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Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-1 TESTING & CALIBRATION OF SINGLE PHASE ENERGY METER Aim: To test and calibrate a single-phase energy meter. Apparatus: Sl.No Apparatus Range Type Quantity 1. Voltmeter 0-300V MI 1No. 2. Ammeter 0-10A MI 1No. 3. Loading rheostat 3KW - 1No. 4. Energy meter 15A,240V Analog 1No. 5. Stop clock ---- Analog 1No. 6. 1- Auto 230V/ 1No. 0-270V,10A Transformer Theory : The energy meter is calibrated by finding error in the meter under different load conditions. In energy meter, there are two fluxes produced by currents, flowing in the series and shunt windings. These alternating fluxes produce emfs in the metallic disc. These emfs inturn circulate eddy currents in the disc. Thus there are two fluxes and two eddy currents and therefore two torques are produced. Total torque is the sum of the torques. The speed of rotation of the disc is proportional to power. Energy consumed = Number of revolutions/Meter Constant 1 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagram: Model graph (Indaicative Only) : %error IL(A) 0 2 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Procedure: 1. Connect the circuit as per the circuit diagram. 2. Apply rated voltage i.e., 230V to the energy meter by varying auto transformer. 3. Switch on the load & vary the load in steps. 4. For each step note down the time taken for 20 revolutions of the disc using stop clock.. 5. Note down the ammeter and voltmeter readings for each step. 6. Repeat the procedure until 10A of load current is reached. 7. Calculate the percentage error. 8. Plot a graph between load current and percentage error taking the load current on x-axis and percentage error on y-axis. Observations: Meter constant K = ______ rev/KWh S. No Current in amps IL(A) Voltage in volts V(V) No.of revoluti ons(N) Time taken for Actual 20 revolutions Energy = VI LT Secs Hrs(T) (Wh) Total energy recorded =N/K×1000 (Wh) % error 1 2 3 4 5 3 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Sample calculations: Voltage = Current = Time for 20 revolutions = Energy measured (actual in Wh) = VILT = Energy recorded = %error = No. of revolutions KWh meter cons tan t energy measured by energy meter actual energy consumed x100 actual energy consumed Precautions: 1.Initially the autotransformer should be kept in minimum position. 2.Initially load resistance should be kept in minimum position. 3.Time taken for 20 revolutions must be determined accurately. Result : 4 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-2 CALIBRATION OF DYNAMOMETER TYPE 1-PHASE POWER FACTOR METER Aim : To calibrate a dynamometer type power factor meter. Apparatus : S.No Apparatus Range Type Quantity 1 Ammeter 0-10A MI 1No. 2 Voltmeter 0-300V/ 150V MI 3No. 3 Powerfactor meter 250V / 10A D.M.T 1No. 4 1- auto transformer 230V/0-270V,10A 1No. 5 1- loading rheostat 3KW,230v 1No. 6 1- loading inductor 230V,15A 1No. 7 SPST Switch 1NO. Theory: Single phase dynamometer type power factor meter consists of a fixed coil which is split into two parts which carries a current of test circuit. Two identical pressure coils are pivoted on spindle of the moving system. One pressure coil has a resistance in series and other has an inductance in series. The two coils are connected across the voltage of the circuit. Current through one pressure coil will be in phase with the voltage and that of the other coil lags by 90 degrees. The torques due to these two coils is in opposite to each other. The pointer will stop when the two torques are equal. The deflection of the instrument depends on phase difference between the main current and currents in the two pressure coils. 5 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagram : Model graph (Indaicative Only) : % error IL(A) 6 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Procedure : 1. Connect the circuit as per circuit diagram. 2. Initially keep the autotransformer in minimum position & close the supply DPST switch. 