EMMI LAB MANUAL_correct.pdf

8/18/2019 EMMI LAB MANUAL_correct.pdf

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Dept. of Electrical & Electronics Engg. College of Engineeri ng, Kidangoor

Exp. No:

Date:

CALIBRATION OF THREE PHASE ENERGYMETER

AIM

To calibrate the given three phase static energy meter at upf

(i) By direct loading.

(ii) By phantom loading

APPARATUS REQUIRED

SL NO. APPARATUS SPECIFICATION QUANTITY

1. Energy meter 1no.

2. Wattmeter 2no.

3. Voltmeter 1no.

4. Ammeter 1no.

5. 3φ autotransformer 1no.

6. 3φ r esistive load 1no.

7. Rheostat 1no.

8. Stop Watch 1no.

< Students are expected to calculate instrument ranges based on the machine ratings before start of

experiment and get it approved before connections are made>


2/41


CIRCUIT DIAGRAM

(i) Direct Loading

Fig. 1

(ii) Phantom Loading

Fig. 2


3/41


PRINCIPLE

In order to check the calibration of a three phase energy meter, reading of energy meter has

to be compared with that of standard instrument. For determining the true energy consumption a

standard wattmeter and an accurate stop watch is used. From the calculated true energy, the error and

percentage error in the energy meter is determined.

(i) Direct Loading

In direct loading, current coil of energy meter and wattmeter are connected to a three phase

supply in series with a loading device, where as in each phase pressure coils of energy meter are

connected altogether to neutral wire. The pressure coils of two wattmeters are connected to Y phase.

By adjusting the auto transformer take the reading of voltmeter which connected in between two phases

as 415V. Supply is given to the circuit. Then by adjusting the loading device required current had got

on the ammeters. Then energy consumption (measured power) is got by observing the time taken for

3 revolutions of energy meter. True energy is calculated from wattmeter reading and time indicated inthe stop watch.

Measured Power = ∗ ∗1000 Watts

K = Energy meter constant in rev/kWh

N = Number of revolutions made by energy meter disc

t = Time for N revolutions of energy meter disc

% error =( . . . ∗ 100) , where M.P is measured power and T.P is true power.(ii) Phantom loading

Calibration is done by phantom loading of three phase energy meter. In phantom loading

using standard wattmeter, the current coil is fed from a low voltage supply and pressure coil

by rated voltage. Hence total power required for conducting the test is small.

Measured Power = ∗ ∗1000 Watts

K = Energy meter constant in rev/kWh

N = Number of revolutions made by energy meter disc

t = Time for N revolutions of energy meter disc

% error =( . . . ∗ 100) , where M.P is measured power and T.P is true power.


4/41


PROCEDURE

(i) Direct Loading

Connections are made as shown in the fig (1). Autotransformer is kept at minimum position

and the supply is switched on. Autotransformer is adjusted to apply rated voltage in the circuit. Then by adjusting the loading devices the required current readings are made on ammeters. Different

Wattmeter readings and the time taken for 3 revolutions of the energy meter using a stopwatch are

noted. The true reading is obtained from the wattmeter reading.


Connections are made as shown in fig (2). Autotransformer is kept at minimum position and

the supply is switched on. Autotransformer is then varied and different currents are passed through

the current coil of the energy meter. All the meter readings, wattmeter reading for all currents and

time taken for three revolutions of the disc are noted.

OBSERVATION

(i) Direct Loading

Sl

no.

Voltmeter

Reading

(V)

Ammeter

Reading

(A)

W1

(W)

W2

(W)

T.P=

W1+W2(W)

Time for 3

revolutions

(sec)

M.P

(W)

Error=M.P - T.P

(W)

% error=.−.

. ×1


5/41



l

o.

Vol

tme

ter

Readin

g

(V)

Am

mete

r

Reading

(A)

W1

(W)

W2

(W)

T.P=

W1+W2

(W)

Time

for 3

revol

utions

(sec)

M.P

(W)

Error

=

M.P-

T.P(W)

% error=

.−.

