Measurements Lab Manual

EMS Lab Plan: S.No (i)

Topic

Week

Equipment used in lab: 1. Single phase energy meter 2. Rheostat 3. wattmeter 4. Voltmeter 5. Ammeter 6. Single phase variac 7. Powerfactor meter 8. DC cromptons potentio meter 9. Kelvin’s double bridge kit 10. Currnt transformer Standard and commercial 11. Phase shifting trasformer 12. Schering bridge kit 13. Anderson’s bridge circuit 14. LVDT Trainer kit 15. Specimen(Liyod fisher)

W1

(ii)

Components used in lab 1. Decade Capacitance box 2. Decade Inductance box 3. Stop watch 4. Sensitive Galvanometer 5. Volt ratio box 6. Hed phones 7. Patch cards 8. Choke Coil 9. 3-Φ Variable Inductive Load Unit-I(Electrical Measurements)

W2

1

Expt-1: Measurement of parameters of choke coil using 3 voltmeter and 3 ammeter method Unit-II(Instrument Transformers)

W3

2

Expt-1: Calibration of Dynamo Power factor meter

W4

3

Expt-2: CT Testing by Silbees’s method

W5

Unit-III(Measurement of Power) 4

Expt-1: Measurement of 3-Φ reactive power with single phase wattmeter.

W6

5

Exp-2: Measurement of 3-Φ active power by using 2 CT’s and single wattmeter method. Unit-IV(Measurement of Energy)

W7

6

Expt-1: Calibration & testing of single phase Energy meter. Expt-2: Calibration of LPF wattmeter by Phantom Loading. Unit-V(Potentio meters)

W8

8

Expt-1:Calibration of PMMC ammeter & voltmeter using crompton’s DC Potentio meter. Unit-VI(Measurement of Resistance)

W10

9

Expt-1: Kelvins Double Bridge

W11

7

W9

Unit-VII(AC Bridges or Measurement of inductance and capacitance) 10

11 12

Expt-1: Measurement of capacitance by Schering bridge & Inductance by Anderson’s bridge. Exp-2: Measurement of Inductance by Anderson’s bridge. Unit-VIII(Magnetic Measurement)

W12

Expt-1: Measurement of Iron loss in bar specimen using a CRO and using a wattmeter Exp-2: LVDT and Capacitance pick-up characterstics and calibration. Supplementary topics

W14

Exp-1: Measurement of 3-Φ reactive power using two and three wattmeter method.

W16

Experiment no.1

W13

W15

Calibration and testing of single phase energy meter AIM : To Calibrate and test the given Single phase energy meter by using Sub-standard or calibrated Wattmeter with direct loading. APPARATUS Sl. NO. 1. 2. 3. 4. 5. 6 7 8

: NAME 1-Phase Energy meter Wattmeter(Sub-standard or calibrated) Voltmeter Ammeter Rheostat Single Phase Variac Connecting wires Stop watch

TYPE AC Dynamometer type, LPF MI MI

Digital

RANGE 230 V/10 A 10A, 300V 300 V 10A 50, 15A 0-270V, 10A

QTY. 1 no. 1 no. 1 no. 1 no. 1no. 1 no 1 set 1no.

CIRCUIT DIAGRAM:

THEORY : Induction type of energy meters are universally used for measurement of energy in domestic and industrial 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 and temperature 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 these electromagnets is made up of silicon steel laminations. The coil of one of the electromagnets is excited by the load current. This coil is called the ‘current coil’. The coil of second electromagnet is connected across the supply and, therefore, carries a current proportional to the supply voltage. This coil is called the ‘pressure coil’. Consequently the two electromagnets are known as series and shunt magnets respectively.

Copper shading bands are provided on the central limb. The position of these banks is adjustable. The function of these bands is to bring the flux produced by the shunt magnet exactly in quadrature with the applied voltage. Moving System: This consists of an aluminium disc mounted on a light alloy shaft. This disc is positioned in the air gap between series and shunt magnets. Braking System: A permanent magnet positioned near the edge of the aluminium disc forms the braking system. 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 shifting the permanent magent to different radial positions as explained earlier. Registering (counting) Mechanism: The function of a registering or counting mechanism is to record continuously 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 the instrument. 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. The breaking torque is produced by the interaction of eddy current and the field of permanent magnet. This torque is directly proportional to the product of flux of the magnet, magnitude of eddy current and effective radius ‘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 adjustments done to bring the errors of the meter with in prescribed limits. AC energy meters should be tested for the following 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. 2. 3. 4. 5. 6. 7. 8. 9 10.

Connections are made as per the circuit diagram. With load DPST open and with variac in minimum (or) zero output position, close the supply DPST switch. Gradually vary the variac to apply the rated voltage (230 volts). Close the load DPST and apply a suitable load (5A). Note down the readings of the meters and time taken for 5 revolutions of energy meter disc. 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. Gradually reduce the load in steps and open the load DPST. Vary the variac gradually to minimum or zero output position and open the supply DPST. Evaluate observed reading, actual reading, %error, %correction. Draw the graph between Load current (vs) % Error.

TABULAR FORM: S. No.