3. Initially keep the inductive load in minimum position & close the SPST switch across it. 4. Vary the variac until voltmeter connected across it reads rated voltage. 5. Vary the resistive load until ammeter shows maximum possible reading ( 9.5A) 6. Note down the readings of all the meters. 7. Open the SPST switch connected across inductive load. 8. Increase the inductive load in steps. 9. Note down the readings of all the meters at each step. 10. Calculate the percentage error for each step. 11. Plot a graph between IL and percentage error. 7 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Observations : S.No Voltage applied VS (Volts) IL (A) Voltage Across R(VR) (volts) Voltage Across L(VL) (volts) Reading of meter Cos (measured value) pf Power Factor Angle =Tan- 1(VL/VR) Actual Value of pf meter Cos % error= Cos Cos Cos ×100 1 2 3 4 5 Sample calculations: Applied voltage Vs= Voltage across inductor VL= Voltage across resistor VR = Power Factor Angle = Tan-1 (VL/VR) = Cos = Cos = % error = Result: 8 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-3 KELVIN’S DOUBLE BRIDGE – MEASUREMENT OF RESISTANCE Aim: To determine the resistance of given unknown resistor.. Apparatus: Sl.No Apparatus Range 1. Kelvin’s double bridge --- Type Quantity ---- 1No trainer kit 2. Unknown resistance --- ---- 1 No 3. Galvanometer ---- Spot light 1 No. Theory: Kelvin’s double bridge is used for the measurement of low resistances (of the order less than 1).Wheatstone bridge is used for the measurement of medium resistances ranging from a few ohms to several megaohms. The lower limit for the measurement is set by the resistance of the connecting leads and by contact resistance of the binding posts. The error caused by leads may be corrected fairly well, but contact resistance presents a source of uncertainty that is difficult to overcome. This is eliminated in the Kelvin’s double bridge by connecting the galvanometer to any intermediate point of the known resistance(r) .Due to this, the resistance(r) is divided into two parts i.e., second set of ratio arms(P,Q) 9 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagram : Procedure: 1. Connect the unknown resistance (DRB) between the terminals C1 and C2 and short circuit the terminals P1 and C1, P2 and C2. 2. Connect the mains lead to 220V A.C. mains. 3. Choose the suitable range multiplier depending upon the magnitude of the unknown resistance connected between terminals C1 and C2. 4. Press the push button provided on kit to bring both battery and galvanometer into the circuit simultaneously. 5. Initially keep the slide wire at zero ohm position and vary the ‘variable standard resistance (main dial)’ and multiplier until the ‘G’ deflection is minimized. 6. By varying the slide wire slowly, adjust the ‘G’ position to zero in initial and final positions of ‘G’ switch for a particular value of sensitivity. Note down the values of variable resistance and slide wire resistance. 7. To eliminate errors, due to thermal e.m.f reverse the direction of the current using reversing switch and take another reading as above. The mean of these two values will give the correct value of unknown resistance 8. Calculate the unknown resistance at each step. 10 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Observations: Variable Resistance S. No resistance r R Normal Unknown slide wire Multiply-ing Factor(M) Reverse Normal Revers Resistance X =(R+r )M Normal e Reve rse 1 2 Sample calculations: Sensitivity= Variable resistance R: Normal = Reverse = Slide wire resistance r: Normal = Reverse = Multiplying factor (M) =1 Unknown resistance, Normal X=(R+r )M = Reverse X=,(R+r)M= Precautions: 1.The galvanometer should not be operated in short circuit position. 