.×100

Power

consumed by

Wattmeter

Power

consumed byEnergymeter

Total

Power

=

T.P+C.C+

P.C(W)

C.C (W)

P.C (W)

C.C (W)

P.C (W)

SAMPLE CALCULATION

(i) Direct Loading

Time for 3 revolutions of energy meter disc = ------------------sec

Energy meter constant, K= ---------------------rev/kWh

Measured Power = ∗ ∗1000 Watts = -------------------W

True Power = W1+W2 = ---------------W

% error = ( . . . ∗ 100)

(ii)

Phantom Loading

Time for 3 revolutions of energy meter disc = ------------------sec

Energy meter constant, K= ---------------------rev/kWh

Measured Power = ∗ ∗1000 Watts = -------------------W


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True Power = W1+W2 = ---------------W

% error = ( . . . ∗ 100) = ----------------Power loss in wattmeter current coil = 2*I2R = --------------W

Power loss in wattmeter pressure coil = 2*V2/R= ----------------W

Power loss in energy meter current coil = 2*I2R = --------------W

Power loss in energy meter pressure coil = 2*V2/R= ----------------W

Total Power = T.P +C.C+P.C = --------------W

Power Savings = True power obtained from direct loading- Total power consumed in

phantom Loading

= ------------W

RESULT

The given three phase energy meter is calibrated at upf by direct loading and phantom loading.

INFERENCE

Exp No:

Date :


7/41


CALIBRATION OF SINGLE PHASE ENERGYMETER

AIM

To calibrate the given single phase static energy meter at upf by direct loading and phantom

loading using standard wattmeter and draw the error and calibration curve.

APPARATUS REQUIRED



2. Wattmeter 1no.

3. Voltmeter 1no.

4. Ammeter 1no.


6. Rheostat 1no.

7. Resistive load 1no.

8. Stop Watch 1no.



CIRCUIT DIAGRAM

(i) Direct Loading

Fig. 1


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(ii)

Phantom Loading

Fig. 2

PRINCIPLE

In order to check the calibration, single phase energy meter is compared with that of standard

instrument. For determining the true energy consumption a standard wattmeter and an accurate stop

watch is used. From the calculated true energy the error and percentage error in the energy meter is

determined. The given energy meter can be calibrated for different loads by comparing the measured

value of power obtained from it and the wattmeter reading. The wattmeter gives the true power.

In direct loading, the load voltage appears across the pressure coil. Hence the energy meter

and wattmeter reads the actual value of energy and power dissipated in the load.

Phantom loading is employed for testing energy meter of high capacity. The phantom loading

method is usually used for calibration of single phase or three phase energy meter. Here external loadis not connected and the current and pressure coils are supplied separately so that it will consume

only less power. In this connection the voltage across pressure coil will be supply voltage even if the

autotransformer is in minimum position and current coil is supplied from a separate low voltage

supply.

For checking the calibration of energy meter at upf, the current coils of energy meter and

wattmeter are connected in series to the supply(R phase) through an autotransformer, which reduces

the voltage to a low value. The pressure coils are connected directly to the supply to the same phase(R

phase).

Energy meter constant, K= 900 rev/KWh which means

900 revolutions = 1 kWh

= 1*1000*3600

ie, one revolution = 1000∗3600 900 = 4000 W


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For ‘N’ revolutions, power = 4000 N W

Time ‘t’ in second is required for N revolutions.

Power expected in one second = 4000 WTrue power = Wattmeter reading

Measured Power = 4000 W% error =( . . . ∗ 100)

PROCEDURE

(1)

Direct loading of energy meter at upf condition.

Made the connections as shown in the fig(1). Supply is switched on and apply full voltage (230 V)

by using autotransformer while keeping the load resistance in the circuit as minimum as possible. Switch

on the load and the voltmeter, ammeter, wattmeter readings and time taken for 3 revolutions of the

energy meter are noted. Experiment is repeated for various load currents up to rated value.

