Part 1: AC MEASUREMENTS

Transcription

1 Part 1: AC MEASUREMENTS OBJECTIVES: The objectives of this experiment are to become familiar with the electrical engineering machinery laboratory and making electrical measurements. MEASUREMENT TECHNIQUES: The following measurement techniques are to be learned from this experiment: Use of multimeters, a current transformer and a wattmeter in ac circuits 1. AC MEASUREMENTS APPARATUS FOR AC MEASUREMENTS: 1. Test table 2. Single phase resistive load bank 3. One current transformer (CT) 4. One 500W wattmeter 5. Two digital multimeters 6. Two Current Clamps 7. Capacitor Bank 8. Ammeter Insertion Plug (a) Plug in the test table. This is necessary to protect the LEMS. (b) Connect a single phase 120V resistive load bank to terminals 1 and 2 on the right side of the test table. Make sure the load bank is set to the no load condition; meaning all switches are in the center position. Plug in the fan for the resistive bank. (c) Connect the 10A input of a multimeter in series with the current connections of the wattmeter. Complete the series connection with the secondary (5A and l outputs) of the current transformer. Remember, never open circuit the secondary of a current transformer. (d) Connect an ammeter insertion plug to the primary side of the CT. For this experiment, use the 50A and L terminals which creates at 10:1 ration of current from the test table to the multimeter and wattmeter. Note: Remember this 10x ratio in your current and power calculations. The metal shorting bar on the top of the CT should be closed until you are ready to take your measurements. (Be sure to use a plastic pen to close the shorting bar if it s not already closed.) Use Figure 1 as a reference. (e) Connect a second multimeter (set to an appropriate AC range) to the voltage connections at the output of the reversing switch. The reversing switch breaker must be open. The reversing switch output allows the measurement of the voltage on the left hand side of the table (input) before it is actually applied to the experiment. This is an extra safety precaution and will be 1/5

2 useful in future experiments. Ammeter Insertion Plug Top View of CT 100A L To test table. It is good practice is to take the voltage reading from the output of the voltage reversing switch. 30A 50A 10A 15A Multimeter to measure the current Multimeter to measure the voltage 10A fused COM Current out of the Wattmeter. V COM 5A l Current in to the Wattmeter Current in to the Wattmeter Note: The multimeter is not necessary to use the wattmeter, it simply provides a visual for the amount of current going in V + Wattmeter A wattmeter needs both voltage and current to measure the power. Be sure to check the top of the wattmeter box lid for scaling factors. Figure 1: CT and Wattmeter Guide. NOTE: The wattmeter and multimeters may have slightly different input and output arrangements to please read the labels carefully. (f) Connect the voltage connections of the wattmeter in parallel with the second multimeter. Note: The wattmeter needs both current and voltage inputs to measure power. Current is measured in series and voltage is measured in parallel. (g) Connect the 120VAC supply to terminals 1 and 2 at the left (input) side of the test table, but do not energize until the instructor approves your setup. (h) Make sure the ammeter shorting switch is open and the second multimeter (which will be measuring the input voltage) is de-energized by at least one switch. Also, do not put in the insertion plug until instructed to do so. (i) Have the instructor check your connections. (j) Energize the system. (k) Close the reversing switch breaker to allow the voltmeter to read the input. Is it what you expect? If yes, set your resistor bank to minimum load current condition, close in the circuit breakers on lines 1 and 2, and then tap the ammeter insertion plug into the connector on Line 1. If the wattmeter pointer moves down scale, flip over the ammeter insertion plug and complete the connection so the wattmeter pointer moves upscale. 2/5

3 (l) Read Steps (l)-(n), and then take five readings between 0 and 25 Amperes. It s good lab procedure to predict the results you expect to see. So, assuming there is 25A coming from the supply, what value would you expect the current multimeter to display? With 25A from the supply and 120V, what would you expect the wattmeter to display? (m) Record the raw data for the current in Line 1. Be sure your multimeter is set for AC current measurement. Make a second column to calculate the current using the CT ratio. (n) Measure and record the voltage at each step using the second multimeter. (o) Measure the power at each step using the wattmeter. Record both the raw data and the calculated data using the CT ratio and the wattmeter scaling ratio. (p) Check your power measurements using the formula: P= V I cosθ. For a resistive load, cosθ = 1. (q) Systematically return the load to an open circuit, open the circuit breakers on the test table, and turn off the power from the main panel before setting up the next part of the experiment. (r) Place the capacitor bank in parallel with the resistive bank. Follow steps (g)-(j) to turn on the power and experiment safely. (s) Take five readings between 0 and 25A for the total current. Use the current clamp to measure the magnitude of the current through the resistor bank and the capacitor bank separately. 2 Remember, I total =sqrt(i r + I 2 c ). (Hint: Take the first reading with the lowest possible current setting for the resistor bank and capacitor bank. Take the second reading with the second lowest settings on each. Take the 3 rd, 4 th and 5 th readings by only increasing the current to the resistor bank.) 3/5

4 REPORT 1. Calculate the percent differences between the readings taken from the wattmeter (Part 2(n)) and the value for power calculated for part 2(o). For both sets of data, include your raw data and then a column that includes CT ratios and scaling factors. Once you ve included the ratios and scaling factor, this will give you the actual power in the system. Calculate the power in the system when the capacitor bank is in parallel with the resistor bank (Part 2(r)). Assume that the angle for the current in the capacitor bank is leading perfectly at 90 degrees. Use the angle of the supply voltage as the reference (assume 0 degrees). Then, calculate the percent difference to compare this to the measured value of power taken from the wattmeter when the capacitor bank was in the system. Don t forget to include your CT ratio and wattmeter scaling factor. Using the measured value of power, the voltage magnitude and total current magnitude in the system, calculate the power factor angle. How does this compare to your ideal power factor angle that you used to calculate power (when you assumed cap bank angle was leading by 90 degrees.) Readings taken from the wattmeter refers to the raw data collect and then accounting for the CT ratio and wattmeter scaling factor. Power calculated uses your voltage measurement, current measurement, and CT ratio. Present the measured and calculated values for power and the percent difference (using the equation below) in a table. Formulae: 4/5

5 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 5/5