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UNIVERSITI MALAYSIA PERLIS ELECTRICAL ENGINEERING TECHNOLOGY EMT 113/4 EXPERIMENT 2 SINGLE PHASE TRANSFORMER – Voltage and Current Ratios MARKS Q6 Q7 Q8 Q9 Q11 Q12 Q13 Q14 Q15 9 1 2 1 3 1 1 3 1 T 1-1 Q19 G19 Q20 Q21 Discussion Conclusion Total % 33 1 5 1 2 4 5 73 100 NAME PROGRAMME MATRIK # GROUP DATE EMT113/4 Single Phase Transformer-Voltage and Current Ratios EXPERIMENT 2 TITLE : SINGLE PHASE TRANSFORMER – VOLTAGE AND CURRENT RATIOS OBJECTIVES 1. 2. To familiar with voltage and current characteristics of a single phase transformer. To be able to use the transformer turns ratio to predict the voltage and current that will flow in the secondary winding. EQUIPMENTS EMC Workstation Model 8110, Single Phase Transformer Model 8341, Power Supply Model 8821 and Data Acquisition Interface Model 9062. INTRODUCTION The windings of a standard single-phase transformer are called the primary winding and the secondary winding. The primary winding is the power input winding and this is the side that is connected to the ac power source. The secondary winding is connected to the load and is physically and electrically isolated from the primary. The voltage and current that flow in the secondary are related to the primary voltage and current by the transformer turns ratio N1/N2 (or Np/Ns) through a very simple relationship. The ratio of primary voltage to secondary voltage equals N1/N2, while the ratio of the primary to secondary is equal to the inverse of the turns ratio, N2/N1. This result in the following, EPRI = N1 ESEC N2 which gives: and which gives: ESEC = EPRI N1 X N2 IPRI = N2 ISEC N1 ISEC = IPRI N1 N2 X Transformers are designed with fixed ratios between primary and secondary voltages and are widely used to step-up (increase) or step-down (decrease) load voltages and currents. The Single-Phase Transformer module used in these exercises has its nominal ratings silk-screened on the front panel and most transformers have markings to indicate their nominal characteristics. Also, many transformers have intermediate taps, or connection terminals on the secondary side, so that different voltage ratios can be obtained using a single transformer. Determining a transformer’s voltage ratio is simple matter. With no load connected to the secondary winding, only small exciting current necessary to create the magnetic flux inside the transformer flows in the primary winding. Transformer losses are minimum and the ratio of primary to secondary voltage equals the turns ratio. The turns ratio can be found by measuring the unloaded PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 1 EMT113/4 Single Phase Transformer-Voltage and Current Ratios secondary voltage with nominal voltage applied to the primary. The current ratio can be evaluated by measuring the short-circuit secondary current with a small ac voltage applied to the primary. The voltage applied to the primary must be low enough to ensure that the nominal current in the primary winding is not exceeded. Otherwise, the windings may overheat and be damaged. The exciting current, which is directly related to the alternating magnetic flux, increases in direct proportion to the applied voltage until core saturation sets in. This occurs when the applied voltage exceeds the rated value of the primary and then the linear relationship between the primary voltage and the exciting current breaks down. The curve of the primary voltage versus exciting current flattens and smaller increases in primary voltage lead to larger increases in exciting current as shown in Figure 2-1. The exciting current is only a few milliamperes in the EMS Single-Phase Transformer module and generally its value is a small percentage of the nominal current of a transformer. Figure 2-1: Saturation Curve of a Transformer PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 2 EMT113/4 Single Phase Transformer-Voltage and Current Ratios PROCEDURE Figure 2-2: Single –Phase Transformer Measurements 1. Install the Power Supply, Data Acquisition Interface (DAI) and Single-Phase Transformer modules in the EMS Workstation. 2. Make sure that the main switch of the Power Supply is set to the O (OFF) position, and the voltage control knob is turned fully counter clock-wise (ccw). Set the voltmeter select switch to the 4-N position. 3. Ensure that the DAI LOW POWER INPUT is connected to the main Power Supply, set the 24V-AC power switch to the 1 (ON) position. 4. Display the Metering application. 5. Set up the transformer circuit shown in Figure 2-2. Connect E1 and I1 as shown and use E2 to measure the different secondary voltages. 6. Turn ON the power and adjust the voltage control for the value of Es given in Figure 2-2. Measure the transformer primary current and the different terminal voltages. After recording the measurements, rotate the voltage control fully ccw and then turn OFF the power. Note: When measuring the various voltages, turn OFF the Power Supply before modifying the connections of the DAI to the circuit. PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 3 EMT113/4 Single Phase Transformer-Voltage and Current Ratios Name:______________________________ Date:_______________________________ Matric No:____________________ Course:______________________ IPRI = ________ A E1-2 = _________ V E3-4 =__________ V E5-6 = ________ V E3-7 = _________ V E7-8 =__________ V E8-4 = ________ V E5-9 = _________ V E9-6 =__________ V 9 marks 7. Do the secondary voltages compare well with the rated values