(2) Phantom loading of energy meter at upf condition.

Keep the autotransformer in minimum position and switch on the supply. Keep the rheostat at

constant value throughout the experiment. Give a voltage so that the ammeter readings do not exceed

the rated current. Take readings for different current by adjusting the autotransformer.

OBSERVATIONS

(1) Direct loading

Sl

No.

Voltmeter

Reading

(V)

Ammeter

Reading

(A)

True

Power

(W)

Time for 3revolutions

(Sec)

Measured

Power

(W)

Error=

M.P-T.P

(W)

% error=

.−.. ×100


10/41


(2) Phantom loading

Sl

No.

Volt

meter

Read

ing(V)

Ammet

er

Readin

g(A)

True

Power

(W)

Time

for 3revoluti

ons(Sec)

M.P

(W)

Error=

M.P-

T.P

(W)

% error=.−.

. ×100Power

consumed by

wattmeter

Power

consumed by

energymeter

Total

power (W)=

T.P+C.C+P.C

C.C

(W)

P.

C

(W)

C.C

(W)

P.C

(W)

SAMPLE CALCULATION

(i)

Direct Loading

True Power = Wattmeter reading = ---------------------W

K= 900 rev/KWh

Number of revolution of energy meter disc, N = 3

Time for 3 revolutions of energy meter disc, t = --------------sec

Measured power =∗∗

∗ = -------------------W

% error = ( . . . ∗ 100) = (ii) Phantom Loading


K= 900 rev/KWh




∗ = -------------------W


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% error = ( . . . ∗ 100) = Power loss in wattmeter current coil = I2R = --------------W

Power loss in wattmeter pressure coil = V2/R= ----------------W

Power loss in energy meter current coil = I2R = --------------W

Power loss in energy meter pressure coil = V2/R= ----------------W

Total Power = T.P +C.C+P.C = --------------W

Power Savings = True power obtained from direct loading- Total power consumed in

phantom Loading

= ------------W

RESULT

The single phase energy meter is calibrated at upf by direct and phantom loading.

INFERENCE


12/41


Exp No:

Date :

CALIBRATION OF SINGLE PHASE ENERGYMETER AT 0.5 pf

AIM

To calibrate the given single phase static energy meter at 0.5 pf lag and lead by phantom

loading.

APPARATUS REQUIRED



2. Wattmeter 1no.

3. Voltmeter 1no.

4. Ammeter 1no.


6. Rheostat 1no.

7 Stop Watch 1no.



CIRCUIT DIAGRAM0.5 pf lag

Fig. 1


13/41


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(i) 0.5 pf lag

Connections are made as shown in the fig (1). Supply is switched on and apply full voltage (230 V)

by using autotransformer while keeping the load resistance in the circuit as minimum as possible. Switch

on the load and the voltmeter, ammeter, wattmeter readings and time taken for 3 revolutions of the

energy meter are noted. Experiment is repeated for various load currents up to rated value.

(iii) 0.5 pf lead

Connections are made as shown in the fig (2). Supply is switched on and apply full voltage (230

V) by using autotransformer while keeping the load resistance in the circuit as minimum as possible.

Switch on the load and the voltmeter, ammeter, wattmeter readings and time taken for 3 revolutions

of the energy meter are noted. Experiment is repeated for various load currents up to rated value.

OBSERVATIONS

(i) 0.5 pf lag

Sl

No.

Voltmeter

Reading

(V)

Ammeter

Reading

(A)

True

Power

(W)

Time for 3

revolutions

(Sec)

Measured

Power

(W)

Error=

M.P-T.P

(W)

% error=

.−.. ×100

(ii) 0.5 pf lead

Sl

No.

