electrical measurements lab

Electrical Measurements Lab III/IV B. Tech; 1ST Semester –EEE ELECTRICAL MEASUREMENTS LAB I-CYCLE 1. Calibration and testing of single-phase energy meter. 2. Kelvin‟s double bridge - Measurement of resistance – Determination of Tolerance. 3. Power factor meter. 4. Measurement of parameters of choke coil using 3 -Voltmeter and 3 -Ammeter methods. 5. Measurement of Mutual Inductance. II- CYCLE 1. Crompton D.C potentiometer. Calibration of PMMC Voltmeter. 2. C.T testing by Silsbee‟s method. Measurement of % ratio error and phase angle of given C.T by comparison. 3. Schering Bridge and Anderson Bridge. 4. Calibration of LPF wattmeter by Phantom testing. 5. Measurement of 3-phase reactive power with single Phase wattmeter.

LIST OF EXPERIMENTS

EEE Department

Page 1

Electrical Measurements Lab S.No.

Name of the experiment

Page No.

1.

Single-phase energy meter

3-6

2.

Kelvin‟s double bridge

6-9

3.

Power factor meter

9-11

4.

Measurement of parameters of choke coil

11-15

5.

Measurement of Mutual Inductance

15-18

6.

Crompton D.C potentiometer

19-23

7.

C.T testing by Silsbee‟s method

23-26

8.

Schering Bridge and Anderson Bridge

27-31

9.

Calibration of LPF wattmeter by Phantom testing.

31-33

10.

Measurement of 3-phase reactive power with single Phase wattmeter.

33-35

EEE Department

Page 2

Electrical Measurements Lab

1.CALIBRATION OF 1 – Φ ENERGY METER Aim: To calibrate given 1- Φ energy meter using direct loading method. Name Plate Details: Energy meter:

Voltage Current Meter constant

: 240V : 10A : 900 rev / Kwh

Apparatus required: S.No 1 2 3 4 5 6 7

Name Energy meter Voltmeter Ammeter Wattmeter Autotransformer Resistive load Stop watch

Type 1- Φ M.I M.I UPF 1- Φ 1- Φ ---

Range 240V, 10A. 0-300V 0-10A 0-300V, 0-10A. 230V/ 0-270V, 10A 230V, 10A ---

Qty 1 1 1 1 1 1 1

Theory: In a single phase energy meter the supply voltage is aided across the pressure coil winding this highly inductive as it has very large number of turns and the reluctance of its magnetic circuit is very low owing to the presence of air gaps of very small length. Thus the current Ip throw the pressure coil is proportional to the supply voltage and lags it by a few degrees less than 90deg. This is because of the winding has small resistance and there are iron losses in the magnetic circuit current Ip produces flux Øpg. This flux divides into Øg and Øp. fluxØp is in phase with Ip and proportional to it.fulx Øp lags voltage by 90deg.

EEE Department

Page 3

Electrical Measurements Lab Circuit Diagram:

Procedure: 1. Connect the circuit as per the circuit diagram. 2. Keep the autotransformer at zero voltage position and make sure all the loads are in off position. 3. Now switch on the supply and variac is varied till to get rated voltage. 4. Apply the load step by step and each step note the readings of voltmeter, ammeter, wattmeter and time taken for 25 rev of the disk of the energy meter. 5. Apply the load below rated current. 6. After note the readings switch off the all loads and minimize the voltage and than switch off the supply. Calculations: The energy meter constant = 900 rev / Kwh. For 900 rev it records 1 unit or 1 Kwh. For 25 rev it records = 25 / 900 Kwh. = 1 / 36 Kwh. = (1000 X 60 X 60) /36 Wsec Energy meter reading (E1) = 1,00,000 Wsec. Actual energy consumed (E2) = Wattmeter reading X Time

EEE Department

Page 4

Electrical Measurements Lab Observation Table:

S. N o

Voltag e (V)

Wattmete r reading (W)

Current (A)

Time taken for 25rev (T)

Energy meter reading (E1)

Actual % Error= energy (E2-E1) / (E2) E2

Model Graph: A graph is drawn between % of error and load current. + % Error

I (amps)

Error -%

Precautions: 1. Loose connections should be avoided. 2. Meter reading should not exceed beyond their rating. 3. Take readings care fully.

