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EE 448 Laboratory Experiment 3 Transformer Experiment EE 448 Fall 2008 Lab Experiment No. 3 04/04/2008 Transformer Experiment -1- EE 448 Laboratory Experiment 3 Transformer Experiment I. INTRODUCTION OBJECTIVES: 1. To learn how real world transformers operate under ideal conditions. 2. To learn what happens to the output voltage when the transformer is loaded. INSTRUMENTS AND COMPONENTS Power Supply Module AC Metering Module (V) AC Metering Module (I) Transformer Module Ammeter (8A) 1.25 Ohm Load Resistor II. EMS 8821 EMS 8426 EMS 8425 EMS 8341 PRELAB EXERCISES All voltages given in Figures 1 – 3 have phase angles of 0 degrees. 1) Consider the following ideal transformer circuit in Figure 1. + 120 VAC V1 I1 11:1 PRI (N1) I2 SEC (N2) + V2 Rload 600 - Ideal Transformer Figure 1: Ideal Transformer Circuit a. Compute the voltage across V2, I1, and I2. b. Compute the power input and power output. c. Compute the reflected impedance seen on the primary side (Hint, you know V1 and I1). -2- EE 448 Laboratory Experiment 3 Transformer Experiment 2) Consider the following circuit (Figure 2) that models a real transformer. R1 X1 1 + 10 X2 2 1 j100 I2` 11:1 2 I1 120 VAC Xm j1000 Rc 10k V1 + - 1 Ic PRI E1 (N1) I2 SEC (N2) R2 2 j1 10 + + E2 V2 - Rload 600 Im - Ideal Transformer Figure 2: Real Transformer Circuit a. Explain how the following equivalent circuit (Figure 3) was obtained. This circuit is functionally identical to the circuit above it (Figure 2). R1 X1 1 + 10 X2 2 1 + j100 2 I1 120 VAC V1 Rc 10k Ic Xm j1000 E1 1 R2 2 j121 1210 + I2` V2` Im - - - Figure 3: Real Transformer Equivalent Circuit b. What is the output voltage V2? c. Calculate the voltage V2 for the following loads. (Remember, you are reflecting these resistances to the primary side of the transformer.) i. 300 Ohms ii. 1200 Ohms iii. 1.25 Ohms -3- Rload 72600 EE 448 Laboratory Experiment 3 Transformer Experiment d. Calculate the real power on the input side of the circuit V1 and the real power on the output side with each of the three load resistances. e. How do these powers compare to those that you calculated in problem 1? f. Calculate the efficiencies of both the real and ideal transformer. Power _ output efficiency (100%) (Remember, these Total _ input _ power quantities are in terms of real power.) Figure 4: Lab Volt Transformer -4- EE 448 Laboratory Experiment 3 Transformer Experiment III. LABORATORY EXPERIMENTS CAUTION: HIGH VOLTAGES ARE PRESENT IN THIS LABORATORY EXPERIMENT! DO NOT MAKE ANY CONNECTIONS WITH THE POWER ON! THE POWER SHOULD BE TURNED OFF BEFORE THE CIRCUIT IS MODIFIED! 1. To perform an open circuit test, the high voltage (primary) side of the transformer is energized by connecting it to the variable voltage supply terminals as shown in the circuit of Figure 5. In order to obtain open circuit test data, adjust the variable AC voltage supply to its zero voltage position before turning on the AC power switch. Next turn on the AC power switch and adjust the variable AC voltage supply until the voltmeter V1 reads 100 volts. Record the measurements of I1, V2, W1, and V1 in Table 1. W1 - - - - + - 4 V1 0 - Pri 117V Sec 10V - Voltmeter 120 V AC Source 8821 N + + Wattmeter Voltmeter V2 - Figure 5: Open Circuit Test Setup How do your calculated voltage ratios compare to the real ratios of N1 = 344 turns and N2 = 32 turns? The wattmeter reading is actually the power lost due to the core resistances and primary winding resistance. -5- - 4 W1 + I1 I1 - + + V1 Digital Multimeter Load Open Circuit Table 1 V2 EE 448 Laboratory Experiment 3 Transformer Experiment W1 - - + - 4 - Sec 10V Ammeter I2 - Figure 6: Short Circuit Test Setup Theoretically what would happen to the output current I2 if the input voltage was increased to 100V? (Do not actually increase the voltage to 100V, just predict what will happen.) 3. Set up the circuit shown in Fig. 7 with the 1.25 ohm resistor. This circuit can be used to measure voltage regulation and power efficiency. Make sure the variable AC voltage supply is in the lowest position and turn the power on. Slowly increase the voltage until V1 equals 100 volts and wait for the resistor to stabilize thermally (about 2 minutes). Record V1, I1, V2, W1, and I2 in Table 3. Table 3 Load V1 V2 I1 1.25 ohms -6- I2 W1 - V1 0 Pri 117V - Voltmeter 120 V AC Source 8821 N + + - + Wattmeter I1 4 - - + + Digital Multimeter 2. A short circuit test can be performed by connecting a voltage source to the primary (high voltage) side of the transformer with the secondary (low voltage) side shorted. In this situation it is VERY IMPORTANT TO ADJUST THE VARIABLE AC VOLTAGE SUPPLY TO ZERO BEFORE TURNING ON THE AC POWER SWITCH. Use the 8 amp range on the ammeter to short the secondary winding as shown in Figure 6. Then increase the supply voltage slowly until the I2 ammeter reads 4.0 amps. A typical value is for I1 is about 0.40 amps. Record the measurements for V1, I1, and I2. W1 - - + - + + - I2 + Wattmeter I1 Ammeter 4 Voltmeter Pri 117V + - 120 V AC Source 8821 Sec 10V + 4 - - + + Digital Multimeter EE 448 Laboratory Experiment 3 Transformer Experiment - Voltmeter V2 - - 0 - V1 N Figure 7: Loaded Circuit Test Setup Use the voltages from Part 3 to calculate the voltage ratio and compare with the winding ratio. How do the ratios in Part 3 compare to the ratios from Part 1? Fill in Table 4 with the following calculations: Calculate percent regulation. (Hint: Vnoload comes from Part 1 of the Procedure.) regulation Vnoload Vload (100%) Vload Calculate the output power W2 in the load resistor. Why is W2 different from the input power W1? What is the efficiency of the transformer? efficiency Power _ output (100%) Total _ input _ power Table 4 Load Percent Regulation W1 1.25 ohms -7- W2 Power Efficiency RL 1.25 EE 448 Laboratory Experiment 3 Transformer Experiment IV. PRESENTATION OF LABORATORY RESULTS WITH SPREADSHEET COMPUTER PROGRAM V. OBSERVATIONS AND COMMENTS -8-