Voltmeter (Volts)

Ammeter Reading Amps

Wattmeter Reading W

Time for 20 rev

Theoretical kWH (O.R)

Practical wxt (A.R)

% error

O.R=OBSERVED READING A.R=ACTUAL READING

%error

MODEL CALCULATIONS: Observed reading = 1/(energy meter constant (k)/no. of revolutions) Where , no. of revolutions = 10 energy meter constant k=900 rev/kwh Actual reading = W *t %error = [(O.R-A.R)/A.R] *100 %correction = - % error = [(A.R-O.R)/A.R] *100 MODEL GRAPH :.

Load current PRECAUTIONS : 1. 2. 3. 4. 5 6 7. 8. 9 10 11 12

Avoid lose connections. Careful while observing the revaluation with stop watch. Do not apply more current, more than the rated energy meter current. Take readings without error. Keep variac at their minimum position initially Vary the variac such that the current and voltage are within the rated value Meter readings should not exceed their ratings. Live terminals should not be touched. If any wattmeter reads, reading, change either current coil or pressure coil connections. Load current should not exceed rated current value. Load should be varied very smoothly. 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. 2. 3. 4. 5. 6. 7. 8. 9.

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

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

EXPERIMENT NO.2 CALIBRATION 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. 1.

p.f. meter

2.

Wattmeter

3. 4. 5. 6 7

Voltmeter Ammeter Rheostat Connecting wires Single Phase Variac

NAME

CIRCUIT DIAGRAM:

TYPE

RANGE

QUANTITY

Dynamometer type, UPF Dynamo meter type LPF MI MI

10A, 300V

1 no.

10A, 600V

1 no.

300 V 10A 50 Ohm/5A

AC

270V, 10A

2 no.s 1 no. 1no. 1 set 1no.

THEORY: Power factor meters indicate directly, by a single reading, the power factor of the circuit to which they are connected. Power factor meters – like wattmeters – have a current circuit and a pressure circuit. The current circuit carries 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 usually split up into two parallel paths – one inductive and the other non-inductive. The deflection of the instrument depends upon the phase difference between the main current and the currents in the two paths of the pressure circuit, i.e. upon the phase angle or power factor of the circuit. The deflection is indicated by a pointer. The deflection of the instrument is a measure of phase angle of the circuit. The scale of the instrument 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 and therefore there is no need for a controlling force. Hence when a power factor meter is disconnected from a circuit 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

5 6. 7. 8. 9

Increase the load in steps and in each step note down the readings of the instruments. Tabulate the readings. Gradually reduce the load in steps and open the load DPST. Vary the variac gradually to minimum or zero output position and open the supply DPST. 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. Draw the graph between Load current (vs) % Error.

TABULAR FORM: S.No.

Voltmeter Reading (Volts)

Ammeter Reading , I (Amps)

Wattmeter Reading, W (Watts)

P.F. meter Reading(S2)

COSØ =W/VI (S1)

%Error =S1-S2*100 S1

%error

MODELGRAPH:

Lead p.f.

Lag p.f.

PRECAUTIONS : 1. 2. 3. 4. 5. 6. 7. 8. 9 10 11. 12. 13 14 15 16 17

There should not be any loose connection Meter readings should not exceed their ratings. Take readings without parallel errors. Note down the readings of voltmeter, wattmeter, and p.f. meter Vary the rheostat by observing the p.f. meter and load current upto rated current of rheostat. Note down all the readings and reduce the input voltage and open the DPST. Now, remove inductance from the circuit, connect only rheostat and follow steps from 2. Connect a variable capacitor in series with rheostat and follow the steps from 1 . Keep variac at their minimum position initially Vary the variac such that the current and voltage are within the rated value Meter readings should not exceed their ratings. Live terminals should not be touched. If any wattmeter reads, zero (or negative) reading, change either current coil or pressure coil connections. Load current should not exceed rated current value. Load should be varied very smoothly. Keep rheostat at its minimum resistance position initially. Take proper care, such that the ammeter reading, should not exceed the current rating of LPF wattmeter.

RESULT :

The dynamometer type power factor meter is tested at different power factors by using phase-shifting transformer and calibration curve is plotted. QUESTIONS: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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

Calculations of dynamo power factor meter: s.no Voltmeter Ammeter(Amps) (Volts) 1 230 0 2 230 1 i)

Wattmeter(Watts) 0 50

Powerfactor meter 1 1

%error=S1-S2/S1 *100 S1= cosФ=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

cosФ=W/VI Error= S1-S2/S1 0 0 0.217 -360.8

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. 1.

NAME

TYPE

DC Crompton Potentiometer Standard cell Volt ratio box

DC

5.

Regulated power supply(RPS) Ammeter

6 7 8

Voltmeter Connecting wires Sensitive galvanometer

9 10 11 12

Standard resistance Rheostat battery DC SUPPLY

2. 3. 4.

CIRCUIT DIAGRAM : CALIBRATION OF VOLTMETER

CALIBRATION OF AMMETER :

DC

RANGE

QTY. 1 no. 1 no. 1 no.

DC

Daniel cell (1.08V Input 0-1.5V Output 0-1.5, 15, 30, 100, 300V 0-30V

MC

0-2A

1no.