EEE Department

Page 5

Electrical Measurements Lab Result: Hence the calibration of energy meter using direct loading is Done. VIVA VOICE QUESTIONS: 1 .why the rotating system of the energy meter is made as small as possible? 2. why use of aluminum disc is preferred over copper disc ? 3. How braking torque can be adjusted in motor meters ? 4. How is the mercury motor meter compensated for fluid friction at high loads ? 5. How is braking torque obtained in a mercury meter ? 6. How does energy meter differ from a watt meter? 7. What type of meter is used for measuring KVARH? 8. Why are clock meters not used as house service meters? 9. How are clock meters superior to induction type energy meters ? 10. What is self braking torque? Why is it important on high loads?

2.KELVIN’S DOUBLE BRIDGE Aim: To measure the resistance of given wire by using Kelvin’s Double Bridge. Apparatus required: 1. 2. 3. 4. 5.

Kelvin‟s Double Bridge experiment board. Standard resistance. Galvanometer. Unknown resistance wire. Connecting wires.

FIG: KELVIN’S DOUBLE BRIDGE

EEE Department

Page 6

Electrical Measurements Lab

FIG: FRONT PANEL Theory: Kelvin‟s double bridge is a modification of wheat stone bridge and provides greatly increased accuracy in measurement of low valve resistance‟s‟ represents the resistance of load that connects the unknown resistance „R‟and standard resistance „S‟ two actual resistance units of correct ratio be connected between en points „m‟ and „n‟ the galvanometer be connected to the junction of resistors . The second set of ratio arms , „p‟ and „q‟ are used to connect the galvanometer to point at the appropriate potential between the points „m‟ and „n‟ to eliminate theeffect of connecting lead of resistance „r‟ between known resistance „R‟ and standard resistance „S‟. P/Q ratio is made equal to p/q such that the resistance of connecting led „r‟ has no effect on the measurement. procedure: 1. The connections are made as shown in the circuit diagram. 2. Connect the known resistance to balance the bridge. 3. Now switch on the supply press battery key Kb and galvanometer key KG.

EEE Department

Page 7

Electrical Measurements Lab 4. Balancing the P/Q ratio by varying the resistance until galvanometer shows null deflection. 5. Compare both Kelvin‟s Double Bridge value & standard value. 6. Now connect the unknown resistance wire in the place of known resistance. 7. Balancing the P/Q ratio by varying the resistance until galvanometer shows null deflection. 8. Note the multipliers and calculate the resistance by using formulas. Calculations: R = (P/ Q) S Where R = Un known resistance P = Variable resistance. Q = Variable resistance. S = Standard resistance. Observation Table: S.no.

P

Q

s

R=(P/Q) S Ω

Result: Hence the unknown resistances of different wires are measured. VIVA-VOICE 1.What do you mean by low resistance? 2. What do you mean by medium resistance? 3. What do you mean by high resistance? 4. HOW does a megger differ from ohm meter? 5. Why is a megger provided with a slipping clutch? 6. What is a megger ? 7. What is the value of low resistance? 8. What is the value of high resistance? 9. What is the value of medium resistance?

EEE Department

Page 8

Electrical Measurements Lab 10. Low resistance are provided with four terminals_________________ 3.CALIBRATION OF DYNAMOMETER POWER FACTOR METER AIM: To calibrate the given dynamo meter power factor meter using given induction regulator. APPARATUS REQUIRED: S.NO Name 1 Power factor meter 2 Phase shift trans former 3 Voltmeter 4 Ammeter 5 Watt meter 6 Auto transformer

Type 1-phase 3-phase

Range 240V,10A 415V,500VA

Qty 1 1

M.I M.I UPF 3-phase

1 1 1 1

7

1-phase

0-300V 0-5A 0-300V,0-5A 415V/0440V,15A 230V/0270V,10A 50 ohm,5A

Auto transformer

8 Rheostat CIRCUIT DIAGRAM:

-----

1 1

Fig: POWER FACTOR METER EEE Department

Page 9

Electrical Measurements Lab

PROCEDURE: 1. Connect the ckt as per the ckt diagram. 2. Keep the auto transformers at zero voltage position and first switch on the 3 phase auto transformer supply 3. Now set the rated voltage appears across the output terminals of phase shift transformer, which are collected to the pressure coil of wattmeter and power factor 4. Now switch on the 1-Φ auto transformer supply and is adjusted to a suitable value say 2A. 5. Check out the reading of power factor meter and wattmeter they just indicating positive reading 6. First adjust the P.F reading to unity using induction regulator ,keep the voltage and current as constant vary the P.F meter reading and corresponding wattmeter reading is noted 7. Repeat the experiment for different values of P.F

CALCULATIONS: Actual reading of P.F

%error

=

=

×100

OBSERVATION TABLE: S.No Voltage(V) Current(I) Wattmeter P.F Actual %of reading reading reading error

RESULT: Hence calibrated the given dynamometer power5 factor meter using induction regulator.