Voltmeter

Reading

(V)

Ammeter

Reading

(A)

True

Power

(W)

Time for 3

revolutions(Sec)

Measured

Power

(W)

Error=

M.P-T.P

(W)

% error=

.−.. ×100

SAMPLE CALCULATION

(i) 0.5 pf lag


15/41



K= 900 rev/KWh




∗ = -------------------W

% error = ( . . . ∗ 100) = (ii) 0.5 pf lead


K= 900 rev/KWh




∗ = -------------------W

% error = ( . . . ∗ 100) = RESULT

The single phase energy meter is calibrated at 0.5 pf lag and lead by phantom loading.

INFERENCE

Exp No:

Date :

CALIBRATION OF SINGLE PHASE ENERGYMETER AT 0.866 pf


16/41


AIM

To calibrate the given single phase static energy meter at 0.866 pf lag and lead using phase

shifting transformer.

APPARATUS REQUIRED



2. Wattmeter 1no.

3. Voltmeter 1no.

4. Ammeter 1no.


6. Rheostat 1no.

7 Stop Watch 1no.



CIRCUIT DIAGRAM

(i) 0.866 pf lag

` Fig.1

(i) 0.866 pf lead


17/41


Fig. 2

Exp. No:

Date:


18/41


MEASUREMENT OF SELF AND MUTUAL INDUCTANCE

AIM:-

To determine self inductance, mutual inductance and coefficient of coupling of coupled coil.

APPARATUS REQUIRED

SL NO APPARATUS SPECIFICATION QUANTITY

1. Iron cored coils (Transformer 1φ) 1no.

Voltmeter 1no.

3. Voltmeter 1no.

4. Voltmeter 1no.

5. Ammeter 1no.

6. Ammeter 1no.

7. Autotransformer 1no.

8. DC regulated power supply 1no.

9. Rheostat 1 no.



CIRCUIT DIAGRAM

Additive Polarity

Fig. 1

Subtractive Polarity


19/41


Fig. 2

To measure resistance of Primary coil

Fig. 3

To measure resistance of Secondary coil

Fig. 4

PRINCIPLE


20/41


Inductance is the property of a circuit element by which energy is capable of being stored in

a magnetic flux field and any circuit element exhibit the property of inductance is called an inductor.

Self Inductance of a coil is the property by which it opposes any flux through it. Mutual

inductance of a coil is the ability to produce an EMF in the neighboring coil by induction, when the

current in the first coil changes.

Consider two magnetically coupled coils of self inductance and . Let M be the mutualinductance of the coils connected in series so that flux is produced by current I through the coils are

in the same direction, then the effective inductance

= + + 2MIf coils are connected such that the flux produced by the current in opposite direction, then

effective inductance

= + - 2MTherefore mutual inductance M= ( - ) / 4Coupling Coefficient k = M / √ In the first case, if and are the applied voltage and current, then = / , = √ , = 2⁄ , where is the DC resistance of primarycoil.

Similarly for the second case

= / , = , = 2⁄ , where is the DC resistance of secondarycoil.

/ = / )2 From the above equations , , M and k can be found out. The experimental determinations ofthe above parameters are carried out for a pair of transformer winding.

PROCEDURE


21/41


For determining the inductance of transformer coil, connections are made as shown in figure

(1) when the windings are connected in series for additive polarity. Supply is switched on and the

rated voltage is applied in the circuit by adjusting the auto transformer. The Voltmeter and Ammeter

readings are noted. Then the terminals of the coils are interchanged as shown in the figure (2) for

subtractive polarity. Auto transformer is adjusted to give a voltage to the primary and all other

readings are taken.(It is desirable to keep V2 constant for both aiding and opposing).

To measure the resistance of the coils, connections are made as shown as figure (3) and (4)

and dc supply is switched on. The rheostat is adjusted for different values of Ammeter and Voltmeter

reading. R = ⁄ After obtaining the voltages and currents, inductance , , M and coupling coefficient k

for a pair of transformer windings are determined.