MC

0-300V

Spot reflecting MC type

30-0-30

1 no 1 set 1 no

0.01 Ohms, 10A 50 Ohms, 5A 2V(freshly charged) 220V

1 no 1 no 1 no 1no.

1 no.

THEORY : A Potentiometer is an instrument designed to measure an unknown voltage by comparing it with a known 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 null condition, no current flows and hence no power is consumed in the circuit containing the unknown emf when the instrument is balanced. Thus the determination of voltage by a potentiometer is quite independent of the source resistance. It can also be used to determine current simply by measuring the voltage drop produced by the unknown current passing through a known standard resistance. The potentiometer is extensively used for a calibration of voltmeters and ammeters and has in fact become the standard for the calibration of these instruments. For the above mentioned advantages the potentiometer has become very important in the field of electrical measurements and calibration. Modern laboratory type potentiometers used calibrated dial resistors and a small circular wire of one or more 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 wire against a standard reference source is known as “Standardisation”. PROCEDURE : STANDARDIZATION: 1. 2. 3. 4.

Connections are made as per the circuit diagram. Keep the function knob of the potentiometer at STD position. Switch on the RPS(1) and adjust to 2 volts. Adjust the slide contact and slide wire of the potentiometer to read standard cell voltage (1.08V). 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 be changed

PMMC VOLTMER 1. Volmeter under test is connected across potential divider in such a way that p.d across voltmeter can be varied 2. V.R box is used in parallel to potentiometer to reduce voltage to the range of potentiometer 3. Change the function knob to E1 position. Switch on RPS and adjust a suitable voltage on V.R box 4 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 RPS 7. 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.

CALCULATION : Calibration of voltmeter: V–R Box ratio = Output voltage / Input voltage Vact = Voltmeter reading Vtrue = Reading obtained from potentiometer = [Coarse voltage + Fine voltage] x Ratio of volt-ratio box

Vact − Vtrue × 100 V true % error = PROCEDURE: STANDARDIZATION: 1. 2.

Connections are made as per the Circuit diagram. Keep the function knob of the potentiometer at STD position. Switch ON the RPS(1) and adjust to 2 volts. Adjust the slide contact and slide wire of the potentiometer to read standard cell voltage (1.08V). 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 of galvanometer.Once standardization is done the position of R1 & R2 should not be changed

3. 4.

PMMC AMMETER: 1 Ammeter to be calibrated is connected in series with variable resistor R & standard resistance S 2 The standard resistance should of such a magnitude that current passed through it doesn’t exceed range of potentiometer 3 V.R box is used in parallel to potentiometer to reduce voltage to the range of potentiometer 4. 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

Vact − Vtrue × 100 Vtrue

FOR AMMETER: S.No

Ammeter reading(A)

E2 value(V)

Iact=E/R

%error

Itrue

I act − I true × 100 I true

CALCULATION : CALIBRATION OF AMMETER: Iact

= Ammeter reading

(Coarse volt + Fine volt ) × Ratio of volt − ratio box S tan dard resis tan ce Itrue = I act − I true × 100 I true % error = PRECAUTIONS : 1. 2. 3. 4. 5. 6.

7.

Connect the circuit without loose connections. Don’t vary the coarse and fine pots, after standardization. After connecting the unknown voltage, Don’t press the final button of Galvanometer directly. Operate the selector switch (P1) ad, slide wire (P2) very smoothly. Take care about current rating which choosing the standard rheostat in calibration of ammeter experiment. Connect the unknown voltage directly to the test terminals, if it has less than 1.5V. 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. The calibration curves for voltmeter and ammeter are drawn. QUESTIONS: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

What do you mean by a potentiometer? What are the types of potentiometer? What is the working principle of a potentiometer? What is standardization of potentiometer? What is the purpose of connecting a standard battery in the circuit? Application of dc potentiometer? What do you mean by calibration curve of the ammeter? What do you mean by a volt-ratio box? What are the types of AC potentiometer? 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)

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%

EXPERIMENT NO.4 KELVIN’S DOUBLE BRIDGE AIM : To determine the low resistance by setting up Kelvin’s double bridge. APPARATUS REQUIRED : Sl. NO. 1 2. 3. 4. 5. 6

NAME Kelvin’s double bridge kit Galvanometer Rheostat. DMM Patch cards/ Connecting wires RPS

TYPE

RANGE

5A,50 OHM DIGITAL DC

2V

QTY. 1 no 1 no. 1 no. 1 no. 1set. 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 through galvanometer G. The first of ratio arms is Q and M. The second set of ratio arms of m is used to connect the galvanometer to a point at the appropriate potential b/w point e and f to eliminate the effect of connecting lead of resistance r between then the unknown resistance X and standard resistance S. If

Q M Q = , thenX = S p m M

PROCEDURE : 1. 2. 3. 4. 5.

Short the terminals which are marked +c,-c & +p , -p. Calibrate the Kelvin’s double bridge by turning the zero adjusting knob until galvanometer indicates zero. During calibration remaining knobs should be kept at zero position. Now short terminals +p,+c and –p,-c. connect unknown specimen resistance between +p and –p terminals. Now adjust the main dial ,slide wire and multiplier until the galvanometer indicates zero position.

6.

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

OBSERVATIONS : S.No.