EEE Department

Page 10

Electrical Measurements Lab VIVA-VOICE 1. How the power factor of a single phase circuit is measured? 2. What is principle of power factor meter? 3. What are the different types of power factor meters? 4. Why is moving iron power factor meter generally used? 5. Why is moving iron PF meters less accurate than dynamometer type? 6. What is power factor? 7. Give expression for the PF? 8. What is synchroscope and where it is used? 9. What is crossed coil PF meter? 10. What is instrument for speed measurements?

4.MEASUREMENT OF CHOKE COIL PARAMETERS Aim: To measure the parameters of a given choke coil using 1. 3 Ammeters. 2. 3 Voltmeters. Apparatus required: S.No 1 2

Name Voltmeter Ammeter

Type M.I M.I

Range 0-75/150/300V 0-5/10A

Qty 3 3

3

Autotransformer

1- Φ

230V/ 0-270V, 10A

1

6

Rheostat

0-230Ω, 1.7A. 0-50Ω, 5A. 230V, 1KVA

1 1 1

---

7 Choke coil --Theory: The parameters of choke coil includes measuring the values of R,L,X,Z..etc. the inductor should be so designed that capacitive effects are negligible . This is particularly important for inductors working at high frequencies where the inter turn capacitance may drastically change effective value of inductance. The inductor should be desirably unaffected by external magnetic fields and should produce a minimum interference field of its own. In the 3voltmeter method the resistance chosen should be very high for measuring the parameters of choker. In 3-ammeter method the resistance chosen was low when compare with the 3-voltmetter method to find the parameters of choke coils. The resistance should be low when

EEE Department

Page 11

Electrical Measurements Lab compared with their inductance or the inductor should be having high L/R ratio. L=XL/2πf 3-Voltmeter Method: Circuit Diagram:

Fig : 3-voltmeter method Procedure for 3-Voltmeter method: 1. The autotransformer in minimum output position. 2. By slowly make the connections as per the circuit diagram. 3. Initially varying the autotransformer, the voltmeter V1 is adjusted at deferent values from 0-150V. 4. Note down the corresponding readings of V2, V3. 3-Ammeter Method:

EEE Department

Page 12

Electrical Measurements Lab Procedure for 3-ammetr method: 1. Make the connections as per the circuit diagram . 2. Initially the autotransformer in minimum output position. 3. By slowly varying the autotransformer, the ammeter A1 is adjusted at deferent values from 0-5A. 4. Note down the corresponding readings of A2, A3. Calculations for 3-Voltmeter method: Supply voltage = V1 Voltage across standard resistance R = V2 Voltage across choke coil = V3 Power consumed by the choke coil P = V1² - V2²- V3² 2R Power factor of the choke coil Cosф = V1² - V2²- V3² 2V2V3 Current flowing through the choke coil I = V2/ R Impedance of the coil (Z) = V3/ I Resistance of the coil (R) = Z Cosф Reactance of the coil (X) = Z Sinф Induction of the coil (L) = X/2Πf Calculations for 3-Ammeter method: Power consumed by the choke coil P = [I1² - I2²- I3²] R 2 Power factor of the choke coil Cosф =

(I1²- I2²- I3²) 2 I2 I3

Voltage across the choke coil V = I2 R Impedance of the coil (Z) = V/ I3 Resistance of the coil (R) = Z Cosф Reactance of the coil (X) = Z Sinф Induction of the coil (L) = X/2 Observation table for 3 – Voltmeter method: S.No