2.While changing the sensitivity of the galvanometer, it should be set in lock position. Result : 11 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-4 SCHERING BRIDGE Aim: To measure capacitance of an unknown capacitor and to determine its dissipating factor using Schering bridge. Apparatus: 1. Schering bridge kit -1 no. 2. Head phones 3. Connecting wires Theory: Schering bridge is used for the measurements of capacitance and dissipating factor. From the balance, the equation to obtain the value of capacitance is C=C1(R2/R1) Since R1and C3 are fixed values, capacitance value can be obtained directly by varying R2.The dissipation factor can be obtained directly from the equation D1=C1r1 Circuit diagram:- 12 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab C = unknown capacitance C1 = standard capacitance C2= variable capacitance R, R1 = non inductive resistances R2 = variable non inductive resistance in parallel with C3 Procedure:1. Connect “AC supply” of 1k Hertz to the “supply” terminals and one unknown capacitor on the kit with the terminals marked C in the circuit diagram and head phone with the terminals marked D. 2. Set the resistance dial R to zero position and also set capacitance dial C 2 to zero position. And also set resistance dial R1 at 1000 ohms. 3. Set the standard capacitor C1 to 0.01 μf position. 4. Now adjust the decade resistance dial R2 to minimize the sound in the head phone. 5. Note the value R1, R2 and C1 and calculate the value of unknown capacitor using above formula. 6. Repeat the same experiment on another value of C1. 7. At each step find out the value of C Where C = C1.R2 / R1 And dissipation factor D = 2 fCr1 where r1 = R1 x C2 / C1 and r1 is the resistance representing loss in C Observations S.no R1() R2() C1(μF) C2(μF) C= r1 = D= C1.R2 / R1 R1.C2 / C1 2fCr1 1 2 3 4 5 13 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Sample calculations: Non-inductive resistance R1 = Variable non-inductive resistance R2 = Standard capacitance C1 = Variable capacitance C2 = Unknown capacitance C= C1.(R2 / R1) = Internal resistance of C = r1= R1.(C2 / C1) = Dissipation factor=2fCr1= Result:Capacitance of given capacitor is S.no C D r1 1. 14 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-5 MEASUREMENT OF 3-PHASE REACTIVE POWER USING SINGLE PHASE WATTMETER Aim : To measure reactive power in a three-phase circuit using single phase wattmeter. Apparatus : Sl.No Apparatus Range Type Qty 1 Voltmeter (0-600V) MI 1 no. 2 Wattmeter 1,600V,10A, D.M.T 1 no. MI 1 no. LPF 3 Ammeter 0-10A, 4 3 autotransformer 415/0-470V, 1no. 10A, 8.14KVA 5 10A, 415V 3 loading inductor 1 no. Theory: Reactive power measurement in 3- circuits using 1- wattmeter can be done only for balanced 3- loads. By connecting the current coil of the wattmeter in one line and the pressure coil across the other two lines of 3- circuit, current through the current coil and voltage across the pressure coil are determined. Now as the current in the current coil lags the voltage by an angle of 90, the wattmeter reads a value proportional to the reactive power of the circuit. 15 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagram : Model graph (Indaicative Only) : 16 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Procedure : 1. Connect the circuit as per the circuit diagram. 2. Keep the variac of the auto-transformer in minimum position. 3. Close supply TPST switch and vary the auto-transformer slowly and apply rated voltage i.e.230V. 4. Vary the load gradually and at different loads, note down readings of ammeter, Voltmeter and Wattmeter. 5. Draw the phasor diagram. Phasor Diagram : Observations : For ideal inductive load = 900 --> Sin= 1 Reactive power S. N o Voltage Current VL volts IL amp Wattmeter reading Reactive power (actualvalue) (measured Qa= value)Qm= (W) 3W (VAR) 3VL I L sin %error = {(Qm-Qa)/ Qa}×100 ( VAR ) 1 2 3 4 5 17 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Sample calculations : Load voltage VL = Load current IL = Watt meter reading W = 3W = Reactive power (actual value in W) = 3VL I L sin = Reactive power (measured value) = % error = measured value - actual value x100 actual value = Result: 18 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-6 MEASUREMENT OF PARAMETERS OF A CHOKE COIL USING 3-VOLTMETER AND 3-AMMETER METHOD Aim : To measure parameters of a choke coil by 3 voltmeter method and 3 ammeter method. Apparatus : S.No Apparatus Range 1 Choke coil 230 V, 0.39A Copper wound 1 No 2 Ammeter 0-1/2 A MI 1 