OBSERVATION

Aiding Circuit

Z A(Ω)

Opposing Circuit

Z B(Ω)

V(V) V 1(V) V 2(V) I(A) V(V) V 1(V) V 2(V) I(A)

Resistance Measurement

Transformer PrimaryWinding

Transformer SecondaryWinding

V(V) I(A) R P (Ω) V(V) I(A) RS (Ω)

Sample Calculation


22/41


= / = ------------- = √ = ------------- = 2⁄ = ------------- = / = -------------

= = ------------- = 2⁄ = -------------M = ( - ) / 4 = -------------

= + + 2M = -------------

= + - 2M = ------------- + = 2 ( + = -------------( + = ( + ) / 2 = -------------

/ = / )2 = / )2 From the above equations the values of , , M and k are calculated.

RESULT

Self inductance, Mutual inductance and Coefficient of coupling for a pair of transformer

windings are determined.

Self inductance of coil 1, = -------------Self inductance of coil 2 , = -------------Mutual inductance , M = -------------

Coefficient of Coupling, k = ---------------

INFERENCE

Expt No:

Date:


23/41


MEASUREMENT OF B-H CURVE USING CRO

AIM

To study the hysteresis loop of a given specimen

APPARATUS REQUIRED


1. Hysteresis loop module ITB-026A 1

2. CRO 1



PRINCIPLE

Magnetic field is a phenomenon where under certain conditions, energy or force transfer canoccur through space. it can be established only by its effective which is used to determine the

magnetic property of the materials.

Hysteresis loop is nothing but a plot of flux density ‘B’ versus magnetizing for ‘H’. Many

other parameters can be determined from this loop. The hysteresis in any process is the

nonconformity of the loading and unloading curve of the process. The reason for occurrence of

hysteresis is that of all energy that has been pumped in to the system during the loading period, is

not being recovered completely due to losses in the system.

The a-b-c-d-e-f-a curve is called hysteresis curve for the magnetic material.

BR is the residual flux density. This is what enables the creation of permanent magnets. The

magnetic force HD is required coerce the material to reduce its flux density level to zero is called

coercive force. The unit for magnetic force ‘H’ is ampere turn per meter At/m. flux density B is

called Tesla (Wb/m2) or gauss. One gauss is 10,000 gauss

Thus the hysteresis loop is often called B-H curve. The understanding of B-H curve is

extremely importance in design of transformer, chokes, coils and inductors

PROCEDURE

(iii)

Connect the variable power supply to the input terminals.

(iv) Connect the X input of the CRO to the terminal T3

(v) Connect the Y input of the CRO to the terminal T6

(vi) Keep the CRO in XY mode

(vii) Hysteresis loop appears as in figure


24/41


(viii) Vary the input AC voltage and calculate VX , VY and tabulate the readings.

(ix) Tabulate the VX and VY from CRO


25/41


OBSERVATION

SL.NO. VX VY I1 H B


26/41


FORMULAE

VX = R 1 * I1 VX=0.22 * I1

I1= VX/0.22

H=N1I1 / L

VY= (1/C) (N2AS/R 2) B

B= VYCR 2/ N2AS

Where

R 1= 0.22 ohms (from manufacturer)

L= length of magnetic path= 0.1139m

N1= no. of turns in primary=2475

N2= no. of turns in secondary+102

As= area of the specimen+0.00045m2

R 2= 4700Ω (from manufacturer)

C= 10μF (from manufacturer)

RESULT

Thus the graph is plotted between B and H

INFERENCE


27/41


CIRCUIT DIAGRAM

a)

Calibration of voltmeter

Fig (1)


28/41


Expt No:

Date:

VERNIER POTENTIOMETER

AIM

To calibrate the given voltmeter, ammeter and wattmeter using Vernier potentiometer and

hence draw the calibration curve.

