Electrical Measurements Lab Manual

7/30/2019 Electrical Measurements Lab Manual

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EMS Lab Plan:

S.No Topic Week

(i) Equipment used in lab:1. Single phase energy meter2. Rheostat3. wattmeter4. Voltmeter5. Ammeter6. Single phase variac7. Powerfactor meter8. DC cromptons potentio meter9. Kelvins double bridge kit10. Currnt transformer Standard and commercial11. Phase shifting trasformer12. Schering bridge kit13. Andersons bridge circuit14. LVDT Trainer kit15. Specimen(Liyod fisher)

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(ii) Components used in lab1. Decade Capacitance box2. Decade Inductance box3. Stop watch4. Sensitive Galvanometer5. Volt ratio box6. Hed phones7. Patch cards8. Choke Coil9. 3- Variable Inductive Load

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Unit-I(Electrical Measurements)

1 Expt-1: Measurement of parameters of choke coil using 3voltmeter and 3 ammeter method

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Unit-II(Instrument Transformers)

2 Expt-1: Calibration of Dynamo Power factor meter W43 Expt-2: CT Testing by Silbeess method W5

Unit-III(Measurement of Power)

4 Expt-1: Measurement of 3- reactive power with singlephase wattmeter.

W6

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5 Exp-2: Measurement of 3- active power by using 2 CTsand single wattmeter method.

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Unit-IV(Measurement of Energy)

6 Expt-1: Calibration & testing of single phase Energy

meter.

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7 Expt-2: Calibration of LPF wattmeter by PhantomLoading.

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Unit-V(Potentio meters)

8 Expt-1:Calibration of PMMC ammeter & voltmeterusing cromptons DC Potentio meter.

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Unit-VI(Measurement of Resistance)

9 Expt-1: Kelvins Double Bridge W11

Unit-VII(AC Bridges or Measurement of inductance and capacitance)

10 Expt-1: Measurement of capacitance by Schering bridge& Inductance by Andersons bridge.

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Exp-2: Measurement of Inductance by Andersonsbridge.

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Unit-VIII(Magnetic Measurement)

11 Expt-1: Measurement of Iron loss in bar specimen usinga CRO and using a wattmeter

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12 Exp-2: LVDT and Capacitance pick-up charactersticsand calibration.

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Supplementary topics

Exp-1: Measurement of 3- reactive power using two andthree wattmeter method.

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Experiment no.1

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Calibration and testing of single phase energy meter

AIM : To Calibrate and test the given Single phase energy meter by using Sub-standard or calibratedWattmeter with direct loading.

APPARATUS :

Sl. NO. NAME TYPE RANGE QTY.1. 1-Phase Energy meter AC 230 V/10 A 1 no.2. Wattmeter(Sub-standard or calibrated) Dynamometer type, LPF 10A, 300V 1 no.3. Voltmeter MI 300 V 1 no.4. Ammeter MI 10A 1 no.5. Rheostat 50, 15A 1no.6 Single Phase Variac 0-270V, 10A 1 no7 Connecting wires 1 set8 Stop watch Digital 1no.

CIRCUIT DIAGRAM:

THEORY :

Induction type of energy meters are universally used for measurement of energy in domestic andindustrial a.c. circuits. Induction type of meters possesses lower friction and higher torque/weight ratio.Also they are inexpensive and accurate, and retain their accuracy over a wide range of loads andtemperature conditions.

There are four main parts of the operating mechanism:(i) Driving system(ii) Moving system(iii) Braking system and(iv) Registering system.

Driving System: The driving system of the meter consists of two electro-magnets. The core of theseelectromagnets is made up of silicon steel laminations. The coil of one of the electromagnets is excited bythe load current. This coil is called the current coil. The coil of second electromagnet is connected acrossthe supply and, therefore, carries a current proportional to the supply voltage. This coil is called thepressure coil. Consequently the two electromagnets are known as series and shunt magnets respectively.

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Copper shading bands are provided on the central limb. The position of these banks is adjustable. Thefunction of these bands is to bring the flux produced by the shunt magnet exactly in quadrature with theapplied voltage.

Moving System: This consists of an aluminium disc mounted on a light alloy shaft. This disc is positionedin the air gap between series and shunt magnets.

Braking System: A permanent magnet positioned near the edge of the aluminium disc forms the brakingsystem. The aluminium disc moves in the field of this magnet and thus provides a braking torque. The

position of the permanent magnet is adjustable, and therefore, braking torque can be adjusted by shiftingthe permanent magent to different radial positions as explained earlier.

Registering (counting) Mechanism: The function of a registering or counting mechanism is to recordcontinuously a number which is proportional to the revolutions made by the moving system.In all induction instruments we have two fluxes produced by currents flowing in the windings of theinstrument. These fluxes are alternating in nature and so they produce emfs in a metallic disc or a drum

provided for the purpose. These emfs in turn circulate eddy currents in the metallic disc or the drum. Thebreaking torque is produced by the interaction of eddy current and the field of permanent magnet. Thistorque is directly proportional to the product of flux of the magnet, magnitude of eddy current and effectiveradius R from axis of disc. The moving system attains a steady speed when the driving torque equals

braking torque.The term testing includes the checking of the actual registration of the meter as well as the adjustmentsdone to bring the errors of the meter with in prescribed limits. AC energy meters should be tested for thefollowing conditions:

1. At 5% of marked current with unity pf.2. At 100% (or) 125% of marked current.3. At one intermediate load with unity pf.4. At marked current and 0.5 lagging pf.

PROCEDURE :

1. Connections are made as per the circuit diagram.2. With load DPST open and with variac in minimum (or) zero output position, close the supply

DPST switch.3. Gradually vary the variac to apply the rated voltage (230 volts).4. Close the load DPST and apply a suitable load (5A).5. Note down the readings of the meters and time taken for 5 revolutions of energy meter disc.6. Increase the load in steps and in each step note down the readings of the meters and also the time

taken for 10 revolutions of energy meter aluminum disc. Tabulate the readings.7. Gradually reduce the load in steps and open the load DPST.8. Vary the variac gradually to minimum or zero output position and open the supply DPST.9 Evaluate observed reading, actual reading, %error, %correction.10. Draw the graph between Load current (vs) % Error.

TABULAR FORM:

S.No. Voltmeter(Volts) AmmeterReading Amps WattmeterReading W Time for20 rev TheoreticalkWH (O.R) Practicalwxt (A.R) %error

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O.R=OBSERVED READINGA.R=ACTUAL READING

MODEL CALCULATIONS:Observed reading = 1/(energy meter constant (k)/no. of revolutions)Where , no. of revolutions = 10

energy meter constant k=900 rev/kwhActual reading = W *t%error = [(O.R-A.R)/A.R] *100%correction = - % error = [(A.R-O.R)/A.R] *100MODEL GRAPH :.

%error

Load current

PRECAUTIONS :

1. Avoid lose connections.2. Careful while observing the revaluation with stop watch.3. Do not apply more current, more than the rated energy meter current.4. Take readings without error.5 Keep variac at their minimum position initially6 Vary the variac such that the current and voltage are within the rated value

7. Meter readings should not exceed their ratings.8. Live terminals should not be touched.9 If any wattmeter reads, reading, change either current coil or pressure coil connections.10 Load current should not exceed rated current value.11 Load should be varied very smoothly.12 Take proper care, such that the ammeter reading, should not exceed the current rating of LPF

wattmeter.

RESULT :

The given single phase energy meter is tested at different loads and calibration curve is plotted.

QUESTIONS:

1. What is an energy meter?2. What are the types of energy meter?3. Which type of energy meters are used in dc circuits?4. Energy meter is an _____________ (i) integrating instrument (ii) indicating instrument5. Can the measured percentage error be negative?6. What do you mean by torque adjustment?7. What is operating torque?8. Define braking torque?9. When does the disc on the spindle rotate with a constant speed?

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10. The operating torque is directly proportional to speed, state true or false.