V1

V2

V3

P= (V1²- V2²- V3²) 2R

EEE Department

CosΦ= (V1²- V2²- V3²) 2 V2 V3

SinΦ

I=V2 R

Z=V3 I

R=ZcosΦ

XL=ZsinΦ

L=XL/2Π f

Page 13

Electrical Measurements Lab Average Inductance =

Average Resistance =

Observation table for 3 – Ammeter method: S.N o

I1

I2

I3

P= (I1²- I2²- I3²) 2R

CosΦ= (I1²- I2²- I3²) 2 I2 I3

SinΦ

V= I2R

Z=V/I3

R=ZcosΦ

XL=ZsinΦ

Average Inductance = Average Resistance= Result: Parameters of a given choke coil by using 3 Voltmeter & 3 Ammeter methods is measured. VIVA-VOICE 1. what is inductance ? 2. what is formula for inductive reactance ? 3. what is formula for capacitive reactance ? 4. what is capacitance? 5. what is rating of dimmer stat? 6. What is meant by choke coil ? 7. What is the difference between MC &MI instruments? 8. what is resistance? 9. what is meant by power factor? 10. what is power?

EEE Department

Page 14

L=XL/2 Π f

Electrical Measurements Lab

5.MEASUREMENT OF MUTUAL INDUCTANCE Aim: To measure the mutual inductance of a given transformer. Name Plate details of transformer: Power Voltage Frequency Phase

3 KVA 230V 50 Hz 1- ф

Apparatus: S.No 1

Name Transformer

Type ---

2

Voltmeter

M.I

3 4

Ammeter Wattmeter

M.I LPF

Range 230V, 3KVA 0-150V 0-300V 0-2A 0-2A, 0-150V

Qty 1 1 1 1 1

Circuit Diagram:

Fig: Theory:

Measurement of mutual inductance

Mutual inductance has been used in great number of bridges for a variety of purposes known mutual inductances are used in some circuits for the measurements of known mutual inductances. Variable

EEE Department

Page 15

Electrical Measurements Lab mutual standard inductances have been used as components in bridges for the measurement of self inductance, capacitance and frequency. Many such bridges are found in literature, but some of the circuits represents minor modifications of other bridges that in changes to achieve greater accuracy or special range of unknown quantities of terminals of two coils whose mutual inductance is to be measured are available. Self inductance= flux linkages per unit current L= (NΦ/I) Mutual inductance is phenomenon but two coils coupled together M= (N2Φ1/I1) Procedure: 1. Make the connections as per the circuit diagram. 2. L.V side is taken as primary for magnification of no load current. 3. Keep the autotransformer at minimum voltage output position and switch on the supply. 4. Now initially apply some voltage to the primary winding and the corresponding no load current, wattmeter reading and e.m.f induced in the secondary side (V2) should be note down. 5. Calculate „M‟ using the formula M= V2 / (ωIm) 6. Repeat the experiment for deferent values of voltage. 7. Take one reading just above the reated voltage. Calculations: No load power factor, CosΦ0 = W/ (I0V1) Ic = I0 CosΦ0 ; Im = I0 SinΦ0 V2 = Induced e.m.f in the secondary side =ω MIm Mutual induction (M) = V2/( ωIm) Where ω=2πf Observation Table: S.No

Primary voltage (V2)

Graph:

Primary current (I0)

Wattmeter reading (W0)

Im = I0 SinΦ0

Secondary voltage (V2)

M= V2/( ω Im)

1. No load losses verses primary voltage. 2. Im verses no load voltage of primary. 3. Mutual induction verses voltage of primary.

EEE Department

Page 16

Electrical Measurements Lab Model graph: M vs V1

W vs V1 Im vs V1

M,W,Im

V1 Result: The Mutual inductance of a given transformer is calculated. Precautions: 1. Loose connections should be avoided. 2. Check the connections before giving supply. 3. Note the readings carefully. VIVA-VOICE 1. What is inductance? 2. What is mutual inductance? 3. What is the formula for voltage across inductance? 4. What is the formula for voltage across capacitance? 5. What is the difference between inductance and mutual 6. Why we are using LPF wattmeter in this experiment? 7. What is LPF wattmeter? 8. What is meant by exciting current? 9. What is meant by magnetizing current? 10. What is the formula for magnetizing current?

6.CROMPTON D.C POTENTIOMETER. Aim: Calibration of PMMC voltmeter and ammeter by using Crompton D.C Potentiometer. Apparatus: 1. 2. 3. 4. EEE Department

DC Potentiometer. Standard cell Volt ratio box Sensitive Galvanometer Page 17

Electrical Measurements Lab 5. 6. 7. 8.