No 0-5 A MI 2 No 3 Voltmeter 0-300V MI 2 No 0-75V MI 1 No 1 4 Phase auto transformer Type 230V/0-270V,10A Quantity 1 No 5 Rheostat 145 /2.8A Wire wound 1No 6 Rheostat 25 /5A Wire wound 1No Theory & Formulae: Inductances of about 50 to 500mH can be measured using this method. It is suitable for iron cored coils, since the full normal current can be passed through it during measurement. From the phasor diagram, VS 2 2 2 VS = VR + VL + 2 VR VL cos VL VS VR VL r , Also cos = 2VRVL r 2 (L) 2 2 cos = 2 2 where r is the resistance and L is the inductance of the coil. VS VR VL 1 r = L= 2VRVL 2f r 2 (L) 2 2 2 2 19 2 VR 2 4r 2VR VL r2 2 2 2 VS VL VR Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagrams : (i) 3 voltmeter method : figure-1 (ii) 3 Ammeter method : figure-2 Procedure : 1. Make the connections as per the circuit shown in figure (1) 2. Initially keep the autotransformer in minimum position. 3. Close supply DPST switch. 4. Vary the applied voltage by varying the auto-transformer until rated current flows through the choke coil. 20 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab 5. Note down the readings of all the meters. 6. Make connections as per the circuit shown in figure(2). 7. Repeat steps 2,3,4 and 5. 8. Draw the phasor diagram for both the methods. Observations : 3-Ammeter method: S. Is Ir IL V Pf Cos = Resistance R Inductive Inducta N (A) (A) (A) (V) Is IR IL 2I R I L reactance V cos IL XL= nce (H) V sin IL XL/2L o 2 2 2 = L = 1 2 3-Voltmeter method : S. Vs Vr VL N (V) (V) (V) o I Pf Cos = Resistance R Inductive (A) Vs VR VL 2VRVL 2 2 2 = reactance VL cos I XL Inducta nce (H) = L = XL/2L VL sin I 1 2 Sample calculations: (i) 3-Ammeter method: cos = Is2 – IR2 – IL2 / 2IRIL = Resistance = (V / IL) cos = Inductive reactance of the coil = XL= [V/IL] sin = 21 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Inductance = [ XL/2f ] = Where w is the frequency of supply in hertz = 50Hz (i) 3-Voltmeter method: Power Factor cos = Vs2 – VR2 – VL2 / 2VRVL = Resistance = (VL / I) Cos = Inductive reactance of the coil = XL= [VL/I] sin = Inductance = [ XL/2f ] = Where f is the frequency of supply in hertz = 50Hz Phasor diagrams : 3-voltmeter method: V1 V3 3-Ammeter method : V2 I2 I3 I1 Precautions : 1. Avoid loose connections 2. Keep autotransformer in minimum position before closing supply DPST. 3. Readings are to be taken without parallax error. Results: The parameters of the given choke coil by 3 voltmeter method and 3 ammeter methods are measured as 3-Ammeter method 3-Voltmeter Method Resistance of the coil R Inductance of the coil L Power factor 22 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-7 CALIBRATION OF LPF WATTMETER BY PHANTOM LOAD TESTING Aim : To calibrate the given LPF wattmeter using phantom load testing. Apparatus : S.No 1 2 3 4 5 6 Apparatus 1- Auto transformer Ammeter Voltmeter Voltmeter Wattmeter Inductive load Range 230V/0-270V, 10A 0-10A 0-300V 0-150V 300V, 10A 250V/1-15 A Type MI MI MI LPF Quantity 2 nos. 1 no. 1 no. 1 no. 1no. 1no. Theory: When the current rating of a meter is high, a test with ordinary loading arrangement would involve a considerable wastage of power. In order to avoid this, phantom or fictitious loading is done. In phantom loading , pressure coil is connected across the supply voltage and current coil is connected in series with low voltage source, but it can supply the rated current because impedance will be low. Due to the above arrangement, the total power consumed for testing the meter is low when compared with actual loading of wattmeter. 23 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagram: Model graph (Indaicative only): % error IL(A) Procedure : 1) Connect the circuit as per the circuit diagram. 2) Initially keep the two autotransformers and inductive load in minimum position. 