EXPERIMENT NO.2CALIBRATION OF DYNAMOMETER POWER FACTOR METER

AIM :Testing and Calibration of the given Dynamometer Power Factor Meter by using a sub-standard wattmeter,ammeter and voltmeter.

APPARATUS :

SL.NO.

NAME TYPE RANGE QUANTITY

1. p.f. meter Dynamometer type, UPF

10A, 300V 1 no.

2. Wattmeter Dynamo meter type LPF

10A, 600V 1 no.

3. Voltmeter MI 300 V 2 no.s

4. Ammeter MI 10A 1 no.5. Rheostat 50 Ohm/5A 1no.6 Connecting wires 1 set7 Single Phase Variac AC 270V, 10A 1no.

CIRCUIT DIAGRAM:

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THEORY:

Power factor meters indicate directly, by a single reading, the power factor of the circuit to which they areconnected.

Power factor meters like wattmeters have a current circuit and a pressure circuit. The current circuitcarries the current (or definite fraction of this current) in the circuit whose power factor is to be measured.The pressure circuit is connected across the circuit whose power factor is to be measured and is usuallysplit up into two parallel paths one inductive and the other non-inductive. The deflection of theinstrument depends upon the phase difference between the main current and the currents in the two paths ofthe pressure circuit, i.e. upon the phase angle or power factor of the circuit. The deflection is indicated bya pointer. The deflection of the instrument is a measure of phase angle of the circuit. The scale of theinstrument can be calibrated in directly in terms of power factor.

The moving system of power factor meters is perfectly balanced at equilibrium by two opposing forces andtherefore there is no need for a controlling force. Hence when a power factor meter is disconnected from acircuit the pointer remains at the position which it occupied at the instant of disconnection.

The instrument must be designed for, and calibrated at, the frequency of the supply on which its to be used.In case the meter is used for any other frequency or if the supply contains harmonics it will give rise to

serious errors in the indication on account change in the value of reactance of choke coil.

PROCEDURE :1. Connections are made as per the circuit diagram.2. With variac in minimum or zero output position, the Rheostat in maximum resistance position,

close the DPST switch.3. Apply rated voltage 230V to pressure coils of the P.F meter and wattmeter with the variac. .4 Close the load DPST and apply a suitable load . Note down the readings of the instruments

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5 Increase the load in steps and in each step note down the readings of the instruments. Tabulate thereadings.

6. Gradually reduce the load in steps and open the load DPST.7. Vary the variac gradually to minimum or zero output position and open the supply DPST.8. The PF measured from PF meter under test and actual PF from the reading of wattmeter, voltmeter

and ammeter calculated and %error, %correction are found.

9 Draw the graph between Load current (vs) % Error.TABULAR FORM:

S.No. Voltmeter Reading(Volts)

AmmeterReading , I(Amps)

WattmeterReading, W(Watts)

P.F. meterReading(S2)

COS=W/VI(S1)

%Error=S1-S2*100

S1

MODELGRAPH:

Lag p.f.Lead p.f.

%error

PRECAUTIONS :

1. There should not be any loose connection2. Meter readings should not exceed their ratings.3. Take readings without parallel errors.4. Note down the readings of voltmeter, wattmeter, and p.f. meter5. Vary the rheostat by observing the p.f. meter and load current upto rated current of rheostat.6. Note down all the readings and reduce the input voltage and open the DPST.7. Now, remove inductance from the circuit, connect only rheostat and follow steps from 2.8. Connect a variable capacitor in series with rheostat and follow the steps from 1 .9 Keep variac at their minimum position initially10 Vary the variac such that the current and voltage are within the rated value11. Meter readings should not exceed their ratings.12. Live terminals should not be touched.

13 If any wattmeter reads, zero (or negative) reading, change either current coil or pressure coilconnections.

14 Load current should not exceed rated current value.15 Load should be varied very smoothly.16 Keep rheostat at its minimum resistance position initially.17 Take proper care, such that the ammeter reading, should not exceed the current rating of LPF

wattmeter.

RESULT :

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The dynamometer type power factor meter is tested at different power factors by using phase-shiftingtransformer and calibration curve is plotted.

QUESTIONS:

1. State the widely used power factor meter?2. What do you mean by true power? State the formula?3. Is there any limit in mechanism to develop the controlling torque in dynamometer pf meter?4. The angular displacement of the coil is proportional to system phase angle. State True/False.5. What are the two different types of power factor meter?6. What is the position of the pointer when no current flows in the circuit?7. Torque acting on the coil P1 is directly proportional to ______________8. Torque acting on coil P2 is directly proportional to ______________9. The condition for the spindle to be in equilibrium?10. The angular displacement of the coils is equal to ____________

Calculations of dynamo power factor meter:

s.no Voltmeter(Volts)

Ammeter(Amps) Wattmeter(Watts) Powerfactormeter

cos=W/VI Error=S1-S2/S1

1 230 0 0 1 0 02 230 1 50 1 0.217 -360.8

i) %error=S1-S2/S1 *100

S1= cos=W/VI

S1=50/230=0.217

Powerfactor meter reading(S2)=1

%Error =0.217-1/0.217 *100

%error = -360.8

ii) S1=100/230=0.434

S2=Powerfactor reading=1

%Error =0.434-1/0.434*100

%error = -130.4

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EXPERIMENT NO.3

CALIBRATION OF PMMC AMMETER AND VOLTMETER USING CROMPTON D.C.POTENTIOMETER

AIM : To calibrate PMMC Ammeter and Voltmeter using Crompton DC potentiometer.

APPARATUS REQUIRED :

Sl.NO.

NAME TYPE RANGE QTY.

1. DC CromptonPotentiometer

DC 1 no.

2. Standard cell DC Daniel cell (1.08V 1 no.3. Volt ratio box Input 0-1.5V Output 0-1.5, 15, 30,

100, 300V1 no.

4. Regulated power supply(RPS)

DC 0-30V 1 no.

5. Ammeter MC 0-2A 1no.

6 Voltmeter MC 0-300V 1 no7 Connecting wires 1 set8 Sensitive galvanometer Spot reflecting MC

type30-0-30 1 no

9 Standard resistance 0.01 Ohms, 10A 1 no10 Rheostat 50 Ohms, 5A 1 no11 battery 2V(freshly charged) 1 no12 DC SUPPLY 220V 1no.

CIRCUIT DIAGRAM :CALIBRATION OF VOLTMETER

CALIBRATION OF AMMETER :

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THEORY :

A Potentiometer is an instrument designed to measure an unknown voltage by comparing it with aknown voltage. The known voltage may be supplied by a standard cell or any other known voltage reference source. Measurements using comparison methods are capable of a high degree of accuracy

because the result obtained does not depend upon on the actual deflection of a pointer, as is the case in

deflectional methods, but only upon the accuracy with which the voltage of the reference source is known.Another advantage of the potentiometers is that since a potentiometer makes use of a balance or nullcondition, no current flows and hence no power is consumed in the circuit containing the unknown emfwhen the instrument is balanced. Thus the determination of voltage by a potentiometer is quite independentof the source resistance.It can also be used to determine current simply by measuring the voltage drop produced by the unknowncurrent passing through a known standard resistance. The potentiometer is extensively used for acalibration of voltmeters and ammeters and has in fact become the standard for the calibration of theseinstruments. For the above mentioned advantages the potentiometer has become very important in the fieldof electrical measurements and calibration.

Modern laboratory type potentiometers used calibrated dial resistors and a small circular wire of one ormore turns, thereby reducing the size of the instrument. The resistance of slide wire is known accurately,the voltage drop along the slide wire can be controlled by adjusting the value of working current. The

process of adjusting the working current so as to match the voltage drop across a portion of sliding wireagainst a standard reference source is known as Standardisation.