DC power supply : (0-30V,1A) Volt meter: (0-30V), MC Ammeter: (0-1A),MC Rheostat: 50Ω,5A

Circuit diagram:

Fig:

DC Crompton Potentiometer

Circuit diagram:

EEE Department

Page 18

Electrical Measurements Lab

Calibration of Voltmeter:

Fig: calibration of voltmeter Theory: Crompton’s DC Potentiometer Crompton‟s DC potentiometer is a slide wire type of potentiometer. The long slide wire is awkward, and ever for the length shown cannot be read to a very great degree of precision. Modern laboratory type potentiometer use calibrated dial resistors and a small circular wire of one (or) more turns, there by reducing the size of the instrument. The circuit of a simple laboratory type potentiometer consists of one dial switch with filter steps ,each having a precession resistors. There is also a single turn wire. This potentiometer consists of unknown cell. A key and a protective resistance is used in the galvanometer circuits. Procedure: 1. Make the connections as per the circuit diagram. 2. Calibrate the DC potentiometer by adjust coarse and fine rheostats. When Galvanometer has null deflection and function must be in STD mode.

EEE Department

Page 19

Electrical Measurements Lab 3. The voltage across voltmeter is stepped down to a value suitable for application to a potentiometer with the help of volt ratio box. 4. For accurse of measurement it is necessary to measure voltage near the maximum range of the potentiometer. Calibration of Ammeter:

Fig: Calibration of Ammeter Procedure: 1. Made the connections as per the circuit diagram. 2. A resistance of suitable value and sufficient current carrying capacity is placed in series with the ammeter under calibration. 3. The voltage across the standard resistor is measured with the help of potentiometer and the current trough the standard resistance can be computed. 4. Current I = VS / R Where VS = Voltage across the standard resistor as indicated by the potentiometer. R = Resistance of standard resistor.

EEE Department

Page 20

Electrical Measurements Lab Tabular column: Calibration of Voltmeter: S.No

Voltage from DC potentiometer (Vact)

Voltmeter (Vin)

% Error = ((I out - I in)/ I

out)

x 100

Calibration of Ammeter: S.No

Iin (Amps)

I out = Vs / S (Amps)

% Error= ((I

out

-I

in)/

I

out)

x 100

RESULT: Thus PMMC Voltmeter and Ammeter using DC Crompton Potentiometer are calibrated. VIVA-VOICE 1. How is dc potentiometer made direct reading? 2. How is the dc potentiometer is standardized? 3. What is dc potentiometer ? 4. What is ac potentiometer? 5. What is the difference between dc and ac potentiometer? 6. What is polar type potentiometer ? 7. What is coordinate type potentiometer? 8. What is difference between polar and coordinate type potentiometer? 9. Why are ac potentiometer not very widely used? 10. On what basis ac potentiometer are classified?

EEE Department

Page 21

Electrical Measurements Lab

7.C.T TESTING BY SILSBEE’S METHOD AIM: To test the given current transformer by Silsbee‟s method and to determine the Phase Angle and Percentage Ratio error. APPARATUS: S.No Name 1 Phase shifting transformer

Type Range ------

Qty ----

2

Precision C.T

(0-5A)

1

3

commercial C.T

(0-5A)

1

4

Ammeters

MI

(0-5A)

4

5

Wattmeter

LPF UPF

(0-2.5A, 0-300V) (0-2.5A, 0-300V)

1 1

6

Rheostat

(50Ω, 5A)

1

THEORY: Silsbee‟s method is a comparison method. There are two types of Silsbee‟s methods: deflection and null. Only deflection method is described here. Here the ratio and phase angle of the test transformer X are determined, in terms of that of a standard transformer S 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 transformer under test. An ammeter is included in the secondary circuit of the standard transformer so that the current may be sent to the desired value . w1 is a wattmeter whose current coil is connected to carry the secondary current of the standard transformer. The current coil of wattmeter w2 carries current ∆I which is the difference between the secondary currents of the standard and test transformer s. The voltage Circuits of the wattmeter‟s (I .e; their pressure coils) are supplied in parallel from a phase shifting transformer at a constant voltage v.