3) By varying the autotransformer2 in the pressure circuit, the voltmeter reading is adjusted to rated value i.e 230V. 4) By slowly varying the autotransformer1 in current coil circuit, the voltmeter reading is adjusted to 150V. 24 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab 5) Apply inductive load in steps and tabulate the readings of all meters and calculate % error at each step. 6) Plot the graph between IL and % error. Observations: cos = 0.2 S.No Voltmeter Reading(V) Ammeter Reading(A) % Error = Wattmeter Reading(Wr) (W) True Power WT=VI cos (W) Wr WT x100 WT 1 2 3 4 5 Sample Calculations: Voltage V = Current IL= Wattmeter reading (Wr) = True power (WT) = % Error = Wr WT x100 = WT Precautions: 1) Loose connections must be avoided. 2) Meter readings should not be exceeded beyond their ratings. 3) Apply the voltage slowly so that the current is within the limited range of ammeter. Result: 25 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-8(a) LINEAR VOLTAGE DIFFERENTIAL TRANSFORMER Aim: To study the operational characteristics of LVDT Apparatus: LVDT TRAINER KIT Theory: Differential Transformer, based on a variable inductance principle, are also used to measure displacement. The most popular variable-inductance transducer for linear displacement measurement is the linear variable differential transformer (LVDT). The LVDT illustrated in the fig. Consists of three symmetrically spaced coils wound on to an insulated bobbin. A magnetic core, which moves through the bobbin without contact, provides a path for magnetic flux linkages between coils. The position of the magnetic core controls the mutual between the center or primary coil and with the two outside or secondary coils. When an AC carrier excitation is applied to the primary coil, voltages are induced in the two secondary coils that are wires in a series-opposing circuit. When the core is centered between the secondary coils, the voltage induces between secondary coils are equal but out of phase by 1800. The voltage in the two coil cancels and the output voltage will be zero. When the core is moves from the center position, an inductance imbalance occurs between the primary coils and the secondary coil and an output voltage develops. The output voltage is a linear function of the core position as long as the motion of the core is with in the operating range of the LVDT. 26 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagram : Procedure : 1. Connect the terminals marked “PRIMARY” on the front panel of the instrument to the terminals marked “PRIMARY” on the transducer itself, with the help of the flexible wires provided along with. Observe the colour code for the wires provided and the colour of the binding posts. 2. Identically establish connections from terminals marked “SECONDARY”. Observe the colour code for wires provided and the colour of the binding posts. 3. Keep pot marked “MAX” in most anticlockwise position. 4. The magnetic core may be displaced and the pointer may be brought to zero position. If the DPM is not indicating zero, use potentiometer marked “MIN” to get zero on DPM at zero mechanical position. If the core is displaced in both directions, the meter must show indications with appropriate polarity. Now displace the core to 19 mm positions in one of the directions. Adjust the “MAX” pot to get an indication of 19.00 on the DPM under these conditions. We may again check for zero position also. 5. Now the core can be displaced by a known amount in the range of +19 and -19 mm and the meter readings can be note down. It may noted that by inter changing the secondary 27 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab terminals or the primary, the polarity of the meter indication can be reversed for a given direction of input displacement. 6. For LVDT provided with dial gage, adjust the magnetic core carefully by rotating the control knob in the clockwise direction. Note that for these type arrangement, displacement in only one direction i.e. positive direction is possible. 