PROCEDURE :STANDARDIZATION:

1. Connections are made as per the circuit diagram.2. Keep the function knob of the potentiometer at STD position. Switch on the RPS(1) and

adjust to 2 volts.3. Adjust the slide contact and slide wire of the potentiometer to read standard cell voltage

(1.08V).4. Press the galvano key of the potentiometer and adjust the coarse and fine rheostats until the

spot reflecting galvanometer gives null deflection. This completes standardization of the

potentiometer. Once standardization is done the position of R1 & R2 should not bechanged

PMMC VOLTMER1. Volmeter under test is connected across potential divider in such a way that p.d across voltmeter

can be varied2. V.R box is used in parallel to potentiometer to reduce voltage to the range of potentiometer3. Change the function knob to E1 position. Switch on RPS and adjust a suitable voltage on V.R box4 Press the galvano key of the potentiometer and adjust the slide contact and slide wire until the spot

reflecting galvanometer gives null deflection.5 Note down the readings of voltmeter, and potentiometer slide contact and slide wire readings.6 Repeat the steps 4 and 5 for different voltages from RPS7. Reduce the voltage of RPS and RPS to zero. Switch off the supply.

8 Draw the graph between Load current (vs) % Error.

NOTE :1. RPS must be at 2 volts only.2. Smoothly vary the knobs on the potentiometer.3. Keep the spot reflecting galvanometer switch to AC mains position and rotating key to free

position during the conduction of experiment.4. At the end of the experiment remove the supply to the galvanometer and move the rotating key to

lock position and short the galvanometer output terminals.

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CALCULATION :

Calibration of voltmeter:VR Box ratio = Output voltage / Input voltageVact = Voltmeter readingVtrue = Reading obtained from potentiometer

= [Coarse voltage + Fine voltage] x Ratio of volt-ratio box

% error =

100V

VV

true

trueact

PROCEDURE:STANDARDIZATION:

1. Connections are made as per the Circuit diagram.2. Keep the function knob of the potentiometer at STD position. Switch ON the RPS(1) and adjust

to 2 volts.3. Adjust the slide contact and slide wire of the potentiometer to read standard cell voltage (1.08V).4. Press the galvano key on the potentiometer and adjust the coarse and fine rheostat until the spot

reflecting galvanometer gives null deflection. This completes standardization ofgalvanometer.Once standardization is done the position of R1 & R2 should not be changed

PMMC AMMETER:1 Ammeter to be calibrated is connected in series with variable resistor R & standard resistance S2 The standard resistance should of such a magnitude that current passed through it doesnt exceed

range of potentiometer3 V.R box is used in parallel to potentiometer to reduce voltage to the range of potentiometer4. Keep the rheostat at maximum position and change the function knob to E1 position. Switch on

RPS(2) and apply 30 volts.5 Vary the rheostat gradually and adjust suitable current.6. Press the galvano key on the potentiometer and adjust the slide contact and slide wire until the

spot reflecting galvanometer gives the null deflection.7. Note down the readings of ammeter, voltmeter, and potentiometer slide contact and slide wire

readings.8. Repeat the steps 5 to7 for different values of current.9 Vary the rheostat to maximum position, reduce the voltage of RPS and RPS to zero. Switch off the

supply.10 Draw the graph between Load current (vs) % Error.

TABULAR FORM:FOR VOLTMETER:

S.No Voltmeter reading(V) , Vtrue

E2 value(V) Vact=(R1+R2/R2)*E2

%error

10V

VV

true

trueact

FOR AMMETER:

S.No Ammeter reading(A)

E2 value(V) Iact=E/R %error

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Itrue10

I

II

true

trueact

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CALCULATION :CALIBRATION OF AMMETER:

Iact = Ammeter reading

Itrue =

ceresisdardS

boxratiovoltofRatiovoltFinevoltCoarse

tantan

)( +

% error =

10I

II

true

trueact

PRECAUTIONS :

1. Connect the circuit without loose connections.2. Dont vary the coarse and fine pots, after standardization.3. After connecting the unknown voltage, Dont press the final button of Galvanometer directly.4. Operate the selector switch (P1) ad, slide wire (P2) very smoothly.5. Take care about current rating which choosing the standard rheostat in calibration of ammeter

experiment.6. Connect the unknown voltage directly to the test terminals, if it has less than 1.5V.7. Since DC Crompton potentiometer measures upto1.86V adjust the voltage at E2 should not exceed

this value while doing the experiment.

RESULT :

The calibration of PMMC Voltmeter and Ammeter is done using Crompton DC potentiometer. Thecalibration curves for voltmeter and ammeter are drawn.

QUESTIONS:

1. What do you mean by a potentiometer?2. What are the types of potentiometer?3. What is the working principle of a potentiometer?4. What is standardization of potentiometer?5. What is the purpose of connecting a standard battery in the circuit?6. Application of dc potentiometer?7. What do you mean by calibration curve of the ammeter?8. What do you mean by a volt-ratio box?9. What are the types of AC potentiometer?10. What are the practical applications of ac potentiometer?

Calibration of PMMC Ammeter & Voltmeter using cromptons DC potentio meter:

Calibration of Voltmeter:

%Error= (Vact-Vtrue) / Vact *100

Vact =R1+R2 / R2 *E2

Vtrue = Voltmeter reading (v)

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Vact = (1+2 / 2) * 5

=7.5V

Vtrue=0.25+51mv

=0.301V

% Error=(7.5-0.301/0.301 ) *100

=23.9%

Calibration of Ammeter:

%Error= (Iact-Itrue) / Iact *100

Iact =E/R

Itrue = Ammeter reading (A)Iact = E/R=5/2

=2.5A

Itrue= 0.1 amps

% Error=(2.5-0.1/0.1 ) *100

=24%

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EXPERIMENT NO.4KELVINS DOUBLE BRIDGE

AIM :

To determine the low resistance by setting up Kelvins double bridge.

APPARATUS REQUIRED :

Sl. NO. NAME TYPE RANGE QTY.1 Kelvins double bridge kit 1 no2. Galvanometer 1 no.3. Rheostat. 5A,50 OHM 1 no.4. DMM DIGITAL 1 no.5. Patch cards/ Connecting wires 1set.6 RPS DC 2V 1 no

CIRCUIT DIAGRAM:

THEORY :

X is the unknown resistance to be measured.S is the standard resistance.

The ratio of Q/q must be equal to M/m under balance conditions; there is no current throughgalvanometer G. The first of ratio arms is Q and M. The second set of ratio arms of m is used to connect thegalvanometer to a point at the appropriate potential b/w point e and f to eliminate the effect of connectinglead of resistance r between then the unknown resistance X and standard resistance S.

If SMQ

thenX,mM

pQ

==

PROCEDURE :

1. Short the terminals which are marked +c,-c & +p , -p.2. Calibrate the Kelvins double bridge by turning the zero adjusting knob until galvanometer

indicates zero.3. During calibration remaining knobs should be kept at zero position.4. Now short terminals +p,+c and p,-c. connect unknown specimen resistance between +p and p

terminals.5. Now adjust the main dial ,slide wire and multiplier until the galvanometer indicates zero position.

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6. Note down the readings of main dial slide wire and multiplier.

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OBSERVATIONS :

S.No. Specimen type (OR) standardor known

un Resistance or practical

1.2.

3.4.

SAMPLE CALCULATIONS :

Unknown resistance = multiplier *(main dial reading + slide wire reading)

PRECAUTIONS :

1. Galvanometer should always be connected to protective resistance to prevent damage togalvanometer.

2. Changes in Q and q should be made immediately to bring the pointer of galvanometer back towithin scale to prevent damage.

3. Resistance r should be in the maximum position to start with and adjusted later if necessary to getlarge deflection.

RESULT :

The unknown resistances of earth wires, having different parameters, are measured by using Kelvinsdouble bridge.

QUESTIONS:

1. Classify resistance?2. Examples of high resistance?3. What are the methods employed in measuring low resistances?4. Which is the most accurate method to measure the low resistances? State the reason?

5. Kelvin double bridge is a modified version of ?6. What is the main problem in measuring low resistances?7. How do we measure high resistances?8. Practical methods to measure earth resistance?9. What are the quantities that are measured by ac bridges?