EEE Department

Page 22

Electrical Measurements Lab

CIRCUIT DIAGRAM:

Procedure: 1. The connections are made as per the ckt diagram. 2. With the (10x4) rheostat cutout the supply switch to the auto transformer is closed. EEE Department

Page 23

Electrical Measurements Lab 3. Using the auto transformer the current through the ct primaries is gradually increased. 4. The current ID1, bring the difference of C.T‟s,s secondary current (Iss-Isx),, should be zero If it is twice that of Iss or Isx, the supply is opened & the connections to any one of the C.T secondaries are reversed. 5. This ensures proper polarity of C.T secondaries the current (Ip) through the C.T primaries is made equal to a fixed value using auto transformer . 6. The burden ( 10x )Ω rheostat is introduced in the secondary ckt . of the C.T to be Compared. This resistance is gradually cut in fill the current Id Is equal to a fixed value. 7. BY operating the phase shifter, the wattmeter Ws is made to indicate zero .The readings Of the ammeter & wattmeter WD2 are noted. 8. Now Ws is adjusted to indicate maximum reading by means of phase shifter and the Reading of the ammeters and wattmeters WD1 are noted. 9. The voltage V applied to the wattmeters potential coils is also noted. The above Procedure is repeated for different primary currents like 15 amp & 20amp . Calculations : Nominal ratio of standard C.T = 2 Ratio error of standard C.T = 0.5% Phase angle error of standard C.T = 8 Actual ratio of standard C.T = 2/(1+0.005) =Na‟ Actual ratio of C.T under test is given by Na = Na‟[1+WD/VIss] %Error = NR-AR/AR Phase angle error for the C.T under test = βx = [βs + (WD2/VIss)(180/Π)] Tabular column: S.NO Ip(A) Iss(A) ID(A) Ws(A) WD1(W) WD2(W) Isx(A) RATIO Phase ERROR angle error

EEE Department

Page 24

Electrical Measurements Lab

Phasor diagram: V

b

V ISX

ISS

a

s Φ

r

IP

RESULT: Hence the phase angle and percentage ratio error are calculated by using Silsbee’s method. VIVA-VOICE 1. What is meant by the term “instrument transformer”? 2. What is instrument transformer? 3. How do instrument transformer is differ from power transformer? 4. What do you understand by ammeter shunt? 5. What do you understand by voltmeter multiplier? 6. Why instrument transformers are used? 7. What is meant by turn “burden “of an instrument transformer? 8. How do current transformer is differ from potential transformer? 9. What is formula for ratio error? 10. What is formula for phase angle error?

8.a SCHERING BRIDGE AIM: To determine the value of given capacitor and obtain its dissipation factor. APPARATUS: 1. Schering bridge 2. Function generator 3. CRO 4. Probes and connecting wires 5. Digital voltmeter THEORY: Schering bridge is widely used for capacitance and the dissipation factor measurement in fact schering bridge is one of the most important of the A.C bridges, it is extensively used in measurement of capacitance in general and in particular in the measurement of proper he of insulators capacitor bushing , insulating oil and other insulating materials . The bridge is particularly suitable for small capacitance and is then usually EEE Department

Page 25

Electrical Measurements Lab supplied from a high frequency or a high voltage sources . The measurement done on small capacitance suffer from many disadvantages it carried out at low voltage high voltage schering bridge is preferable for such measurement.

Fig:

Circuit diagram of schering bridge

TABULAR FORM: R1(Ω) R2(Ω)

C3(µf)

CX=(R1/R2)C3 Dissipation factor

Average capacitance = Dissipation factor= Average Dissipation factor=

EEE Department

Page 26

Electrical Measurements Lab PROCEDURE: 1) The bridge is connected as shown in circuit diagram. 2) The oscilloscope is connected across the point b and d. 3) The bridge is balanced by adjusting Cy and Ry. 4) The reading are tabulated and C4 and R1 is calculated. 5) A number of reading are taken and average values are calculated. CALCULATIONS: C1=Unknown capacitance R1= Resistance representing losses in the unknown capacitance C3=Standard capacitor R3=Standard non-inductive resistance R4=Variable non-inductive resistance C4= Variable capacitor CX=(R1/R2)C3 Dissipation factor=2∏fCxR1 RESULT: Thus the value of given capacitance and Dissipation Factor are measured. VIVA VOICE QUESTIONS: 1.

