AC / DC MEASUREMENTS OBJECTIVES: The objectives of this experiment are to become familiar with the electrical engineering machinery laboratory, making electrical measurements, analyzing voltage and current waveforms and determining quantities like r.m.s, and harmonic components present in current and voltage waveforms. MEASUREMENT TECHNIQUES: The following measurement techniques are to be learned from this experiment: Use of ammeter and voltmeter in ac and dc circuits Use of current transformer and wattmeter in ac circuits Use of harmonic analyzer Use of oscilloscope. APPARATUS: 1. Test Table 2. Resistive load bank 3. 0-50 A D.C ammeter 4. One Current Transformer 5. One Wattmeter 6. Two digital multimeters 7. Harmonic Analyzer 8. Short extension cord dog bone (for measuring current in harmonic analyzer) 9. Current Probe 10. Oscilloscope. Figure 1: Picture of Dog Bone (short extension cord) 1/6
PROCEDURE 1. DC 2. AC (a) Plug in the test table. (b) Connect a single phase 120 Volt resistive load to terminals 1 and 2 at the right side of the test table. (c) Connect a 0-50 Amp dc ammeter to the ammeter- insertion -plug. All shorting switches must be closed. (d) Connect a multimeter (set to an appropriate range ) to the voltage connections at the output of reversing switch. (e) Connect the dc supply through the appropriate distribution box to terminals 1 and 2 on the left side of the test table, but do not energize(turn-on ) the breaker. (f) Take zero readings for the LEMS in lines 1 and 2. (g) Have the instructor check your connections. (h) Plug in the fan in the resistive bank. (i) After you have the instructor s permission, you may turn on the breaker. (j) Take five readings between zero and 50 Amperes. Remember to open the ammeter shorting switches. Record the output of the volt and current LEMS (k) Turn off the breaker at the main dc panel and disconnect the power source. (l) Remove the dc meters from the test table. (a) Use the same resistive load as for the dc portion of this lab. (b) Connect the 10A input of a multimeter in series with the current connections of a wattmeter and connect the combination to the secondary of a current transformer (CT). Connect an ammeter-insertion-plug to the 10 Amp primary of the CT. The instructor will show you how to change this connection as needed. (c) Connect a multimeter (set to an appropriate range) to the voltage connections at the output of the reversing switch. The reversing switch must be open. (d) Connect the voltage connections of the wattmeter in parallel with the voltmeter. (e) Connect the 3-phase 240Volt-ac supply to terminals 1,2,3 and N at the left side of the test table, but do not energize until the instructor approves your setup. Only the voltage between lines 1 and 2 will be used in this experiment; i.e., single-phase ac will be used. (f) Make sure the ammeter-shorting switch is closed and the voltmeter is deenergized by at least one switch. (g) Have the instructor check your connections. (h) Energize the system. 2/6
(i) Close the reversing switch to allow the voltmeter to read. Note any movement of the wattmeter when the circuit is energized. If the meter pointer moves down scale, reverse the voltage with the reversing switch. (j) Take five readings between 0 and 50 Amperes. (k) Record the currents in lines 1 and 2 using the ammeter. (l) Measure the voltage using the voltmeter. (m) Measure the power using the wattmeter. (n) Check your power measurements using the formula: P= V I cosθ. Remember that the power factor, cosθ, is 1 for a resistive load. 3. OSCILLOSCOPE: (a) Use the same experiment setup as in part 2. (b) Connect an inductor bank to terminals 1 and 2 on the right side of the test table. (c) Choose WYE connection for the load. (d) Set the position knob to 1-2, so that the meters read line voltage and line current. (e) Connect the voltage probe of oscilloscope across the load, using a voltage isolator. (f) Use current probe and clamp it to the wire with the current you want to measure. (g) Connect the current probe and voltage probe to an oscilloscope. (h) Record waveforms and the readings obtained from the oscilloscope and compare them with the readings obtained by meters. (i) Calculate the phase shift. (j) Now, set the position knob to 1-N, so that the meters read phase voltage and phase current. (k) The waveforms and reading obtained in the oscilloscope are recorded and checked with the meter readings. (l) Open the breaker and replace the inductor bank with a capacitor bank. (m) Repeat the above experiment, with the capacitor bank. (n) Open the breaker at the main A.C panel and disconnect the power source. (o) Remove the D.C meters from the test table. 3/6