KELVINS DOUBLE BRIDGE CALCULATIONS:

Unknown resistance = multiplier * (main dial reading + slide wire reading)

Unknown resistance = 100 *(0.1+ (15*0.01))

= 11.5

Known resistance = 14

Tolerance = Known resistance - Unknown resistance

= 14 - 11.5

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Tolerance = 2.5

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EXPERIMENT NO. 5CT TESTING BY SILSBEES METHOD

AIM:

To measure the % ratio error and phase angle error of given CT by Silsbees method.APPARATUS:

SL.NO.

NAME TYPE RANGE QUANTITY

1. CT Precision (Standard) 5/5 1 no.2. CT Commercial 5/5 1 no.3. Voltmeter MI 300 V 1 no.s4. Ammeter MI 10A 2 no.5. Wattmeter Electrodynamometer

Type LPF10A, 600V 2no.

6 Connecting wires 1 set

7 Rheostat 50 Ohms, 5A8 Loading Burden Rheostat 50 Ohms, 5A 1 no9 Phase-shifting transformer 3 Ohms, 420V, 10A 1 no10 Single Phase supply AC 230V, 10A 1no.

CIRCUIT DIAGRAM:

THEORY :

Methods of experiment testing instrument transformers i.e., finding their ratio and phase angle errors maybe broadly classified into two groups. Absolute methods and comparison methods each of these testmethods can be classified according to measurement technique employed as Deflecting methods and Null-methods.

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Silsbees method is a comparison method. There are two types of Silsbees methods deflectionaland here the ratio and phase angle of the test transformer are determined, in term of a standard transformershaving the same nominal ratio.

The two transformers are connected with their primaries in series. An adjustable burden is put inthe secondary circuit of the transformers under test. An ammeter is included in the secondary circuit of thestandard transformers so that the current may be set to the desired value. W1 is a wattmeter whose current

coil is connected to carry the secondary current of the standard transformers. The current coil of wattmeterW2 carries a current which is the difference between the secondary currents of the standard and testtransformers. The voltage circuits of the wattmeters lies their pressure coils are supplied in parallel from a

phase shifting transformer at a constant voltage V.

PROCEDURE:

1. Connections are made as per circuit diagram.2. Switch on the supply and by using single phase variac and rheostat adjust primary

current to some value say 3 A.3. By varying the phase shifting transformer the phase of the voltage is so adjusted that wattmeter

W1 reads maximum value (Phase condition).4. Now note down the readings of all meters. The readings of wattmeter in phase condition are taken

as W1p and W2p5. Again vary the phase shifting transformer so that the wattmeter W1 reads zero value (90 phaseshift Quadrature condition).

6. At this instant also note down all the meter readings and the wattmeter readings are taken as W1qand W2q.

7. % Ratio error and phase angle error are calculated by using the formulae given below.

TABULATION:

S.No. Ammeter ReadingI1(Amps)

AmmeterReadingI2(Amps)

W1p(Watts)

W2p(Watts)

W1q(Watts)

W2q(Watts)

Rs =I1/I2% Ratio error (Rx) = Rs {(W2p / W1p) +1}Phase angle error (x) = W2q/ (W1p W2p) + s

FORMULAE USED :Nominal ratio of standard CT = 10/5 = 2Phase angle error of standard CT = 5

Actual ratio of standard CT = 1.99

Actual ratio of CT under test =

+= sssx IV

W

RR

10

1

% ratio error =x

s

R

RR

Phase angle error for CT under test

+=

18020

sssx

IV

W

PRECAUTIONS :1. Take care, while varying the phase angle with phase shifting transformer.

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2 Avoid lose connections.3 Meter readings should not exceed their ratings.4. Take readings without error.5 Keep variac at their minimum position initially6 Vary the variac such that the current and voltage are within the rated value7 Live terminals should not be touched.

8 If any wattmeter reads, reading, change either current coil or pressure coil connections.9 Take proper care, such that the ammeter reading, should not exceed the current rating of LPFwattmeter.

10 Load current should not exceed rated current value.11 Load should be varied very smoothly.

RESULT :

The current transformer is tested by using Silsbees method and, the ratio and phase angle errors arecalculated.

QUESTIONS:

1 How types of Silsbees methods ? And what are those ?2 Silsbees methods----------- method3 What is Burden of transformer?4 Define (C.T&P.T) A. Transformation ratio

B. Turns ratioC .Nominal ratioD .RCF

5 Comparison between C.T & P.T

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EXPERIMENT NO.6A. MEASUREMENT OF CAPACITANCE BY SCHERING BRIDGE

AIM : To find the capacitance of unknown capacitor using Schering bridge.

APPARATUS :

Sl. NO. NAME TYPE RANGE QTY.1 Schering bridge circuit 1 no2. Head phones 1 no.3. Decade capacitance box 1 no.4. DMM digital 1 no.5. Patch cards 1set6 RPS 230v 1 no7 Galvanometer 1 no

CIRCUIT DIAGRAM:

THEORY:The Schering bridge is very widely used for the measurement of capacitance, dielectric loss and powerfactor of capacitors. The advantage of using this bridge is that it can be employed in both low voltage andhigh voltage measurements.

From the circuit :C1 is the unknown capacitance andR1 is a resistance representing its loss componentC2 is a standard air capacitor and it is loss freeR3 & R4 are non-inductive standard resistors.R3 is fixed and R4 is variableC4 is a variable capacitors.The detector may be a head phone or vibration galvanometer

E is a low voltage a.c. source r 230v,50hz,supplyThe bridge is balanced by adjustingC4 and R4At balance, we have V1 = V2 & V3 = V4

42431321

11 &,1

12,1

ZIVRIVjwc

Vjwc

rV =

=

+=

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Where Z4 =144

4

41

4

41

4

+=

+

CjwR

R

jwcR

jwcR

Or V4 =44

42

1

1

CjwR

R

+

11

=

+

22

11

11

jwcI

jwcr . (1) and

I1R3 =44

42

1 CjwR

RI

+ .. (2)

From (1) & (2)

31

1

1R

jwCr +

=))((

1

42

44

Rjwc

CjwR+

or

+

+ 44

41 1

1

RjwC

R

jwgr= R3

2

1jwc

Simplifying, r1R4 - 1

4

wC

jR

= 2

443

C

CRR

- 2

3

wC

Rj

Equating real and imaginary partsSeparately, we get :

r1R4= 2

443

C

CRR

& C1= Cx =(R4/R3) x C2NOTE: C1 =Cx1= Cx2=C2=Cs1=Cs2=.

PROCEDURE :1. Connections are made as per the circuit diagram.2. Connect the unknown capacitance Cx between the terminals A & D.3. Switch on the supply and vary the Resistance knob (R1) until minimum sound is obtained from the

loud speaker.4. Switch off the supply and measure the Resistance (R1) between C & B by using DMM.5. Repeat the above steps for different values of capacitance & tabulate the readings.6. Switch off the supply.

PRECAUTIONS :1. Avoid loose connections.2. Resistance should be varied very smoothly.3. Switch off the supply when the resistance R1 is measured.

FORMULA USED:

r1R4=2

443

C

CRR& C1= Cx =(R4/R3) x C2

NOTE: the value of unknown capacitors is

C = 0.01F0.02 F0.03 F

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0.04 f

s.no Cs (knowncapacitance)

R4 R3 Cx=CsR4/R3

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QUESTIONS:

1. What do you mean by high voltage Schering Bridge?2. State some of the errors that occur in bridge measurements?3. Anderson Bridge is a modified version of __________4. In Anderson Bridge the self inductance is measured in comparison with ___________

5. What are the resistors need to be adjusted to get the balance6. At what condition the galvanometer detector will be replaced by the head phone.7. Schering bridge is used for the measurement of ______________8. What is meant by loss angle ?9. Why we are doing electrostatic shielding for high voltage Schering bridge?10. What are the elements need to be adjusted to obtain balance in Schering bridge?