7. Plot the graph of input displacement and the output indication on the X and Y axes respectively. Observations : Input displacement Output (mm) Indication S.No. Precautions: 1. While connecting lead wire from panel to transducer, make proper connections following colour code. 2. Move the core with a gentle fashion by operating the knob for core movement very carefully. Do not try to effect the core movement beyond the given range. Result : 28 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-8(b) STRAIN GUAGE Aim: To measure the strain by using the strain guage. Apparatus: Strain measurement trainer - 1 Strain guage cantilever beam - 1 Weights - 10gm,100gm,etc. Theory : Strain guage is a transducer which converts the applied load strain in to the change in resistance of the materials. It is connected as one of the arms of the wheat stone bridge. According to change in resistance of material used in strain guage breidge unbalanced voltage will be appeared and this voltage is calibrated in to the values of strain. In tail type strain guage, nichrome, which has low temperature coefficient, is used. This wire is designed in such a way that its thickness will be 0.005mm. It is connected on paper bakelite sheet of thickness 0.05mm and there it is attached to the cantilever beam with adhesive material. Here cantilever beam is tee material whose strain is to be found. The nichrome is designed in such a way that its natural strain should be zero. 29 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit diagram : supply Procedure: 1. Connect the flexible wires provided with the strain gage cantilever beam between terminals 1-1, 2-2 and 3-3 and 4-4. 2. Amp. Gain pot may be kept in the position of 100. 3. Keep the switch SW2 in the downward position (4 arm configuration). 4. Turn on the mains supply. By gently moving the balance pot P1 and P2, Obtain initial balance on the meter and wait for 5 minutes to allow the strain gage temperature to stabilize. 5. Now apply a weight of 1kg on the cantilever and adjust the gain pot so that reading of 1.00 is obtained on the DPM. Now remove the weight and check 30 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab for the bridge balance. After one or two such adjustments we will be able get a reading of 1.00 on the DPM. 6. Apply the load on the sensor using loading arrangement provided in steps up to 5kg. 7. Note down the readings in the tabular form. The % error is calculated by comparing the theoretical values. 8. Plot graph of applied load versus the indicated meter reading. Observations : S.No. Weight on the DPM cantilever (kg) readings(kg) Practical Theoretical % Error value strain value strain (V) (V) Sample Calculations: Theoretical : L Efficient length of beam (15.5cm) b Width of bean (2.0 cm) t thickness of beam (0.59 cm) G Gauge factor (2.0) Eout strain, in V. 31 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab R strain gage resistance. Exc strain gage excitation (5V) E Modulus of Elasticity (2*106 kg/cm2) S Sensitivity of strain gauge (100mV/Kg) By definition stress = f = (W*L)/(0.166*bt2) = strain = stress / E = f/E = ΔR/R = strain * Gauge factor = We can consider Eout = Exc * (ΔR/R) = Practical: Eout = S*W = % Error = {(Practical Eout – Theoretical Eout )/ Theoretical } x 100 Precautions: Avoid loose connections Result: 32 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-9 DIELECTRIC OIL TESTING USING H.T.TESTING KIT Aim: To determine break over voltage of given dielectric oil, using H.T testing kit. Apparatus: 1. Dielectric oil testing kit – 1No. 2. Dielectric oil. Theory: The dielectric strength of an oil is the potential at which it starts behaving as a conducting medium. In the HT testing kit, the oil to be tested is placed in an acrylic box consisting of two metal electrodes. By varying the distance between electrodes and by applying high voltage across the electrodes, the break over voltage of the oil is determined. Dielectric strength of oil = break over volta ge (kV/cm) distance Dielectric strength of oil decreases with moisture. 33 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Circuit Diagram: 34 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Procedure: 1. Take the oil cup and adjust the gap between the electrodes with the help of gauge. 2. Fill up oil test cup with oil to be tested, close it with the lid and place it on the HT horns under the hinged acrylic cover and close the acrylic cover properly. 