APPARATUS REQUIRED


1. Vernier potentiometer

2. STD cell

3. Fixed 2V supply

4. DC regulated power supply

5. Voltmeter

6. Galvanometer

7. Volt ratio box

8. Standard Resistance

9. Rheostat

10. Ammeter



PRINCIPLE

The potentiometer is an instrument used for measurement of an unknown EMF or potential difference

by balancing it, wholly or partially by a known potential difference produced by the flow of a known

current in a network or circuit of known characteristics. Potentiometers are extensively used in

measurements where the precision required is higher than that can be obtained by ordinary deflection

instruments. EMFS are measured directly with a potentiometer in terms of the EMF of a standard

cell. By using, in addition, a standard resistor, current can also be measured. From the current and

voltage measurements, power can be calculated. The


29/41


b) Calibration of ammeter

c)

Calibration of Wattmeter

Fig (2)

Fig (3)


30/41


potentiometer is widely employed for calibration of voltmeters, ammeters and wattmeters.

The potentiometer works on the principle of opposing the unknown EMF by a known EMF with the

– VE terminals of two EMFS connected together and also the +VE terminals connected together

through a galvanometer. Galvanometer gives no deflection if two EMFS are equal.

PROCEDURE

a) Standardization

The potentiometer is energized by giving a 2V dc supply to the terminals given in fig. The

selector switch is thrown to position ‘STD’ to include standard cell in the circuit. Range selector key

should be in position X0.1. In position X0.1, all dial readings should be multiplied by a factor 0.1 in

addition to factors shown near dials. The dials are adjusted to read standard cell voltage i.e. if standard

cell voltage is 1.0186, the first dial should read 10; the second dial 18 and third dial 60. The

galvanometer is connected to the terminal pair marked as ‘GALV’. The galvanometer key is pressed

and observed the galvanometer. The coarse, medium and fine rheostats are adjusted until the

galvanometer gets null deflection.

b)

Calibration of voltmeter

The voltmeter to be calibrated is connected across the variable dc source. The voltmeter is

connected to the high voltage side of volt ratio box. The low voltage side of volt ratio box is connected

to the potentiometer at appropriate terminals. The selector switch is thr own to ‘TEST’ position and

voltage at output of volt ratio box is measured using potentiometer. Then the true value of voltage

across voltmeter can be determined by multiplying the voltage at low voltage side of the volt ratio

box by multiplication factor. The voltmeter reading is noted. The ratio of difference between

measured value and true value is the percentage error. The errors at various voltmeter readings are

calculated.

% Error = − × 100


31/41


OBSERVATIONS

a) Calibration of voltmeter

Sl

No.

Volts

V

X1 ×0.1 X2 ×0.001 X3 ×0.00001 V(V) − × 100

b)

Calibration of ammeter

Sl

No.

Current I X1×0.1 X2×0.001 X3×0.00001 V I =/ − × 100

c) Calibration of Wattmeter

Sl

No V Wind X1

×0.1

X2

×0.001

X3

×0.00001E1

X1

×0.1

X2

×0.001

X3

×0.00001E2

I= 2/

Wact

W−WW× 100


32/41


a)

Calibration of ammeter

An Ammeter to be calibrated is connected in series with a standard resistance R s of suitable value.

The current supplied by a D.C. supply passes through ammeter as well as standard resistance R s.

using D.C. potentiometer; a voltage across standard resistance can be measured. Thus, current

flowing through R s (and ammeter also) is given by,

I =R ; where Vs = voltage across Rs measured using potentiometer

Rs = resistance of standard resistor

Since the resistance of standard resistance is known accurately and also the voltage across Rs is

measured using standardized potentiometer, the method of calibrating ammeter is very accurate. If

the actual current Iact and the current indicat5ed by ammeter Iind are not matching, error is

indicated. The percentage error is given by

% error =−

× 100 ; where Iind = current indicated by ammeter and Iact = I =R

The calibration curve of ammeter can be obtained by plotting % error against the reading of

ammeter i.e. Iind.

Calibration of Wattmeter

For the calibration of a Wattmeter, a low voltage supply is given to the current coil (CC)

whose current can be adjusted by using a rheostat R h in series with low voltage supply and a high

voltage supply is given across the potential coil (PC). The voltage is stepped down by volt-ratio box.