RESULT : the value of unknown capacitance is calculated by using Schering bridge

. MEASUREMENT OF CAPACITANCE BY SCHERING BRIDGE

Schering Bridge

At balance condition V1=V2 & V3=V4Here V1= (r1+ (1/jC1))V2 = (1/jC2)

V3 = I1R3

V4 = I2Z4

Where Z4 = (R4 (1/jC4))/ (R4+ (1/jC4)) = (R4 / jR4C4)Therefore V4 = (I2R4)/( jR4C4)At balance V1=V2i.e., (r1+ (1/jC1)) = (1/jC2) (1)

V3 = V4i.e., I1R3 = (I2R4)/( jR4C4)..(2)From equation (1) & (2)

V1 /V3 = V2 /V4(r1+ (1/jC1))/ (R4 / jR4C4) = (R3/ jC2)

By equating real & imaginary parts we get,

r1 R4 = (R3 R4 C4)/ C2

and C1 = Cx = (R4 C2)/ R3

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Calculation:By taking C2 value as 0.1f

We get R4 = 10.3 kR3 = 560m

Cx = (R4 C2)/ R3

Then, Cx = (10.3 k*0.1f)/ 560m= 1.83pf

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B. MEASUREMENT OF INDUCTANCE BY ANDERSONS BRIDGE

AIM : To find the unknown inductance of a coil or inductor using Andersons bridge.

APPARATUS :

Sl. NO. NAME TYPE RANGE QTY.1 Andersons bridge circuit 1 no2. Head phones 1 no.3. Decade inductance box 1 no.4. DMM DIGITAL 1 no.5. Patch cards 1set6 RPS 230 1 no7 Galvanometer 1 no

CIRCUIT DIAGRAM:

THEORY :

Andersons bridge is a modification of the Maxwells inductance capacitance bridge. In this method, theself-inductance is measured in terms of a standard capacitor. This method is applicable for precisemeasurement of self-inductance over a very wide range of values.Figure shows the connections and the phasor diagram of the bridge for balanced conditions:Let L1 = Self-inductance to be measure

R1 = resistance of self-inductor,r1 = resistance connected in series with self-inductor,r, R2, R3, R4 = known non-inductive resistances, andC = fixed standard capacitor.

At balance, I1 = I3 and I2 = Ic + I4

Now I1R3 = Lc xCj

1 Ic = I1jCR3.

Writing the other balance equations

I1 (r1+R1+jL1) = I2 R2 + Icr and Ic

+

Cjr

1= (I2 Ic) R4.

Substituting the value of Ic in the above equations, we haveI1(r1+R1+jL1) = I2R2+I1jC R3rOr

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I1(r+R1+jL1-jCR3r) = I2R2 (i)and

jCR3 I1

+

Cjr

1= (I2 IjCR3)R4 or I1(jCR3r + jCR3R4 +R3) = I2R4 (ii)

From Eqns. (i) and (ii), we obtain

I1 (r1 + R1 + jl1 jCR3r) = I1

++ 234

32

4

32 RCRjR

rRCRj

R

RR

Equating the real and the imaginary parts : R1 = 14

32 rR

RR

and L1 = C4

3

R

R[r(R4 + R2) + R2R4]

An examination of balance equations reveals that to obtain easy convergence of balance, alternateadjustments of r1 and should be done as they appear in only one of the two balance equations.

ADVANTAGES:

1. In case adjustments are carried out by manipulating control over r1 and r, they become independent ofeach other. This is a marked superiority over sliding balance conditions met with low Q coils whenmeasuring axwells bridge. A study of convergence conditions would reveal that it is much easier toobtain balance in the case of Andersons bridge than in Maxwells bridge for low Q-coils.

2. A fixed capacitor can be used instead of a variable capacitor as in the case of Maxwells bridge.

3. This bridge may be used for accurate determination of capacitance in terms of inductance.

DISADVANTAGES:

1. The Andersons bridge is more complex than its prototype Maxwells bridge. The Andersons bridgehas more parts and is more complicated to set up and manipulate. The balance equations are not simple

and in fact are much more tedious.

2. An additional junction point increases the difficulty of shielding the bridge.

Considering the above complications of the Andersons bridge, in all the cases where a variable capacitor ispermissible the more simple Maxwells bridge is used instead of Andersons bridge.

PROCEDURE :1. Connections are made as per the circuit diagram with an audio oscillator and head phones

connected to proper terminals of the Andersons bridge.2. Connect the unknown inductor L as shown in the circuit diagram.3. Switch on the supply and select a certain value of C say 0.01 F.4. Adjust R1and r1alternately till the head phones give minimum or no sound.

5. Note down the values of S, M and C at this balanced condition.6. Repeat steps (4) and (5) for the same inductance by selecting different value of C.7. Repeat the above steps for different values of unknown inductance.8. Switch off the supply.

NOTE :1. The value of C is so chosen that there is sufficient adjustment available in the value of M.2. When C is small, M will be large.3. The bridge is useful for measuring small values of inductor such as 50, 100, 150 and 200 mH.

Note the value of unknown inductances

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1. 10mH2. 100mH

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CALCULATION :

L value is calculated by the given formula.

L1 = C4

3

R

R[r1(R4+ R2) + R2R4]

R1 = 14

32 rR

RR

S.NO C(KNOWMCAPACITANCE)

r1 R1 R2 R3 R4 L1

RESULT :

The value of unknown inductance is calculated by using Anderson Bridge

QUESTIONS:

1. What do you mean by high voltage Schering Bridge?2. State some of the errors that occur in bridge measurements?3. Anderson Bridge is a modified version of __________4. In Anderson Bridge the self inductance is measured in comparison with ___________5. What are the resistors need to be adjusted to get the balance6. At what condition the galvanometer detector will be replaced by the head phone.7. Schering bridge is used for the measurement of ______________8. What is meant by loss angle ?9. Why we are doing electrostatic shielding for high voltage Schering bridge?10. What are the elements need to be adjusted to obtain balance in Schering bridge?

B. MEASUREMENT OF INDUCTANCE BY ANDERSONS BRIDGE

ANDERSONS BRIDGEAt balance condition I1=I3 I2 = IC+I4Here IC = I1 jR3CFrom the circuit balance equations areI1(r1+ R1+jL1) = I2 R3+ IC rIC(r+ (1/jC)) = (I2 - IC) R4By substituting of the value of IC in the above equation we get,

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I1 (r1+ R1+jL1 - jCR3r) = I2 R2......... (1)

I1(jCR3r+ jCR3R4+ R3) = I2 R4.........(2)From equation (1) & (2)I1 (r1+ R1+jL1 - jCR3r) = I1((R3 R2 /R4)+( jCR3 R2 r /R4)+ jCR3 R2)

By equating real & imaginary parts we get,

R1 = 14

32 rR

RR

and L1 = C4

3

R

R[r1(R4+ R2) + R2R4]

Calculation:In Andersons Bridge we have to calculate the unknown inductance value by using theformula

L1 =C4

3

R

R[r1 (R4+ R2) + R2R4]

By taking C value as 0.02fWe get R2 = 0.13 k

R3 = 0.06 kR4= 0.11 kr1= 0.2 k

By using the above formula we get the value of inductancei.e., L1 = (0.02f *(0.2 k (0.11 k+0.13 k) + 0.13 k * 0.11 k))/ 0.11 k

= 0.56mh

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EXPERIMENT NO.7

MEASUREMENT OF 3-PHASE REACTIVE POWER WITH SINGLE PHASE WATTMETER

AIM : To measure the 3-Phase reactive power using single wattmeter.

APPARATUS :Sl. NO. NAME TYPE RANGE QTY.

1. Wattmeter Dynamometer type, LPF 10A, 300V 1 no.2. Voltmeter MI 300 V 1 no.3. Ammeter MI 10A 1 no.4. 3 Phase variable inductive load 0-10A, 230 V, 6-steps

50, 15A1no.

5 Three Phase Variac 415/0-470V, 15A 1 no6 Connecting wires 1 set

CIRCUIT DIAGRAM:

THEORY :The basic principle used for measuring active as well as reactive power in an a.c. circuit is the

Blondels theorem.If a network is supplied through n conductors, the total power is measured by summing the

readings of n wattmeters is in each line and the corresponding voltage element is connected between theline and a common point.