3. Keep the variac in minimum position. 4. Connect the mains lead to the 220V, single phase AC 50Hz supply. 5. Switch ON the power supply by operating the toggle switch, then yellow neon bulb glows indicating that the HT kit is switched off. 6. Press the HT ‘ON push’ switch. The red Neon lamp will start glowing and the HT transformer circuit will be energized, the green neon bulb start glowing. 7. In case the red indication does not glow, check up the hinged acrylic cover is properly closed and the variac knob is fully rotated in the anticlockwise direction for ‘0’ start. 8. Now start rotating the variac knob slowly in the clockwise direction till the flash over occurs across electrodes in the oil test cup. The speed of ratio should be such that the voltage rises at the rate of 2 kv/sec. 9. As soon as flash over occurs, the supply of the high voltage transformers, will be cut off and the voltage pointer will also stop indications the flash over level. Note down the reading of voltmeter and distance between the electrodes. 10. To repeat test on the sample, switch OFF the mains supply and stir the test pot with the help of a clean rod and let it cool for sometime and close the acrylic cover properly. 11. Repeat the steps 2 to 10. 12. Switch OFF the mains supply after the tests are over. Observations: S.NO Distance between electrodes (Cm) the Break over voltage Dielectric strength of of oil (KV) oil = break over volta ge distance (kV/cm) 1 2 3 35 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Precautions: 1. The lid of the HT testing kit should be closed properly. 2. The variac should be kept in minimum position initially. 3. Oil cup must be kept on the HT testing horns properly. Result : 36 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Exp-10 Measurement of 3-Phase Power Using 2 CTs and 1 Wattmeter Aim: To measure the 3-φ power in a balanced circuit using 2 CTs and 1 Wattmeter. Apparatus: Sl.No Apparatus Range Type Quantity 7. Voltmeter 0-600V MI 1No. 8. Ammeter 0-10A MI 1No. 9. Current Transformer 5/5 A - 2 No. 3KW - 1No. 10. Loading rheostat Theory : In this method the power absorbed in a 3-φ balanced circuit is measured using a single wattmeter in conjunction with 2 CT's. Usually 2-Wattmeter method is used measure the 3-φ power for both balanced and unbalanced load, but method like this requires only one wattmeter. The CT's used for this method should be of 1:1 ratio. The primaries are connected in series with the 2 phases. The secondaries are connected to the current coil of wattmeter such that the difference in the two phase currents will flow through the current coil. The wattmeter pressure coil is connected between the same two phases. Any wattmeter measures the product of Voltage across pressure coil VRY = VR - VY Current through the current coil IRY = IR - IY 37 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Cosine of phase angle between voltage and current is φ. ω = VRY. IRY.cos φ VR = VY = VB = V IR = IY = IB = I VRY = VR - VY = √3 V IRY = IR - IY = √3 I ω = 3V.I.cos φ = 3× phase active power The circuit is assumed to be connected in star, for delta connection also the procedure is valid and the wattmeter reads directly total power absorbed. Circuit diagram: Phasor diagram 38 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab Procedure 1. Give the connections as per the circuit diagram. 2. Apply 415 V, 3-φ, 50Hz supply closing the TPST switch. 3. Vary the load in suitable steps. 4. At each load note down the wattmeter readings. 5. Tabulate the results. Expected example tabulated results are as follows: S.No VL (Volts) IL (Amps) Wattmeter ω Calculated Reading (1.732VLIL (Watts) Cosφ) 1. 2 3 4 5 Sample calculations: Line Voltage VL= Line Current VR = 3- Phase Power = 1.732VLIL Cosφ = Precautions : 1. Avoid loose connections 2. Readings are to be taken without parallax error. Result: 39 Department of Electrical & Engineering Vignan’s University, Vadlamudi Electrical Measurements Lab ELECTRICAL MEASUREMENTS LAB MANUAL 40 Department of Electrical & Engineering