Why a spark is connected across resistance arms in a Schering bridge? 2. The most useful ac bridge for comparing capacitances of two air capacitor is______________________________ 3. Dissipation factor of a capacitor can be determined by using a _______________________ 4. The capacitance and dielectric loss of a capacitor is generally measured by ___________ 5. A bridge used for measurement of dielectric loss and power factor is _________________ 6. The bridge used for measuring inter –electrode capacitance is____________________ 7. The bridge used for measuring dissipation factor of a capacitor is___________________ 8. Most commonly used AC bridge circuit for the measurement of capacitance is________ 9. The bridge suitable for measurement of capacitance of a capacitor at high voltage is ___________ 10. Why is Schering bridge particularly suitable for measurement at high voltage?

EEE Department

Page 27

Electrical Measurements Lab

8.b ANDERSON BRIDGE AIM: To calculate the value of unknown inductance by using Anderson Bridge. Apparatus: 1. Anderson Bridge trainer circuit. 2. Connecting wires. 3. Head phones. 4. Unknown inductance. 5. Auto frequency Oscilloscope. CIRCUIT DIAGRAM:

FIG: ANDERSON BRIDGE Procedure: 1. Connect the audio oscillator and Head phones to proper terminals. 2. 230 V supply is given to the Bridge oscillator. 3. In the Bridge the value of P, Q, R is equal to 1000 Ω. Minimum sound can be obtained by varying „S‟ and „M‟ alternatively. 4. The best way to get balance is to vary „S‟ first to dicers the sound in the headphones. The final minimum sound can be obtained by varying „M‟. EEE Department

Page 28

Electrical Measurements Lab 5. It is to be note that the perfect silence cannot be obtained in the headphones. But only minimum of sound can be achieved. 6. The value of M, S, and C can be noted. The value of L can be calculated from formula. L= C [RQ + (R+S) M] Where „L‟ is in henries „C‟ is in farads Other resistances are in Ohms. „S‟ includes the resistance of self-inductance also for all calculations. 7. The experiment for the same inductance can be reputed By selecting of deferent value of „C‟ and gating balance by varying „S‟ and „M‟. The mean vale of „L‟ can be calculated. Bridge arms: P = Non inductive resistance of 1000 Ω Q = Non inductive resistance of 1000 Ω R = Non inductive resistance of 1000 Ω S = A variable non inductive resistance in the form of 3 decades of 10x1, 10x10, 10x100 Ω. M = A variable non inductive resistance in the form of 3 decades of 10x1, 10x10, 10x100 Ω. C = A standard capacitance in the form 4 values of 0.005, 0.01, 0.02, 0.5 µfd selected by a selector switch Precautions: 1. The value of „C‟ should be small so as to allow sufficient variations of „M‟. 2. The A.C balance should be obtained by varying „S‟ and „M‟ alternately. Result: Hence the unknown value of inductance is measured by using Anderson Bridge .

EEE Department

Page 29

Electrical Measurements Lab VIVA VOICE 1. Maxwell bridge is used for measurement of _________________________ 2. Maxwell‟s bridge is very convenient and useful bridge for determination of Inductance of a coil having ________________________ 3. why there are two conditions of balance in ac bridges ? 4.why is high grade insulation employed in high impedance bridges? 5. why are highly sensitive detectors undesirable for the operation of ac bridges ? 6. In an Anderson bridge , the unknown inductance is measured in terms of________________ 7.Anderson bridge is used for the measurement of ______________________ 8Anderson bridge is a modification of _________________________________ 9.Anderson bridge is used to measure______________________ 9.CALIBRATION OF LPF WATTMETER BY PHANTOM LOAD TEST Aim: To calibrate the given LPF wattmeter using phantom loading. Apparatus: Name Type Range Qty Voltmeter

M.I

0-150V

1

ammeter

M.I

0-5/10A

1

wattmeter LPF Autotransformer ….. CIRCUIT DIAGRAM :

EEE Department

0-150V,5A 0-270V,10A,1-Ph

1 2

Page 30

Electrical Measurements Lab Fig: Calibration of LPF wattmeter Theory: Calibration of LPF watt meter by phantom loading : When the current rating of a meter under test is high as test with actual loading arrangements‟ would involve considerable wastage of power. In order to avoid this”phantom” or “frictions” loading is done. Phantom loading consist of supply the pressure circuit from a circuit of a required normal voltage and a current circuit from a low voltage supply. It is possible to circulate the rated current throw the current circuit with a low voltage supply as the impedance of the 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, pulse that due to the circuit current supplied at low voltage. The total power , required for testing the meter is comparatively very small. Procedure: 1. Connect the circuit as per the circuit diagram. 2. Initially keep the two autotransformers at minimum output voltage. 3. By varying the autotransformer in pressure circuit the voltmeter reading is adjusted to a rated value. 4. By slowly varying the autotransformer reading is adjusted at different values in steps. 5. The same procedure is repeated by connecting ammeter and wattmeter 10A range. Calculations: True power=VI COSΦ %Error=(W-Wt/Wt) X 100 Observation table: For Current 5A: Voltmeter Ammeter (V) (A)