A voltmeter measures voltage V and ammeter measures current I which gives true power as,

Wind = V I

This value can be compared with a value indicated by watt meter. If two values are not matching, a

positive or negative error is indicated which is given by,

% error =W−W

W × 100

RESULT

The given voltmeter, ammeter and wattmeter were calibrated the using vernier potentiometer

and hence the calibration curve were drawn.

INFERENCE


33/41


Expt.No.

Date:

WHEATSTONE BRIDGE AND KELVIN’S DOUBLE BRIDGE

AIM

a) To use wheatstone bridge for the measurement of medium resistance

b) To use Kelvin Double bridge for the measurement of low resistance.

APPARATUS REQUIRED

SL No APPARATUS SPECIFICATION QUANTITY

1. Wheatstone Bridge 1 no.

2. Voltmeter 1 no.

3. Voltmeter 1 no.

4. Rheostat 1 no.

5. Kelvin Double Bridge 1 no.

6. Galvanometer 1 no.

7. DC regulated power supply 1 no.

8. Ammeter 1 no.

9. Ammeter 1 no.



CIRCUIT DIAGRAM

Wheatstone Bridge

Fig.1


34/41


Fig.2

PRINCIPLE

a) Wheatstone Bridge

This is the best and common method of measuring medium resistances (1Ω to 0.1MΩ).Majority of the electrical apparatus and equipments have resistances within these limits and hence

the wheat stone bridge is a very useful instrument for resistance measurement.

The General circuit arrangement is given in the figure (1), where P and Q are two known fixed

resistances, S being a known variable resistance and R be the unknown resistance (i.e. voltmeter

or rheostat). At balanced condition, no current flows through the galvanometer, and the unknown

resistance R is given by R = .The arms P and Q are the ratio arms of decade dial is provided in the portable bridge

for this purpose. S is the known standard resistance whose values may be varied. Decade dials in

different ranges are provided in the portable bridges. The product of the range selector ( ) andthe total value of variable resistance (S) gives the unknown resistance R.


35/41


Kelvin Double bridge

Fig .4

Fig .3


36/41


Kelvin Double Bridge

This method is the best available for the precise measurement of low resistances (less than

1Ω). It is a development of the wheat stone bridge by which the errors due to contact and lead

resistance are eliminated. Kelvin Double Bridge is suitable for measuring resistance fitted withfour terminals i.e. two current terminals C1 and C2 and two potential terminals P1 and P2. This is

to reduce contact and lead resistances completely. It incorporates the idea of a second set of ratio

arms. Hence the name double bridge and the use of four terminals.

Figure (2) shows the schematic diagram of Kelvin’s double bridge. The first set of ratio arms

is Q and M. The second set of ratio arms q and m is used to connect the galvanometer to a point

at the appropriate potential between Q and M to eliminate the effect of connecting lead resistance

between the known resistance R and standard resistance S. In the figure (2), R is the low resistance

to be measured (i.e. resistance of ammeter, given length of wire etc.), S is the standard variable

resistance and Q, M, q and m are four non inductive resistances, one pair of each (M and m Or

Q and q) are variable. These are connected to two sets of ratio arms which are used for range

selection. The ratio ⁄ is kept same as ⁄ under balanced condition. These ratios along withS being varied until zero deflection of the galvanometer are obtained. Then ⁄ = ⁄ = ⁄ or R = ⁄ × S. So the product of ratio arms ⁄ and variable resistance S gives the unknownresistances.