Specimen type (OR) standard or known

un Resistance or practical

1. 2. 3. 4. SAMPLE CALCULATIONS : Unknown resistance = multiplier *(main dial reading + slide wire reading) PRECAUTIONS : 1. 2. 3.

Galvanometer should always be connected to protective resistance to prevent damage to galvanometer. Changes in Q and q should be made immediately to bring the pointer of galvanometer back to within scale to prevent damage. Resistance r should be in the maximum position to start with and adjusted later if necessary to get large deflection.

RESULT : The unknown resistances of earth wires, having different parameters, are measured by using Kelvin’s double bridge. QUESTIONS: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Classify resistance? Examples of high resistance? What are the methods employed in measuring low resistances? Which is the most accurate method to measure the low resistances? State the reason? Kelvin double bridge is a modified version of ? What is the main problem in measuring low resistances? How do we measure high resistances? Practical methods to measure earth resistance? What are the quantities that are measured by ac bridges?

KELVIN’S 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

Tolerance = 2.5

EXPERIMENT NO. 5 CT TESTING BY SILSBEE’S METHOD AIM: To measure the % ratio error and phase angle error of given CT by Silsbee’s method. APPARATUS: SL. NO. 1. 2. 3. 4. 5.

CT CT Voltmeter Ammeter Wattmeter

6 7 8 9 10

Connecting wires Rheostat Loading Burden Rheostat Phase-shifting transformer Single Phase supply

NAME

TYPE Precision (Standard) Commercial MI MI Electrodynamometer Type LPF

RANGE

5/5 5/5 300 V 10A 10A, 600V

QUANTITY

1 no. 1 no. 1 no.s 2 no. 2no. 1 set

AC

50 Ohms, 5A 50 Ohms, 5A 3 Ohms, 420V, 10A 230V, 10A

1 no 1 no 1no.

CIRCUIT DIAGRAM:

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

Silsbee’s method is a comparison method. There are two types of Silsbee’s methods deflectional and here the ratio and phase angle of the test transformer are determined, in term of a standard transformers having the same nominal ratio. The two transformers are connected with their primaries in series. An adjustable burden is put in the secondary circuit of the transformers under test. An ammeter is included in the secondary circuit of the standard 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 wattmeter ‘W2’ carries a current which is the difference between the secondary currents of the standard and test transformers. 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. 2.

3. 4. 5. 6. 7.

Connections are made as per circuit diagram. Switch on the supply and by using single phase variac and rheostat adjust primary current to some value say 3 A. By varying the phase shifting transformer the phase of the voltage is so adjusted that wattmeter ‘W1’ reads maximum value (Phase condition). Now note down the readings of all meters. The readings of wattmeter in phase condition are taken as W1p and W2p Again vary the phase shifting transformer so that the wattmeter ‘W1’ reads zero value (90° phase shift –Quadrature condition). At this instant also note down all the meter readings and the wattmeter readings are taken as W1q and W2q. % Ratio error and phase angle error are calculated by using the formulae given below.

TABULATION: S.No.

Ammeter Reading I1(Amps)

Ammeter Reading I2(Amps)

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

W1p (Watts)

W2p (Watts)

W1q (Watts)

+ θs

FORMULAE USED : Nominal ratio of standard CT = 10/5 = 2 Phase angle error of standard CT = 5’ Actual ratio of standard CT = 1.99 W01   Actual ratio of CT under test = R x = R s 1 +   V I ss  Rs − R % ratio error = Rx    W02  180   Phase angle error for CT under test θ x = θ s +   ×  V I π    ss     PRECAUTIONS : 1. Take care, while varying the phase angle with phase shifting transformer.

W2q (Watts)

2 3 4. 5 6 7 8 9

Avoid lose connections. Meter readings should not exceed their ratings. Take readings without error. Keep variac at their minimum position initially Vary the variac such that the current and voltage are within the rated value Live terminals should not be touched. If any wattmeter reads, reading, change either current coil or pressure coil connections. Take proper care, such that the ammeter reading, should not exceed the current rating of LPF wattmeter. Load current should not exceed rated current value. Load should be varied very smoothly.

10 11

RESULT : The current transformer is tested by using Silsbee’s method and, the ratio and phase angle errors are calculated. QUESTIONS: 1 2 3 4

5

How types of Silsbee’s methods ? And what are those ? Silsbee’s methods----------- method What is Burden of transformer? Define (C.T&P.T) A. Transformation ratio B. Turns ratio C .Nominal ratio D .RCF Comparison between C.T & P.T

EXPERIMENT NO.6 A. MEASUREMENT OF CAPACITANCE BY SCHERING BRIDGE AIM : To find the capacitance of unknown capacitor using Schering bridge. APPARATUS : Sl. NO. 1 2. 3. 4. 5. 6 7

NAME Schering bridge circuit Head phones Decade capacitance box DMM Patch cards RPS Galvanometer

TYPE

RANGE

digital 230v

QTY. 1 no 1 no. 1 no. 1 no. 1set 1 no 1 no

CIRCUIT DIAGRAM:

THEORY: The Schering bridge is very widely used for the measurement of capacitance, dielectric loss and power factor of capacitors. The advantage of using this bridge is that it can be employed in both low voltage and high voltage measurements. From the circuit : C1 is the unknown capacitance and R1 is a resistance representing its loss component C2 is a standard air capacitor and it is loss free R3 & R4 are non-inductive standard resistors. R3 is fixed and R4 is variable C4 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,supply The bridge is balanced by adjusting C4 and R4 At balance, we have V1 = V2 & V3 = V4

  1  1  V1 = r1 + ,V 2 = 1 ,V3 = I1 R3 & V4 I 2 Z 4 jwc1    jwc 2 

 1  R 4  R4  jwc 4  = Where Z4 = 1 jwR 4C 4 + 1 R4 + jwc 4 12 R4 Or V4 = 1 + jwR 4 C 4   1  1   ……. (1) and ∴ 11 r1 +  = I 2  jwc1    jwc 2  I 2 R4 I1R3 = …….. (2) 1 + jwR 4 C 4 From (1) & (2)

1 jwC1 1 + jwR4 C 4 r1 + R3 = ( jwc 2 )( R4 ) or   R4 1    r1 +  jwg  1 + jwC 4 R4  

 1   jwc 2   = R3  R jR4 R3 R4 C 4 j 3 wC1 = C 2 wC 2 Simplifying, r1R4 Equating real and imaginary parts Separately, we get :

R3 R4 C 4 C2 r1R4= & C1= Cx =(R4/R3) x C2 NOTE: 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 (R 1) 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: R3 R 4 C 4 r1R4= & C1= Cx =(R4/R3) x C2 C2 NOTE: the value of unknown capacitors is C = 0.01μF 0.02 μF 0.03 μF

0.04 Μf s.no

Cs (known capacitance)

R4

R3

Cx=CsR4/R3

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 balance 6. 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=V4 Here V1= (r1+ (1/jωC1)) V2 = (1/jωC2) V3 = I1R3 V4 = I2Z4 Where Z4 = (R4 (1/jωC4))/ (R4+ (1/jωC4)) = (R4 / jωR4C4) Therefore V4 = (I2R4) /( jωR4C4) At balance V1=V2 i.e., (r1+ (1/jωC1)) = (1/jωC2) ……… (1) V3 = V4 i.e., I1R3 = (I2R4) /( jωR4C4)………..(2) From equation (1) & (2) V1 /V3 = V2 /V4 (r1+ (1/jωC1))/ (R4 / jωR4C4) = (R3/ jωC2) By equating real & imaginary parts we get, r1 R4 = (R3 R4 C4)/ C2 and C1 = Cx = (R4 C2)/ R3

Calculation: By taking C2 value as 0.1μf We get R4 = 10.3 k R3 = 560mΩ Cx = (R4 C2)/ R3 Then, Cx = (10.3 k*0.1μf)/ 560mΩ = 1.83pf

B. MEASUREMENT OF INDUCTANCE BY ANDERSON’S BRIDGE AIM : To find the unknown inductance of a coil or inductor using Anderson’s bridge. APPARATUS : Sl. NO. 1 2. 3. 4. 5. 6 7

NAME Anderson’s bridge circuit Head phones Decade inductance box DMM Patch cards RPS Galvanometer

TYPE

RANGE

DIGITAL 230

QTY. 1 no 1 no. 1 no. 1 no. 1set 1 no 1 no

CIRCUIT DIAGRAM:

THEORY : Anderson’s bridge is a modification of the Maxwell’s inductance capacitance bridge. In this method, the self-inductance is measured in terms of a standard capacitor. This method is applicable for precise measurement 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, and C = fixed standard capacitor. At balance, I1 = I3 and I2 = Ic + I4 1 ∴ Ic = I1jωCR3. Now I1R3 = Lc x jω C Writing the other balance equations  1   = (I2 – Ic) R4. Ic  r + jωC   Substituting the value of Ic in the above equations, we have I1(r1+R1+jωL1) = I2R2+I1jωC R3r Or I1 (r1+R1+jωL1) = I2 R2 + Icr and

I1(r+R1+jωL1-jωCR3r) = I2R2 …(i) and  1   = (I2 – IjωCR3)R4 or I1(jωCR3r + jωCR3R4 +R3) = I2R4 … (ii) jωCR3 I1  r + jωC   From Eqns. (i) and (ii), we obtain  R 2 R3 jωCR 2 R3 r  + + jωCR3 R2  I1 (r1 + R1 + jωl1 – jωCR3r) = I1  R4  R4  Equating the real and the imaginary parts : R1 =

R2 R3 − r1 R4

R3 [r(R4 + R2) + R2R4] R4 An examination of balance equations reveals that to obtain easy convergence of balance, alternate adjustments of r1 and should be done as they appear in only one of the two balance equations. and L1 = C

ADVANTAGES: 1. In case adjustments are carried out by manipulating control over r1 and r, they become independent of each other. This is a marked superiority over sliding balance conditions met with low Q coils when measuring axwell’s bridge. A study of convergence conditions would reveal that it is much easier to obtain balance in the case of Anderson’s bridge than in Maxwell’s bridge for low Q-coils. 2. A fixed capacitor can be used instead of a variable capacitor as in the case of Maxwell’s bridge. 3. This bridge may be used for accurate determination of capacitance in terms of inductance. DISADVANTAGES: 1. The Anderson’s bridge is more complex than its prototype Maxwell’s bridge. The Anderson’s bridge has 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 Anderson’s bridge, in all the cases where a variable capacitor is permissible the more simple Maxwell’s bridge is used instead of Anderson’s bridge. PROCEDURE : 1. Connections are made as per the circuit diagram with an audio oscillator and head phones connected to proper terminals of the Anderson’s 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

1. 2.