written on the front panel? Yes No 1 mark 8. The transformer windings between terminals 1 and 2 and between terminals 5 and 6 each have 500 turns of wire. The number of turns in the winding between terminals 3 and 4 is 865. Calculate the turns ratios between the primary and secondary windings for each case. N1-2 = ________ N5-6 N1-2 = ________ N3-4 2 marks 9. Using the measured values in step 6, compare these transformer turns ratios with the corresponding voltage ratios. Are they approximately the same? Yes No 1 mark Figure 2-3: Determining the ratio of Primary Current to Secondary Current 10. Connect I2 as shown in Figure 2-3 and note that it short-circuits secondary winding 5-6. Turn on the power and slowly adjust the voltage control to obtain the value of current Is given in Figure 2-3. PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 4 EMT113/4 Single Phase Transformer-Voltage and Current Ratios Name:______________________________ Date:_______________________________ Matric No:____________________ Course:______________________ 11. Record the values of primary voltage and current and the value of the shortcircuit secondary current in winding 5-6. EPRI = ________ V IPRI = _________ A ISEC =__________ A 3 marks 12. Return the voltage control to zero and turn off the power. Calculate the ratio of primary current to secondary current. IPRI = ________ ISEC 1 mark 13. Is the ratio approximately equal to N2/N1? Yes No 1 mark 14. Connect I2 so that it now short-circuits secondary terminals 3-4. Turn on the Power Supply and slowly adjust the voltage control knob for the same value of current used in step 10. Once again, record the values of primary voltage and current and the secondary winding current. EPRI = ________ V IPRI = _________ A ISEC =__________ A 3 marks 15. Return the voltage control to zero and turn off the power. Again, calculate the ratio of primary current to secondary current. It is equal to N2/N1? Yes No 1 mark Figure 2-4: Effect of Core Saturating on Exciting Current PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 5 EMT113/4 Single Phase Transformer-Voltage and Current Ratios Name:______________________________ Date:_______________________________ Matric No:____________________ Course:______________________ 16. Set up the transformer circuit shown in Figure 2-4. It will be used to show how exciting current is affected when the transformer core becomes saturated. Since the exciting current is so small, the corresponding voltage across a sense resistor R will be used to illustrate its variation. Connect the transformer primary terminals to Power Supply terminals 4 and 5 through sense resistor R. Connect E1, E2 and E3 to measure the transformer voltages. 17. Turn on the power and use the output voltage control to obtain values for E2 equally spaced at about 10% intervals over the complete control knob range. At each voltage adjustment, click the Record Data button to enter the measurements in the Data Table. Then fill in the data into Table 2-1 below. Interval, % 0 Sensed Voltage (E1), V Primary Voltage (E2), V Secondary Voltage (E3), V 10 20 30 40 50 60 70 80 90 100 33 marks Table 2-1 Transformer Voltages 18. When all data values have been recorded, rotate the voltage control fully ccw, and turn off the Power Supply. 19. Display the Graph screen, select E1 as the X-axis parameter and E2 as the Yaxis parameter. Click the Line Graph button to observe the curve of primary voltage versus exciting current, represented by E1. Then copy this graph into the graph in the RESULT Section. Does the exciting current increase more rapidly after the rated voltage is exceeded? Yes No 1 mark PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 6 EMT113/4 Single Phase Transformer-Voltage and Current Ratios Name:______________________________ Date:_______________________________ Matric No:____________________ Course:______________________ 20. Does the curve illustrate that the transformer core becomes saturated? Yes No 1 mark 21. Review the measured data to determine how the primary-to-secondary voltage ratio was affected when the transformer core become saturated. ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 2 marks 22. Ensure that the Power Supply is turned off, the voltage control is fully ccw and remove all leads and cables. PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 7 EMT113/4 Single Phase Transformer-Voltage and Current Ratios Name:______________________________ Date:_______________________________ Matric No:____________________ Course:______________________ RESULT 5 marks PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 8 OP-AMP SINEWAVE 'NON-INVERT' output voltage input EMT113/4 Single Phase Transformer-Voltage and Current Ratios Name:______________________________ Date:_______________________________ Matric No:____________________ Course:______________________ DISCUSSION The primary winding in standard single-phase transformer is _____________________ time ______________________________________________________________________ ______________________________________________________________________ 2 marks The secondary winding in standard single-phase transformer is ___________________ ______________________________________________________________________ ______________________________________________________________________ 2 marks CONCLUSION The current ratio can be evaluated by _______________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 5 marks PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK 9