4. HARMONIC ANALYZER: (a) Connect a P.C to the supply wall outlet on table. (b) Harmonic analyzer is used to measure rms. and peak values of voltage and current waveforms and can also be used to compute the magnitude and phase of harmonic components in the waveform. (c) Connect Harmonic Analyzer voltage probe across the input voltage to the PC. (d) Connect the current probe also to the supply with the help of dog bone. Use the clamp or current probe around the black or white wire. (e) Record the readings and graphs obtained with harmonic analyzer. (f) Sample readings and graphs are shown in the next page for reference. Calculation of phase shift in a oscilloscope: Definition: The phase shift is the difference between zero crossings of the voltage and current waveforms, when shown without dc component or dc offset. Use the voltage and current waveforms obtained using an oscilloscope. Use the GND button of oscilloscope, to match the reference levels of both the current and voltage waveforms. The rising or falling parts of both the waveforms are considered, then the number of time divisions between zero crossings are noted, say t. Then, the phase shift = t*360/16.67 sec. 4/6
REPORT 1. Using the data gathered in Part 1(j) of the procedure section, (a) Calculate the percent difference between the readings taken using the two LEMS. (b) Calculate the average value of the two readings taken using the LEMS. (c) Calculate the percent differences between the average of the two readings taken using the LEMS and the readings taken from the ammeter. Do not forget to correct the LEM readings for the zero current offset. Document the two LEMS current amplitudes, the percent difference between the LEMS currents, the average LEM current, the ammeter current, and the percent difference between the average LEM current and the ammeter current in a table. 2. Calculate the percent differences between the readings taken from the wattmeter (Part 2(m)) and the value for power calculated for part 2(n). Present the two values and the percent difference in a table. 3. Using the data gathered from the oscilloscope and meters, with the position of knob in 1-2 and 1-N position, the difference in the readings of voltages and currents can be calculated for the inductive and resistive loads. Also it verifies that V (line) = 3 V ( phase). Record the graphs of voltage and current obtained in the oscilloscope. Calculate the phase shift value in degrees and milliseconds. 4. The R.M.S value, peak value, THD, crest factor, harmonic component amplitudes of both the current and voltage were obtained from harmonic analyzer. The following equations can be used. Verify the following formulas with your data: Formulae: h max ( THD = Mh) * Mh) \ M1 h = = 2 RMS = ( 1+ ( THD) * ( THD) *M1. CREST FACTOR= (peak value of measured waveform)/(rms value) 5/6
Description of equipment used: WATTMETER A wattmeter is used to measure power. Hence it will have a current coil to measure current and a voltage coil to measure voltage. The current coil is always in series with the line and the voltage coil in parallel to the supply source. The resistive load and the voltage coil of the wattmeter are in parallel. Hence the voltage across load is equivalent to voltage across the voltage coil of the wattmeter. The voltage across load is obtained by measuring voltage across reversing switch. Hence, the voltage across the reversing switch and the voltage coil of the wattmeter are the same. The working principle: The wattmeter has two coils; one fixed and the other moving. The fixed coil carries the load current and the moving coil acts more as a potential element with the voltage of the load across it, say V. The power measured by a wattmeter is proportional to the torque produced on the moving coil. The torque on the moving coil depends on the product of field-flux density times the current in the moving coil; i.e. depends on the product of the currents in the two coils. The current through the fixed coil is load current I. The current through the potential circuit is practically V/R at any instant, where R is the resistance of the voltage coil. Therefore, torque depends on I*V/R; Average torque is proportional to average value of power in a steady state. In the sine wave case, V= Vm sin(ωt) I= Im sin(ωt+φ) P=(Vm sinωt) (Im sin(ωt+φ)) P=(Vm*Im)/2* (cosφ-cos(2ωt+φ)) P=(Vm/ 2)(Im/ 2)cosθ 6/6