10mH 100mH

CALCULATION : ‘L’ value is calculated by the given formula. L1 = C

R3 [r1(R4+ R2) + R2R4] R4

R2 R3 − r1 R4

R1 = S.NO

C(KNOWM

r1

R1

R2

R3

R4

L1

CAPACITANCE)

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 balance 6. 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 ANDERSON’S BRIDGE ANDERSON’S BRIDGE At balance condition I1=I3 I2 = IC+I4 Here IC = I1 jωR3C From the circuit balance equations are I1(r1+ R1+jωL1) = I2 R3+ IC r IC(r+ (1/jωC)) = (I2 - IC) R4 By substituting of the value of IC in the above equation we get,

I1 (r1+ R1+jωL1 - jωCR3r) = I2 R2......... (1) I1(jωCR3r+ jωCR3R4+ R3) = I2 R4.........(2) From equation (1) & (2) I1 (r1+ R1+jωL1 - jωCR3r) = I1((R3 R2 /R4)+( jωCR3 R2 r /R4)+ jωCR3 R2) By equating real & imaginary parts we get, R1 =

R2 R3 − r1 R4 R3

and L1 = C R [r1(R4+ R2) + R2R4] 4

Calculation: In Andersons Bridge we have to calculate the unknown inductance value by using the formula R3

L1 =C R [r1 (R4+ R2) + R2R4] 4

By taking C value as 0.02μf We get R2 = 0.13 k R3 = 0.06 k R4= 0.11 k r1= 0.2 k By using the above formula we get the value of inductance i.e., L1 = (0.02μf *(0.2 k (0.11 k+0.13 k) + 0.13 k * 0.11 k))/ 0.11 k = 0.56mh

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. 1. 2. 3. 4. 5 6

:

NAME Wattmeter Voltmeter Ammeter 3 Phase variable inductive load Three Phase Variac Connecting wires

TYPE Dynamometer type, LPF MI MI

RANGE 10A, 300V 300 V 10A 0-10A, 230 V, 6-steps 50, 15A 415/0-470V, 15A

QTY. 1 no. 1 no. 1 no. 1no. 1 no 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 the line 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 to some 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 reactive power the current coil of the wattmeter is connected in one line and pressure coil is connected across other two lines. Reading of wattmeter = V13 I 2 cos(90 + φ )

3 VI cos(90 + φ )

=

− 3 VI sinφ

= Q = 3 VI sinΦ = − 3 x reading of wattmeter.

(

)

−1  Q  Phase angle Φ = tan   . P PROCEDURE :

1. 2.

Connections are made as per the circuit diagram. 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. Note down the readings of all the indicating instruments at no-load position. 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. Vary the load gradually to no-load position, vary the 3-phase variac to minimum or zero output position and open the TPST switch. Calculate the three phase reactive power.

3. 4. 5. 6.

TABULAR FORM: S.N o.

Voltmeter reading V(volts)

Ammeter reading I(Amps)

Wattmeter reading W (watts)

Three phase reactive power Q=√ 3 *W (VAR)

sinø= 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 value 9. 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 loading conditions. QUESTIONS: 1. 2. 3. 4. 5. 6. 7.

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

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 cosΦ Hence one wattmeter issued to measure the single phase power & then it is to be multiplied by 3 Q = 3VIsinΦ Q = √ 3 *(Wattmeter reading) Calculation: Take the voltage, ammeter reading and wattmeter reading from the circuit V = 300 v I = 2 amp W = 480 watts Then 3-Φ reactive power is Q =√ 3 *W VAR i.e., Q = √ 3 * 480 = 831.2 VAR and sinΦ = Φ/(√ 3VI ) i.e., sinΦ = 831.2/ (√ 3*300*2) = 0.799

EXPERIMENT NO.8 MEASUREMENT 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 3 Ammeter methods. APPARATUS Sl. NO. 1. 2. 3. 4. 5. 6 7

: NAME

Choke coil .Regulated D.C Power supply Voltmeter Ammeter Rheostat Single Phase Variac Connecting wires

TYPE

RANGE 230 V, 10 A (0-30V)

QTY. 1 no. 1 no.