PROCEDURE

a) Wheatstone bridge circuit is shown in figure ( 1 ). Initially, set the four knobs of the decade

resistance box at zero position. Then adjust the galvanometer pointer to zero by using ‘SET

ZERO’ knob after checking the position of key 10, which should be in the normal raised position

and connect the given voltmeter to the external terminal ( 3 and 4) of the bridge.[Ensure that the

terminals 1 and 2 and also 7 and 8 are shorted.]. The range selector ( ) is properly selected andresistance S (i.e. by varying the four decade resistances) is varied and the galvanometer is

checked. This is continued until the balanced condition of the bridge is obtained. The readings

of the range selector and the four dials of the variable resistance S are noted. The experiment is

repeated for different values of range selector.

Kelvin Double bridge circuit is shown in figure ( 3 ). Provision has been given in

the bridge for current terminals (C and C1) and potential terminals (P and P1) separately. But

when connecting the unknown resistance having two terminals (like ammeters), they are

connected to only C and C1 while C to P and C1 to P1 are short circuited. An external dc power

supply (at ‘CURRENT INPUT’ terminal) and a galvanometer (between G1 and G2) are

connected. After setting the CURRENT SWITCH to NOMAL and adjusting the current to the


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required value, select a proper value for range multiplier. Then vary the resistance S by adjusting

the decade dials. This is continued until the balanced condition of the bridge is obtained. The

readings of the range selector and the four dials of the variable resistance S are noted. The

experiment is repeated for the REVERSE CURRENT SWITCH condition also. The mean of the

two readings should be taken as the correct value.

OBSERVATIONS

a) For Wheatstone bridge

Sl

No.

S1 × 1000 S2 × 100 S3 ×10 S4 × 1 R3 = S1+S2+S3+S4 R =

b) For Kelvin Double bridge

Sl No.

Remarks ⁄ S1 S2 S = S1 + S2 R = ⁄ × S

RESULT

1. Resistance of Voltmeter =

2. Resistance of Ammeter =

INFERENCE


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Expt.No.

Date:

B - H CURVE USING SINGLE PHASE TRANSFORMER

AIM

To plot the B- H characteristics of a transformer core.

APPARATUS REQUIRED

SL No APPARATUS SPECIFICATION QUANTITY

1. Autotransformer 1 no.

2. Voltmeter 1 no.

3. Ammeter 1 no.

4. Wattmeter 1 no.

5. Transfomer 1 no.



CIRCUIT DIAGRAM

Fig. 1


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Expt.No.

Date:

DETERMINATION OF HYSTERISIS LOOP

AIM

To trace the hysteresis loop using CRO.

APPARATUS REQUIRED


1. Hysteresis loop module ITB-026A 1

2. CRO 1



CIRCUIT DIAGRAM

Fig. 1


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PRINCIPLE

Magnetic field is a phenomenon where under certain conditions, energy or force transfer can

occur through space. it can be established only by its effective which is used to determine the

magnetic property of the materials.

Hysteresis loop is nothing but a plot of flux density ‘B’ versus magnetizing for ‘H’. Many

other parameters can be determined from this loop. The hysteresis in any process is the

nonconformity of the loading and unloading curve of the process. The reason for occurrence of

hysteresis is that of all energy that has been pumped in to the system during the loading period, is

not being recovered completely due to losses in the system.

The a-b-c-d-e-f-a curve is called hysteresis curve for the magnetic material.

BR is the residual flux density. This is what enables the creation of permanent magnets. The

magnetic force HD is required coerce the material to reduce its flux density level to zero is called

coercive force. The unit for magnetic force ‘H’ is ampere turn per meter At/m. flux density B is

called Tesla (Wb/m2) or gauss. One gauss is 10,000 gauss

Thus the hysteresis loop is often called B-H curve. The understanding of B-H curve is

extremely importance in design of transformer, chokes, coils and inductors

PROCEDURE

(i) Connect the variable power supply to the input terminals.

(ii) Connect the X input of the CRO to the terminal T3

(iii)

Connect the Y input of the CRO to the terminal T6(iv) Keep the CRO in XY mode

(v) Hysteresis loop appears as in figure

(vi) Vary the input AC voltage and calculate VX , VY and tabulate the readings.

(vii) Tabulate the VX and VY from CRO


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