EEE Department

Wattmeter (W)

True power VICOSØ

%Error (W-Wt/Wt) X 100

Page 31

Electrical Measurements Lab Observation table: For Current 2.5A: Voltmeter Ammeter (V) (A)

Wattmeter (W)

True power VICOSØ

%Error (W-Wt/Wt) X 100

Model Graph: + %Error

(0-5A)

(0-25A)

W -%Error

Result: Hence calibrations of the given LPF Wattmeter using Phantom loading are done.

VIVA-VOICE 1.What is meant by phantom loading? 2.What is meant by low power factor? 3.How is electrostatic wattmeter superior to other types of wattmeter? 4.How does LPF wattmeter are differ from ordinary wattmeter? 5.What is formula for low power factor? 6.What are the errors in wattmeter? 7.What is meant by fictitious load? 8.What are the special features of a wattmeter suitable for working on LPF circuits? 9.What is the rating of dimmer stat?

EEE Department

Page 32

Electrical Measurements Lab

10.MEASUREMENT OF 3-Φ REACTIVE POWER WITH 1- Φ WATTMETER AIM: To measure the reactive power and to draw a characteristics curve between KVAR and P.F with load current of 3 - Φ system with 1- Φ wattmeter method. NAME PLATE DETAILS: power --- 3 KVA voltage --- 415V current --- 4.5A frequency -----50 HZ speed -----1440 rpm APPARATUS: NAME

TYPE

RANGE

QTY

Voltmeter

M.I

0-5A

1

Ammeter

M.I

0-600V

1

Wattmeter

UPF

600V,5A

Tachometer

---------

----------

2 1

Theory:This method can be used only when the load is balanced. The connections are shown in the figure. The current coil is connected in one of the lines and one end of the pressure coil to the same line, other end being connections alternatively to the other two lines. Power measurements are made in high voltage circuit connecting the wattmeter to circuit through current and potential transformers. Voltmeters and Ammeters are affected by only ratio errors while watt meters are influenced in additional by phase angle errors. CIRCUITDIAGRAM:

FIG:Measurement of 3 – Φ Reactive Power with 1- Φ Wattmeter

EEE Department

Page 33

Electrical Measurements Lab Procedure: 1. Connect the ckt as per the ckt diagram. 2.The motor is started with the help of starter. 3.Varying the load on the motor by tighten the spring balance over the pully of the motor. 4. Take the corresponding reading of the voltmeter, ammeter and wattmeter. 5.Repeat the process for different values up to rated current 6.Calculate the reactive power. Calculations: Power factor, cosΦ = W/VI Reactive power, Q = VI sinΦ KVAR = Q/1000 Model graph: P .F

KVAR KVAR

F

P.F

IL(A) Precautions: 1. loose connections should be avoided. 2. operate the starter care fully. 3. loads should not exceeding beyond their rating. 4. proper rating meters should be used. 5. check the connections before giving supply. Result: Hence the 3- phase reactive power is measured by 1-phase wattmeter method and characteristics curve between KVAR and P.F with load currents are plotted. VIVA-VOICE 1. What is electrodynamometer type wattmeter ? 2. What is meant by KVAR ? 3. What is formula for reactive power? 4. What is meant by UPF wattmeter? 5. What is rating of three phase induction motor? 6. What is meant by TPST switch? 7. What is meant by DOL starter? 8. What is meant by balanced load? 9. What is meant by unbalanced load? 10. What is model graph of reactive power measurement?

EEE Department

Page 34

Electrical Measurements Lab

EEE Department

Page 35

Electrical Measurements Lab

EEE Department

Page 36

Electrical Measurements Lab

EEE Department

Page 37

Electrical Measurements Lab

EEE Department

Page 38

Electrical Measurements Lab

EEE Department

Page 39

Electrical Measurements Lab

EEE Department

Page 40