MI MI

300 V 10A 50, 15A 0-270V, 10A

3 no. 4 no. 1no. 1 no 1 set

AC

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 their parameters 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: V12 = V22 + V32 − 2V2V3 cos φ cosΦ =

r V12 − V22 − V32 , but cosΦ = 2 [ r + ( 2 π fL) 2 ]1 / 2 (−2V2V3 )

V12 − V22 − V32 r = ∴ 2 (−2V2V3 ) r + (2πfL ) 2

[

2

2

2

r + (2πfL ) = r × (2πfL) 2 =

L=

1 2πf

(

(

− V22

1/ 2

4V22V32

( V12 − V22 − V32 ) 2

4V22V32 r 2

V12

]

− r2

)

2 − V32

4V22V32r 2

)

2 V12 − V22 − V32

− r2

For 3-Ammeter method:

I 12 = I 22 + I 23 − 2I 2I 3 cosφ cos  =

r I 12 − I 22 − I 32 ; 2 (−2 I 2 I 3 ) r + (2πfL ) 2

[

I12 − I 22 − I 32 r = 2 − 2 I 2 I3 r + (2πfL ) 2

[

∴L=

1 2πf

(

4 r 2 I 22 I 32 I 12

−

I 22

−

)

2 I 32

]

]

1/ 2

1/ 2

− r2

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 voltmeter1 4. Note down the readings of meters in each step. Tabulate the readings as per tabular form1 5. Gradually vary the variac to minimum position and open the DPST switch. 6 Now calculate the parameters of the choke coil. 3 Ammeter 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 upto the rated voltage (230V).OR Apply current less than (500mA) indicated by ammeter 2 4. Note down the readings of 3 Ammeters in each step. Tabulate the readings as per tabular form 2 5. 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. 2. 3. 4

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

5

Calculate resistance values R10v, R15v, R20v and take average of 3 resistance values as resistance of choke coil. TABLE 1 S.NO

V1

TABLE 2 S.NO V

V2

V3

IL

P

coso

ZL

RL

XL

L

I1

I2

I3

P

COSO

ZL

RL

XL

L

PRECAUTIONS : 1 2 3 4 5 6 7 8

Measure the exact magnitude of external rheostat. Avoid lose connections. Take readings without parllox error. Keep variac at their minimum position initially Vary the variac such that the current and voltage are within the rated value Meter readings should not exceed their ratings. Live terminals should not be touched. If any wattmeter reads zero (or negative) reading, change either current coil or pressure coil connections. Load current should not exceed rated current value. Load should be varied very smoothly. Take proper care, such that the ammeter reading, should not exceed the current rating of LPF wattmeter.

9 10 11

RESULT : The experiment is conducted by 3-voltmeter and 3-ammeter methods and inductance of the given choke coil is calculated QUESTIONS: 1. 2. 3. 4. 5.

What are the choke coil parameters? What is the function of choke? What are the methods are there to find choke coil parameters? What are the methods are there to find choke coil parameters? 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 cosΦ CosΦ = (V12 = V22 +V32)/(- 2 V2 V3 ) But CosΦ = r/(r2+(2ΠfL)2)1/2 So that (V12 -V22 -V32)/ (- 2 V2 V3 ) = r/(r2+(2ΠfL)2)1/2 By solving we get L = (1/2Πf) *√ ((4 V22 V32 r2)/ (V12 -V22 -V32)) - r2

Calculation: At on load i.e., IL = 1 amp V2 = 100 v V3 = 106 v P = 230 watts CosΦ = 0.9 Therefore V12 = V22 +V32- 2 V2 V3 cosΦ = 220 v L = (1/2Πf) *√ ((4 V22 V32 r2)/ (V12 -V22 -V32)) - r2 Here f = 50Hz L =(1/2Πf) *√ ((4 I22 I32 r2)/ (I12 -I22 -I32)) - r2

EXPERIMENT NO.9 CALIBRATION OF LPF WATTMETER BY PHANTOM LOADING AIM : Testing and Calibration of the given LPF wattmeter by using a sub-standard Wattmeter by phantom loading method. APPARATUS : Sl. NO. 1 2. 3. 4. 5. 6 7

NAME Wattmeter(Testing or standard) Wattmeter(Sub-standard or calibrated) Voltmeter Ammeter Rheostat Single Phase Variac Connecting wires

TYPE Dynamometer type, LPF Dynamometer type, UPF MI MI

RANGE 10A, 300V 10A, 300V 300 V 10A 350, 1.2A 0-270V, 10A

QTY. 1 no 1 no. 1 no. 1 no. 1no. 1 no 1 set

CIRCUIT DIAGRAM:

THEORY : Electrodynamometer type wattmeter has two coils connected in different circuits for measurement of power. The fixed coils or “field coils” are connected in series with the load and so carry the current in the circuit. The fixed coils, therefore, form the “current coil” or simply C.C of the wattmeter. The moving coil is connected across the voltage and, therefore, carries a current proportional to the voltage. A high non-inductive resistance is connected in series with the moving coil to limit the current to a small value. Since the moving coil carries a current proportional to the voltage, it is called the ‘pressure coil” or “voltage 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 by the power requirements of the voltage circuit since most of the power is absorbed by the resistance in series with the moving coil and considerable heat is generated. For higher voltages, the pressure coil circuit is designed 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 Φ = Angle between V& I, then P = Power being measured = VI cosΦ