If the common point is located on one of the lines, then the power may be measured by (n-1)

wattmeters.One wattmeter method can be used for the measurement of power but this method is applicable

only for balanced loads the current coil is connected in one of the lines and one end of the pressure coil tosome line, other end being connected alternatively to other two lines.

In case of balanced three phase circuits it is simple to use a single wattmeter to read reactivepower the current coil of the wattmeter is connected in one line and pressure coil is connected across othertwo lines.

Reading of wattmeter = )90cos(213 +IV

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= )90cos(3 +VI

= sinVI3

Q = 3 VI sin = ( )3 x reading of wattmeter.

Phase angle =

P

Q1tan .

PROCEDURE :

1. Connections are made as per the circuit diagram.2. Keep the 3-Phase variac to minimum or zero output position. Close the TPST switch and

gradually vary the variac until the rated voltage of 415V is applied.3. Note down the readings of all the indicating instruments at no-load position.4. Apply the load in steps by varying the 3-phase inductive load upto the rated current (10Amps) and

notedown the readings of instruments in each step.5. Vary the load gradually to no-load position, vary the 3-phase variac to minimum or zero output

position and open the TPST switch.6. Calculate the three phase reactive power.

TABULAR FORM:

S.No.

VoltmeterreadingV(volts)

AmmeterreadingI(Amps)

Wattmeterreading W(watts)

Three phase reactive powerQ= 3 *W(VAR)

sin= Q/ 3 VI

PRECAUTIONS :1. Keep the 3-phase variac in zero position initially.2. Keep inductive load at no-load position initially.3. Live terminals should not be touched.4. Connect the circuit without any loose connection.

5. If any wattmeter reads, reading, change either current coil or pressure coil connections.6. Take all meter readings without parallel error.7. Load current should not exceed rated current value.8 Vary the variac such that the current and voltage are within the rated value9. Meter readings should not exceed their ratings.10 Load should be varied very smoothly.11 Take proper care, such that the ammeter reading, should not exceed the current rating of LPF

wattmeter.

RESULT : Reactive power absorbed by the inductive load is calculated & tabulated for various loadingconditions.

QUESTIONS:

1. How do you measure power ?2. State the difference between wattmeter and an energy meter?3. Types of wattmeters?4. Which types of wattmeter is widely used?5. How is the controlling torque obtained?6. What are the errors in dynamometer type wattmeters? State a few.7. How many wattmeters do we require to measure 3-phase power?

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8. What is reactive power ? State the formula.9. How many wattmeters are required to measure 3-phase reactive power?10. How do we minimize the errors due to eddy currents in wattmeters?

Measurement of 3-phase reactive power with single phase wattmeter calculations:

When the load is balanced,Total power is P = 3*(Wattmeter reading)i.e., P = 3*Vph*Iph cosHence one wattmeter issued to measure the single phase power & then it is to be multipliedby 3

Q = 3VIsinQ = 3 *(Wattmeter reading)

Calculation:Take the voltage, ammeter reading and wattmeter reading from the circuitV = 300 v

I = 2 ampW = 480 wattsThen 3- reactive power is

Q = 3 *W VARi.e., Q = 3 * 480 = 831.2 VARand sin = /( 3VI )

i.e., sin = 831.2/ ( 3*300*2) = 0.799

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EXPERIMENT NO.8MEASUREMENT OF PARAMETERS OF CHOKE COIL USING 3VOLTMETERS &

3AMMETERS METHOD

AIM : To obtain the parameters (R, X, L,Z, power and PF)of given choke coil using 3 Voltmeter and 3Ammeter methods.

APPARATUS :

Sl. NO. NAME TYPE RANGE QTY.1. Choke coil 230 V, 10 A 1 no.2. .Regulated D.C

Power supply(0-30V) 1 no.

3. Voltmeter MI 300 V 3 no.4. Ammeter MI 10A 4 no.5. Rheostat 50, 15A 1no.6 Single Phase Variac AC 0-270V, 10A 1 no7 Connecting wires 1 set

CIRCUIT DIAGRAM:

THEORY :An inductive transducer works on the principle of variation of inductance using multiple coils.

The coils that are being used need to be evaluated and their parameters so defined such that the use of theirparameters may be regarded as constant and accurate.Thus emphasis need to be laid upon the method of measurement of inductance of choke coil by using 3

voltmeter meter method and 3 ammeter method.

For 3-Voltmeter method:

cos2 322

32

22

1 VVVVV +=

cos =)2( 32

23

22

21

VV

VVV

, but cos = 2/122 ])2([ fLr

r

+

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[ ] 2/12232

23

22

21

)2()2( fLr

r

VV

VVV

+=

( ) 2232221

23

22222

VVV

VV4r)fL2(r

=+

( )2

223

22

21

223222 4)2( rVVV

rVVfL

=

=L ( )2

223

22

21

223

22 r

VVV

rVV4

f2

1

For3-Ammeter method:

+= cosII2III 3223

22

21

cos =)2( 32

23

22

21

II

III

;

[ ]2/1

22 )2( fLr

r

+

[ ] 2/12232

23

22

21

)2(2 fLr

r

II

III

+=

( )

222

322

21

23

22

24

21

r

III

IIr

fL

=

Hence by using the above formulae we can calculate the inductance of a choke coil.

PROCEDURE :

3 Voltmeter method

1. Connections are made as per the circuit diagram.2. Keep the variac in minimum position and close the DPST switch.3. Vary the variac gradually in steps up to rated current capacity of conductor that used for coil or up

to voltage less than rated voltage indicated by voltmeter14. Note down the readings of meters in each step. Tabulate the readings as per tabular form15. Gradually vary the variac to minimum position and open the DPST switch.6 Now calculate the parameters of the choke coil.

3 Ammeter method1. Connections are made as per the circuit diagram.2. Keep the variac in minimum position and close the DPST switch.3. Vary the variac gradually in steps upto the rated voltage (230V).OR Apply current less than

(500mA) indicated by ammeter 24. Note down the readings of 3 Ammeters in each step. Tabulate the readings as per tabular form 25. Gradually change the variac to minimum position and open the DPST switch.6. Now calculate the parameters of the choke coil.

MEASUREMENTS OF RESISTANCE OF CHOKE COIL:

1. Make the connection as per the circuit diagram.2. Keep the variac in minimum position and close the DPST switch.3. Vary the variac gradually in steps say 10V,15V and 20V4 Note down the ammeter readings . Tabulate the readings as per tabular form 3

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5 Calculate resistance values R10v, R15v, R20v and take average of 3 resistance values as resistanceof choke coil.

TABLE 1

S.NO V1 V2 V3 IL P coso ZL RL XL L

TABLE 2S.NO V I1 I2 I3 P COSO ZL RL XL L

PRECAUTIONS :

1 Measure the exact magnitude of external rheostat.2 Avoid lose connections.3 Take readings without parllox error.4 Keep variac at their minimum position initially5 Vary the variac such that the current and voltage are within the rated value6 Meter readings should not exceed their ratings.7 Live terminals should not be touched.

8 If any wattmeter reads zero (or negative) reading, change either current coil or pressure coilconnections.9 Load current should not exceed rated current value.10 Load should be varied very smoothly.11 Take proper care, such that the ammeter reading, should not exceed the current rating of LPF

wattmeter.

RESULT :

The experiment is conducted by 3-voltmeter and 3-ammeter methods and inductance of the given chokecoil is calculated

QUESTIONS:

1. What are the choke coil parameters?2. What is the function of choke?3. What are the methods are there to find choke coil parameters?4. What are the methods are there to find choke coil parameters?5. Which method is very important for finding the choke coil parameters?

