ECE 321 Experiment No: 4 Energy Systems Lab 1 Fall 2009 TRANSFORMERS-1

Transcription

1 TRANSFORMER: EXPERIMENT NO 4 TRANSFORMERS-1 The transformer, which is made up of two or more coils or windings linked magnetically, with or without a core to shape and enhance the magnetic flux, is used in many applications to transform voltages, current, and impedance, and to isolate circuits from each other electrically. Transformers have no moving parts and rely on the interaction between electrical and magnetic phenomena for their operation. PRELIMINARY: The primary and secondary windings of the transformer studied in this laboratory consist of two windings that may be connected in parallel or in series. One of the sets of low and high voltage windings will be designated by the subscript V Xa V Ha respectively and the other by V Xb V Hb. To understand the circuits better, the following cases can be considered: 1. With a voltage, V LV, applied across the two low voltage windings connected in series a current of I LV Amperes and a power of P LV Watts is measured with the high voltage windings open circuited, as shown in Fig.1 2. If one set of the low voltage windings are reconnected in parallel; V Xa = V Xb = V X Find I TOTAl (I TOTAL = I Xa +I Xb ) and P TOTAL (P TOTAL = P a +P b ) with the high voltage windings open circuited, as shown in Fig.2. OBJECTIVE: The objective of this experiment is to find the approximate equivalent circuit parameters of a transformer using short circuit and open circuit tests. APPARATUS 1. Test table 2. 2 Multimeters 3. 1 Wattmeter 4. One phase of a three phase variac kva single-phase transformer 6. Current transformer 1/8

2 PROCEDURE: Open Circuit Test: 1. Connect the two low voltage windings of the transformer in series, and the two high voltage windings of the same transformer in parallel as shown in Figure 1. Make sure that the transformer is connected according to nameplate specifications. 2. Connect the low side series connected windings to the 230 VAC power panel through a test table (use lines 1 and 2). 3. Connect the high side windings to the test table's voltage back connections on the right side of the table, and use a multimeter to measure the voltage across the lines. Make sure the output windings are open circuited (switch on the front of the table)! 4. Connect the 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 appropriate primary setting of the CT. 5. Connect a multimeter to the voltage connections at the output of the reversing switch. The voltage circuit breaker for the reversing switch must be open. 6. Connect the voltage connections of the wattmeter in parallel with the multimeter that is measuring the output of the reversing switch. 7. After the TA approves your connections, energize the circuit. 8. Measure and record the low side quantities V OCX, I OCX, P OCX, and the high side voltage V OCH. 9. Deenergize the circuit and disconnect from the source. 10. Reconnect the low voltage windings in parallel as shown in Figure Connect these windings through the test table to the output of one phase of the variac. 12. Connect the input of the variac to the 120 VAC power panel. 13. After the TA approves your connections, energize the circuit. 14. Apply a voltage that is approximately half the voltage used in part a. 15. Measure and record V OCX, I OCX, P OCX, and V OCH. 16. Set the variac output to 0V. 17. Deenergize the circuit and disconnect from the source. 2/8

3 Note: Remember to change the primary setting of the Current Transformer for every figure. Figure 1. Open Circuit Test, Series Connection of the Low Voltage Windings Figure 2. Open Circuit Test, Parallel Connection of the Low Voltage Windings 3/8

4 Short Circuit Test: 1. Connect the high voltage windings in series and the low voltage windings in parallel as shown in Figure 3. Make sure that the transformer is connected according to nameplate specifications. Connect the low voltage windings to the test tables current back connection. (If you re facing the front of the table, it s on the right hand side.) 2. Connect the high voltage windings through the test table to the output terminals of the variac (use lines 1 and 2 of the test table) as shown in Figure 3. Make sure that all test table line switches are open. 3. Connect the input terminals of the variac to the 120 V AC power panel. Do not turn on power. Have the lab instructor check your set up. 4. Calculate the rated current for the input windings. Check your value with the lab instructor. Make sure that your current transformer is set up to handle this current. 5. With the test table's line switches still open, turn on power and adjust the variac until its output voltage is zero. 6. Close the test table line switches in a manner such that the measuring instruments are protected from high currents. Immediately after closing the line switches check the current in Line 1. The current should be close to zero. If the current is not close to zero, de-energize your circuit and recheck it. 7. Slowly increase the output voltage of the variac until the ammeter in Line 1 indicates rated current. Remember to include your CT ratio in this measurement. Measure and record: V SCH, I SCH, P SCH. 8. Protect your measuring instruments, zero out the variac. 9. Deenergize the circuit, and disconnect from the source. 10. Repeat steps 2 through 5 with the high voltage windings connected in parallel as shown in Figure 4. Before the next lab session, complete the report for this lab. 4/8

5 H1 TEST TABLE TEST TABLE LEFT SIDE BACK CONNECTION X1 X3 H2 120 V H3 X2 X4 VARIAC Figure 3. Short Circuit Test, Series Connected HV windings H4 TEST TABLE TEST TABLE LEFT SIDE BACK CONNECTION H1 H3 X1 X3 120 V X2 X4 VARIAC H2 H4 Figure. 4. Short Circuit Test, Parallel Connected HV windings. 5/8

6 TABULAR FORM: (BE SURE TO INCLUDE THESE RESULTS IN YOUR REPORT) Open Circuit Test: V OCLV I OCLV P OCLV R CLV θ OCLV X MLV Short Circuit Test: V SCHV I SCHV P SCHV R EQHV Z EQHV X EQHV SUMMARY: The part 1 of experiment is open circuit of a transformer, which means the transformer is under no load conditions. The primary input current I OCLV under no-load conditions is not at 90 degrees behind V OCLV but lags it by an angle θ OCLV < 90. No load input power (P OCLV ) is P OCLV V OCLV I OCLV cos( θ OCLV ) Under O.C test, primary no load current I OCLV is small, so copper loss is negligibly small and hence wattmeter reading represents practically core loss. Under S.C test, applied voltage is a small percentage of normal voltage, core losses are very small And hence wattmeter reading represents practically full load copper loss. 6/8

7 REPORT: 1. Using the data found in each of the open circuit tests calculate G CX and B CX. G CX is given by G CX =P OCX /V OCX 2 (1) B MX is given by Y OCX I OCX V OCX (2) 2 2 B MX Y OCX G CX (3) 2. Using the data found in each of the short circuit tests calculate R EQH and X EQH. R EQH is given by R EQH P SCH 2 I SCH (4) X EQH is given by Z EQH V SCH I SCH (5) 2 2 X EQH Z EQH R EQH (6) 3. Draw the approximate transformer equivalent circuits with all parameters referred to the low voltage side with the windings in parallel. Show the calculated values of each parameter. Remember to include units. Remember the short circuit test was performed with the voltage applied to the high voltage side! 4. Consider the basic transformer equation E = 4.44Nfφmax and the magnetic circuits involved. Is the core flux of Figure 1, the same as the core flux of Figure 2. 7/8

8 a = N N LV HV I LV I HV a R eq,lv X eq,lv 120 V V LV R c,lv X c,lv av HV 2 a Z LOAD,HV Figure 5. Transformer Equivalent Circuit (Note: LV and X both indicate the Low Voltage side. HV and H both indicate the High Voltage side.) 8/8