The deflection is directly proportional to the power being measured and the scale is essentially uniform over the range in which (dM/dӨ ) is constant. By suitable design, the mutual inductance between fixed and moving coil and be made to vary linearly with angle over a range of 40o to 50o on either side of zero mutual inductance position. On lagging loads the wattmeter will read high, as the effect of the inductance of the pressure coil circuit is to bring the pressure coil current more nearly into phase with the load current than would be the case if this inductance were zero. Very serious errors may be introduced by pressure coil inductance at low power factors unless special precautions are taken. Many wattmeters are compensated for errors caused by inductance of pressure coil by means of a capacitor connected in parallel with a portion of multiplier (series resistance). Measurement of power in circuits having low power factor by ordinary electrodynamometer wattmeter is difficult and inaccurate because: i) The deflecting torque on the moving system is small (owing to low power factor) even when the 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 of wattmeter. 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 coil current in a low power factor wattmeter may be as much as 10 times the value employed for high power factor wattmeter. 2. Compensation for Pressure coil Current. The power being measured in a low power factor circuit is small and current is high on account of low power factor. It is absolutely necessary to compensate for 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 a considerable waste of power. In order to avoid this Phantom loading as Fictitious loading is done. Phantom loading consists of supplying the pressure coil from a circuit of required normal voltage and the current coil from a separate low voltage supply. It is possible to circulate the rated current through the current coil with a low voltage supply as the impedance of this circuit is very low, with this arrangement. The total power supplied for the test is that due to the small pressure coil current at normal voltage, plus that due to the current 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 meter and wattmeter 4 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 readings of all the instruments 5 Now vary the current (upto 4A) in current coils of both wattmeter and p.f meter in steps using single 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 and open the DPST switches. 7. Evaluate True power (S1) , Actual power (S2) , %error, %correction.

8.

Draw the graph between

Load current (vs) % Error.

TABULAR FORM: S.No

Voltmeter Reading V (volts)

Ammeter Reading, I (Amps)

P.F meter reading

Wattmeter Reading(S2) (Watts)

True power (S1) VIcosø(Watts)

% error (S1-S2)/S1

MODEL CALCULATIONS: True power (S1) Actual power (S2) % error %correction

= V * I *cosø = Wattmeter reading.lpf = [(S1- S2) / S1] * 100 = [(S2 – S1) /S1] * 100

MODEL GRAPH :

%error

Load current

PRECAUTIONS: 1. 2. 3 4 5 6 7 9 10 11 12

Avoid lose connections. Take readings without parllox error. Keep variac at their minimum position initially Vary the variac such that the current and voltage are within the rated value Meter readings should not exceed their ratings. Live terminals should not be touched. If any wattmeter reads, reading, change either current coil or pressure coil connections. Load current should not exceed rated current value. Load should be varied very smoothly. Keep rheostat at its minimum resistance position initially. 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 plotted for the given wattmeter QUESTIONS: 1. 2. 3. 4. 5. 6. 7. 8. 9.

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

%Correction (S2-S1)/S1

10. Explain the working principle of induction wattmeter? .

Calibration Of Lpf Wattmeter By Phantom Loading

Calculations: True power S1 = VICosΦ Actual power S2 = wattmeter reading LPF % error = ((S1-S2)/S1)*100 %correction = ((S2-S1)/S1)*100 By taking values V = 230 v I = 1 amp S1 = 230 * 1* 0.95 =218.5 watts S2 = 90 watts % error = ((S1-S2)/S1)*100 = 58.81% %correction = ((S2-S1)/S1)*100 = -58.81%

EXPERIMENT NO.10 MEASUREMENT OF 3-PHASE ACTIVE POWER BY USING 2 CT’S AND SINGLE WATTMETER METHOD AIM : To measure the 3-phase active power by using a single wattmeter and 2 CT’s. APPARATUS : Sl. NO. 1 2. 3. 4. 5. 6 7

NAME Wattmeter CT’s Voltmeter Ammeter 3-phase variable inductive load Three Phase Variac Connecting wires

TYPE Dynamometer type, LPF MI MI Balanced star

RANGE 10A, 300V 1:1, 5/5A 300 V 10A 350, 1.2A 0-270V, 10A

QTY. 1 no 2 no. 1 no. 2 no. 1no. 1 no 1 set

CIRCUIT DIAGRAM

THEORY : To measure 3- phase power using two CT’s. 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 wattmeter method is used to measure the 3- phase power. The primary windings of CT’s are connected in series with two phases. The secondary of both the CT’s are connected as shown in figure the current coil of wattmeter connected across both secondaries of CT’s. 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:

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 initially 5. Vary the variac such that the current and voltage are within the rated value 6. 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 coil connections. 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-Φ active power of the inductive load is calculated using single phase wattmeter and two CT’s method. at balanced load condition. QUESTIONS: 1. 2.

3 4

What is Burden of transformer? Define (C.T&P.T) A. Transformation ratio B. Turns ratio C .Nominal ratio D .RCF Why C.T secondary should not be opened? Comparison between C.T & P.T

EXPERIMENT NO.11 LVDT 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 transformer similar to a potential transformer. LVDT consists of three windings, one primary and two secondaries of equal turns. Primary is wound centrally between two secondaries. All three windings are wound on a hollow tubular former through which magnetic core slides. Core affects magnetic coupling between primary and the secondaries while primary is connected to an AC signal. 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 null position induces unequal voltage from both secondaries and hence the difference signal of it is a non zero quantity, this non zero quantity varies with core position. Ideally displacement versus change in difference signal should be linear. When ES1=ES2 (core at null position or central position) Ediff=0 When core is moved left ES1>ES2 & Ediff (ES1-ES2) is in phase with ES1 When core is moved right ES1