Measurement Of Parameters Of Choke Coil Using 3voltmeters & 3ammeters Method

calculations:

For 3-ph Voltmeter method

V12= V22 +V32- 2 V2 V3 cos

Cos = (V12= V22 +V32)/(- 2 V2 V3 )

But Cos = r/(r2+(2fL)2)1/2

So that (V12-V22 -V32)/ (- 2 V2 V3 ) = r/(r2+(2fL)2)1/2

By solving we get L = (1/2f) * ((4 V22 V32 r2)/ (V12-V22 -V32)) - r2

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Calculation:At on loadi.e., IL = 1 ampV2 = 100 vV3 = 106 v

P = 230 wattsCos = 0.9Therefore V12= V22 +V32- 2 V2 V3 cos = 220 v

L = (1/2f) * ((4 V22 V32 r2)/ (V12-V22 -V32)) - r2

Here f = 50Hz

L =(1/2f) * ((4 I22 I32 r2)/ (I12-I22 -I32)) - r2

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EXPERIMENT NO.9CALIBRATION OF LPF WATTMETER BY PHANTOM LOADING

AIM : Testing and Calibration of the given LPF wattmeter by using a sub-standard Wattmeter by phantomloading method.


1 Wattmeter(Testing or standard) Dynamometer type, LPF 10A, 300V 1 no2. Wattmeter(Sub-standard or calibrated) Dynamometer type, UPF 10A, 300V 1 no.3. Voltmeter MI 300 V 1 no.4. Ammeter MI 10A 1 no.5. Rheostat 350, 1.2A 1no.6 Single Phase Variac 0-270V, 10A 1 no7 Connecting wires 1 set

CIRCUIT DIAGRAM:

THEORY :

Electrodynamometer type wattmeter has two coils connected in different circuits for measurementof power. The fixed coils or field coils are connected in series with the load and so carry the current inthe circuit. The fixed coils, therefore, form the current coil or simply C.C of the wattmeter. Themoving coil is connected across the voltage and, therefore, carries a current proportional to the voltage. Ahigh non-inductive resistance is connected in series with the moving coil to limit the current to a smallvalue. Since the moving coil carries a current proportional to the voltage, it is called the pressure coil orvoltage coil or simply called P.C. of the wattmeter.

Both fixed and moving coils are air cored. The voltage rating of the wattmeter is limited to about 600 V bythe power requirements of the voltage circuit since most of the power is absorbed by the resistance in serieswith the moving coil and considerable heat is generated. For higher voltages, the pressure coil circuit isdesigned for 110V, and a potential transformer is used to step down the voltage.

If V= Voltage across the pressure coil,I = Current through the current coil and = Angle between V& I, thenP = Power being measured = VI cos

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The deflection is directly proportional to the power being measured and the scale is essentially uniformover the range in which (dM/d ) is constant. By suitable design, the mutual inductance between fixedand moving coil and be made to vary linearly with angle over a range of 40o to 50o on either side of zeromutual inductance position.

On lagging loads the wattmeter will read high, as the effect of the inductance of the pressure coil circuit isto bring the pressure coil current more nearly into phase with the load current than would be the case if thisinductance were zero. Very serious errors may be introduced by pressure coil inductance at low powerfactors unless special precautions are taken.

Many wattmeters are compensated for errors caused by inductance of pressure coil by means of a capacitorconnected in parallel with a portion of multiplier (series resistance).

Measurement of power in circuits having low power factor by ordinary electrodynamometer wattmeter isdifficult and inaccurate because:

i) The deflecting torque on the moving system is small (owing to low power factor) even whenthe current and pressure coils are flly excited;

ii) Errors introduced because of inductance of pressure coil tend to be large at low power factors.

Special features are incorporated in an electrodynamometer wattmeter to make it a low power factor type ofwattmeter. These features are discussed in details below:

1. Pressure Coil Current. The pressure coil circuit is designed to have a low value of resistance,so that the current, flowing through it, is increased to give an increased operating torque. The pressure coilcurrent in a low power factor wattmeter may be as much as 10 times the value employed for high powerfactor wattmeter.

2. Compensation for Pressure coil Current. The power being measured in a low power factorcircuit is small and current is high on account of low power factor. It is absolutely necessary to compensatefor the pressure coil current in low power factor wattmeter.

When the current rating of meter under test is high a test with actual loading arrangements would involve aconsiderable waste of power. In order to avoid this Phantom loading as Fictitious loading is done. Phantomloading consists of supplying the pressure coil from a circuit of required normal voltage and the current coilfrom a separate low voltage supply. It is possible to circulate the rated current through the current coil witha low voltage supply as the impedance of this circuit is very low, with this arrangement. The total powersupplied for the test is that due to the small pressure coil current at normal voltage, plus that due to thecurrent circuit current applied at low voltage. The total power therefore, required for testing the meter with

phantom loading is comparatively small.

PROCEDURE :1. Connections are made as per the circuit diagram.2. With variac I and II in minimum position and the rheostat in minimum position, close the

DPST-I switch to connect the supply-I.

3. Vary the variac-I gradually to apply the rated voltage (230V). to pressure coils of both p.f meterand wattmeter4 Close the DPST-II switch and vary the variac-II to adjust a suitable current of (1A or 2A) in the

current coils of LPF wattmeter and p.f meter. as indicated by ammeter. Note down the readingsof all the instruments

5 Now vary the current (upto 4A) in current coils of both wattmeter and p.f meter in steps usingsingle phase variac & rheostat and note down readings for each step. Tabulate the readings.

6 Bring the rheostat to minimum position, vary the variac-II and variac-I to minimum position andopen the DPST switches.

7. Evaluate True power (S1) , Actual power (S2) , %error, %correction.

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8. Draw the graph between Load current (vs) % Error.

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TABULAR FORM:

S.No VoltmeterReadingV (volts)

AmmeterReading,I (Amps)

P.Fmeterreading

WattmeterReading(S2)(Watts)

True power (S1)VIcos(Watts)

% error(S1-S2)/S1

%Correction(S2-S1)/S1

MODEL CALCULATIONS:

True power (S1) = V * I *cosActual power (S2) = Wattmeter reading.lpf% error = [(S1- S2) / S1] * 100%correction = [(S2 S1) /S1] * 100

MODEL GRAPH :

%error

Load current

PRECAUTIONS:

1. Avoid lose connections.2. Take readings without parllox error.3 Keep variac at their minimum position initially4 Vary the variac such that the current and voltage are within the rated value

5 Meter readings should not exceed their ratings.6 Live terminals should not be touched.7 If any wattmeter reads, reading, change either current coil or pressure coil connections.9 Load current should not exceed rated current value.10 Load should be varied very smoothly.11 Keep rheostat at its minimum resistance position initially.12 Take proper care, such that the ammeter reading, should not exceed the current rating of LPF

wattmeter.

RESULT: The LPF wattmeter is calibrated using phantom loading and calibration curve is plottedfor the given wattmeter

QUESTIONS:

1. What is meant by correction factor ?2. The load current in LPF wattmeter is high / low ?3. Why are the LPF wattmeter designed to have a smaller controlling torque ?4. What is the need of introducing compensating coil?5. State a few errors in dynamometer wattmeter?6. Applications of LPF wattmeter?7. Why more operating torque is produced in LPF wattmeter?8. Why the controlling torque in an LPF wattmeter is less?9. What are the different methods used for measurement for 3-phase power?

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10. Explain the working principle of induction wattmeter?

.

Calibration Of Lpf Wattmeter By Phantom Loading

Calculations:True power S1 = VICosActual power S2 = wattmeter reading LPF

% error = ((S1-S2)/S1)*100%correction = ((S2-S1)/S1)*100By taking values

V = 230 vI = 1 ampS1 = 230 * 1* 0.95 =218.5 wattsS2 = 90 watts% error = ((S1-S2)/S1)*100 = 58.81%%correction = ((S2-S1)/S1)*100 = -58.81%

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EXPERIMENT NO.10MEASUREMENT OF 3-PHASE ACTIVE POWER BY USING 2 CTS AND SINGLE WATTMETER

METHOD

AIM : To measure the 3-phase active power by using a single wattmeter and 2 CTs.


