch S backward
and forward by 20 times
Open the key K ,measure H by reversing the switch S
Value of flux density is calculated for corresponding value of H
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Step by step
Circuit diagram is shown above
Construction:
Magnetizing winding is supplied through potential divider
Potential divder is having no . Of tappings
Specimen before testing is magnetized
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Procedure for Step by step method
Close Switch s1 set S2 in tapping 1.
Throw of the galvanometer, note the value of B1.
Calculate H1 from the current flowing in magnetizing winding
at tap1.for the corresponding value of B.
Set S2 to tap 2 to increase H1 to H2.
Determine ∆B from the throw of the galvanometer.
Calculate B2 corresponding H2 is B1 + ∆B
Repeat the process until H reaches maximum value.
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Determination of hysterisis loop
Step by step method
Same procedure is followed until H reaches maximum
point at tap 10 as explained before.
Reduce the magnetizing current in steps to zero by moving
tapping to 9,8,7,…..,3,2,1.
After magnetizing current to zero, to obtain negative H
Reverse the supply of potential divider
Move the switch S2 up in the range again 1,2,….,10
Note down the values of B and H and draw the loop.
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Determination of hysterisis loop
Method of reversals:
Note the value of +Bm to lower value
Iron specimen is passed through the remainder of the cycle
of magnetization back to flux density Bm.
Cycle of magnetization is preserved.
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Method of reversal
Procedure:
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Connections are given as in circuit diagram.
R1, R2 and R4 are resistances in magnetizing winding
R3 = variable shunting resistance connected across
magnetizing winding
Form the BB-H curve as explained earlier.
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Measurement of Iron loss
Three types of methods
Wattmeter method
Bridge method
Potentiometer method
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Wattmeter method
Most common used method
Material to be tested is assembled in the form of square.
There are two forms of forming square
Epstein square
Lloyd fisher square
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Epstein square
Four stacks of strips
Stacks are bound and insulated
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Lloyd
Lloyd-- fisher square method
Most commonly used magnetic square
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