1 Wattmeter Dynamometer type, LPF 10A, 300V 1 no2. CTs 1:1, 5/5A 2 no.3. Voltmeter MI 300 V 1 no.4. Ammeter MI 10A 2 no.5. 3-phase variable inductive load Balanced star 350, 1.2A 1no.6 Three Phase Variac 0-270V, 10A 1 no7 Connecting wires 1 set

CIRCUIT DIAGRAM

THEORY :To measure 3- phase power using two CTs. Power can be measured in many ways, for an balanced load,only one wattmeter is enough to measure the 3- phase power, and for an unbalanced load, two wattmetermethod is used to measure the 3- phase power. The primary windings of CTs are connected in series withtwo phases. The secondary of both the CTs are connected as shown in figure the current coil of wattmeterconnected across both secondaries of CTs. The pressure coil is connected between the two phases.

PROCEDURE :1. Make the connections as per the circuit diagram.2. Keep the 3 phase variac in minimum position and close the TPST switch.3. Vary the 3- phase variac gradually and apply the rated voltage (415V).4. Note down all the meter readings and tabulate them.5. Vary the inductive load in steps upto the rated current and tabulate the meter readings in each step.6. Reduce the voltage to zero gradually by varying the 3 variac and open the TPST switch.

TABULATION:

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S.NO V W Ict1 Ict2 load

PRECAUTIONS :1. Avoid loose connections.

2. The secondary of CT should not be kept open.3. Take readings without error.4. Keep variac at their minimum position initially5. Vary the variac such that the current and voltage are within the rated value6. Meter readings should not exceed their ratings.7. Live terminals should not be touched.8 If any wattmeter reads zero or negative reading, change either current coil or pressure coilconnections.9. Load current should not exceed rated current value.10. Load should be varied very smoothly.13. Take proper care, such that the ammeter reading, should not exceed the current rating of LPF

wattmeter.

RESULT :The 3- active power of the inductive load is calculated using single phase wattmeter and two CTsmethod. at balanced load condition.

QUESTIONS:

1. What is Burden of transformer?2. Define (C.T&P.T) A. Transformation ratio

B. Turns ratioC .Nominal ratioD .RCF

3 Why C.T secondary should not be opened?4 Comparison between C.T & P.T

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EXPERIMENT NO.11LVDT AND CAPACITANCE PICKUP-CHARACTERISTICS AND CALIBRATION

AIM:To measure the displacement using linear variable differential transformer.

APPARATUS:.

CIRCUIT DIAGRAM:

THEORY:

Linear variable differential transformer LVDT is a transducer. Basically it is passive inductive transformersimilar to a potential transformer.

LVDT consists of three windings, one primary and two secondaries of equal turns. Primary is woundcentrally between two secondaries. All three windings are wound on a hollow tubular former throughwhich magnetic core slides.

Core affects magnetic coupling between primary and the secondaries while primary is connected to an ACsignal.

Normal / null position of core causes equal induced voltage in both the secondaries. Hence the total

difference voltage of both the secondaries becomes zero. Any deviation in core position from its nullposition induces unequal voltage from both secondaries and hence the difference signal of it is a non zeroquantity, this non zero quantity varies with core position. Ideally displacement versus change in differencesignal should be linear.When ES1=ES2 (core at null position or central position)

Ediff=0When core is moved left

ES1>ES2 &Ediff (ES1-ES2) is in phase with ES1

When core is moved right ES1


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Ediff (ES1-ES2) is in phase with ES2.

Amount of Ediff. is proportional to the displacement of core. Phase angle of the output voltage decides thedirection of core from its normal null position.Electronic circuit can be used to recover appreciable difference signal from LVDT. LVDT offers linearityin the output for certain Range of displacement. Change in its output voltage is stepless and resolution

depends on test equipment. Higher magnitude output is possible. LVDT can tolerate high degree ofvibration and shorts and hence more rugged. Repeat accuracy is better due to low hysteresis.

Disadvantages of LVDT are its sensitivity towards stray magnetic field (magnetic shielding is possible),large displacement is required for appreciable differential output.

PROCEDURE:

1. Connections are made as per the circuit diagram.2. Switch on the supply keep the instrument in ON position for 10 minutes for initial warm up.3. Rotate the micrometer core till it reads 20.0 mm and adjust the CAL potentiometer to display 10.0

mm on the LVDT trainer kit.4. Rotate the micrometer core till it reads 10.0 mm and adjust the zero potentiometer to display 20.0

mm on the LVDT trainer kit.5. Rotate back the micrometer core to read 20.0 mm and adjust once again the CAL potentiometer

till the LVDT trainer kit display reads 10.0 mm. Now the instrument is calibrated for 10mmrange.

6. Rotate the core of micrometer in steps of 2 mm and tabulate the readings of micrometer, LVDTtrainer kit display and multimeter reading.

Observations:

GRAPH:

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PRECAUTIONS:

1. Allow the instrument in ON position for 10 min. for initial warm up.2. While calibrating vary the cal/span pot slowly and smoothly.

RESULT:Displacement is measured using linear variable differential transformer and graphs of indicateddisplacement Vs actual displacement, %Error Vs actual displacement and output voltage Vs actualdisplacement.

QUESTIONS:

1. What is LVDT?2. What is transducer?3. How many transducers are there?4. How many windings the transformer in LVDT have in its construction?5. How the secondaries are connected in the transformer of LVDT?

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EXPERIMENT NO.12MEASUREMENT OF IRON LOSS IN A BAR SPECIMEN USING A CRO AND USING A

WATTMETER

AIM:To measure the iron losses in strip (sheet) material using Lloyd - Fisher Square.

APPARATUS:

CIRCUIT DIAGRAM

THEORY:The voltage across the secondary i.e. voltmeter reading given the rms value of the induced voltage.

E = 4 kfm f N2N2 = No. of turn of the secondary winding.

Bm1

=2fAsfNAK

E

(apparent value)As = area of specimen; m2

Bm = Bm1 0Hm

1As

Ac

Ac = Cross sectional area of coil; m2

Hm = Magnetizing force corresponding to maximum flux density; A/m. (obtained from B.H. curve of

specimen)

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Wattmeter reading = iron loss in the specimen + Copper loss in the secondary

Cu loss = rcrpE 2

+

Where rp = resistance of pressure coilRc = resistance of coil S1.

Total iron loss = Pi =rcrp

EVPE

2

+ watt

Pi = eddy current loss + hysterias loss

Eddy current loss = Volume x 3P

t2KfBm4f 222

Hysteresis current loss = Volume x fBmK

Volume =3Kg/miironofDensity

KginWeight

Steinmetz coefficient = 1.6F = 50 Hz

i.e. = specific resistivityKf= form factor (1.11)

T = thickness (given)

Lloyd fisher-square is most commonly used for measurement of iron loss in strip material. The stripmaterial to be tested is assembled as a closed magnetic circuit in the form of a square, called magneticsquare. The strips used are usually 0.25m long & 50 to 60mm wide, and ferromagnetic material.

PROCEDURE:

1. Connect the circuit as shown in figure.2. Adjust the voltage applied to the primary till the ammeter reads gives the required value of Bm.

3. Observe the wattmeter and voltmeter reading.

4. Reduce the current insteps of 0.5A and note down the readings.5. Switch OFF the circuit and calculate iron loss.

OBSERVATIONS :

CALCULATIONS:

P = Wattmeter reading

Pi = iron loss =rcrp

V

readingV

readingVP

2

2

1

2

+

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PRECAUTIONS:

1. Connect the circuit, without any loose connection.2. V1 & V2 voltmeter used in the secondary should be either electro static on electro dynamometer

type.3. Take the meter readings without parallel error.

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RESULT:

Iron loss are calculated in a bar specimen by using Lloyed fisher square method, by conducting theexperiment.

QUESTIONS:

1. What are the methods of measuring iron losses?2. What are the two types of squares used to measure iron losses?3. Which square is preferred to measure iron losses?4. How the strips are located in Epstein square and Lloyd-Fisher square?5. What are iron losses?6. What are the types of iron losses?7. How the hysteresis loss is minimized?8. How the eddy current loss is minimized?9. Define form factor?

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Electrical Measurements Lab Manual

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Transcript of Electrical Measurements Lab Manual