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Electrical Machines & Applications 1 (DET 204) Laboratory Module EXPERIMENT 1 PARAMETERS DETERMINATION OF A TRANSFORMER Objective: The objective of the experiment is to perform open and short circuit tests on a singlephase transformer a. to find out the core and copper losses. b. to find out the parameters of the equivalent circuit. c. to draw the equivalent circuit. Introduction A transformer is a static machine having two or more windings coupled by a mutual magnetic field. Ferromagnetic cores are commonly used to provide tight magnetic coupling and high flux densities. Single-phase transformers are widely used in low power electronic or control circuits to isolate one circuit from another. They are also used as instrument transformers to measure voltages and currents. In a practical transformer there are core and copper losses due to the winding resistance and leakage flux. These losses can be conveniently represented by an equivalent circuit with resistances and inductances where all the quantities are referred to one side, preferably high voltage side. The equivalent circuit of a single-phase transformer is given in figure 1. Figure 1 The resistance Rp represents the ohmic resistance of the primary winding and Rs – of the secondary winding. The reactance Xp and Xs model the leakage flux of the primary and UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 1 Electrical Machines & Applications 1 (DET 204) Laboratory Module secondary windings, respectively. The resistance Rc is responsible for the core losses due to hysteresis and eddy currents, and XM for the generation of the main flux (magnetising reactance). All impedances on the secondary side of the transformer can be recalculated for the primary side. This is also know as the referring to the primary side and results in the equivalent circuit shown in Figure 2. Figure 2 The transformer equivalent models shown above are often more complex then necessary. A more desirable equivalent circuit that works as well as the original circuit are shown in Figure 3. In this model the excitation branch is moved to the front of the transformer, and the the primary and secondary impedances are left in series with each other. Figure 3 Experimental determination of all elements in the transformer equivalent circuit involves 2 tests: a. open-circuit test b. short circuit test. UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 2 Electrical Machines & Applications 1 (DET 204) Laboratory Module Measurement of the Primary Resistance A DC ohmmeter should be connected across the primary terminals and Rp should be recorded. Open-Circuit Test The transformer’s secondary should be open-circuited and primary winding supplied with a full rated voltage (Figure 4). The input voltage (Voc), primary current (Ioc) and power (Poc) are measured. This test is sufficient to calculate the core resistance and magnetising reactance, using Equations (1.0) to (1.7). The conductance of the core-loss resistor is given by: Figure 4 Gc = 1/Rc ………………………………………….1.1 Bm = 1/Xm ………………………………………….1.2 Since these two elements are in parallel, their admittances add and the total excitation admittance is YE = Gc – jBm = 1/Rc – j1/Xm ……………………………1.3 The magnitude of the excitation admittance can be determined by the Equation (1.4) ……………………………………………1.4 The angle of the admittance can be found from knowledge of a circuit power factor. ………………………………………….1.5 and the power factor angle is given by Equation (1.6) UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 3 Electrical Machines & Applications 1 (DET 204) Laboratory Module ………………………………………………. 1.6 The power factor is always lagging for a real transformer, so the angle of the current always lags the angle of the voltage by degrees. Therefore, the admittance YE is .............................................1.7 Short Circuit Test The transformer’s secondary terminals are short-circuited and the primary voltage is supplied with the voltage, much reduced comparing with the rated value (Figure 5). In practical situation, this voltage is adjusted so that the primary current is approximately rated, and the primary voltage (Vsc), primary current (Isc) and power (Psc) are measured. Figure5. The following Equations (4.12) to (4.16) can be used to calculate elements in the primary and secondary branches. The magnitude of the series impedances referred to the primary side of the transformer is …………………………………………….1.8 The short circuit power factor (PF) is given by: ………………………………………..1.9 and the power factor angle è is given by Equation (4.14) …………………………………………...2.0 Therefore, UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 4 Electrical Machines & Applications 1 (DET 204) Laboratory Module ……………………………………...2.1 The series impedance Zsc is equals to ………………………………………2.2 Equipment 1. Single Phase Transformer. 2. Variable AC Source (Variac) 3. Two Digital Multimeters. 4. AC Wattmeter (Use Voltech PM100) Procedure 1. Rated Quantities From the transformer nameplate, note the rated values of S1, V1, and V2. Calculate the rated maximum currents for the ammeters on the primary and secondary sides. Do not exceed these values. 2. Open Circuit Test Connect the apparatus as shown in Figure 1.5. Apply the rated input voltage to the primary winding and measure input voltage, current, power, and output voltage. Do not turn the power on before your circuit has been checked by your Engineers! Figure 1.5 3. Starting from VOC =20V turn the variac knob and slowly increase the input voltage. Complete all the data in Table 1.1. UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 5 Electrical Machines & Applications 1 (DET 204) Laboratory Module 4.Short Circuit test Short-circuit the secondary winding through an ammeter as shown in Figure 6 Do not turn the power on before your circuit has been checked by your Engineers! 5.Slowly and gradually increase the applied voltage and carefully watch the primary and secondary currents. Measurements and calculations ratio should be done for short circuit current values specified in Table 1.2 Figure 6 6.Calculate the short circuit impedance as measured from the primary using Equations (1.8) to (1.12). Complete all the measurements and calculations in Table 1.2. UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 6 Electrical Machines & Applications 1 (DET 204) Laboratory Module EXPERIMENT 1 OPEN-CIRCUIT AND SHORT-CIRCUIT TESTS OF A TRANSFORMER Name : _______________________________ Group : ____________ Matric No: ____________________ Date : _______________ 1. What are the rated voltage and current of the transformer? ______________________________________________ 2 Table 1.1: Transformer Open circuit Test and Measurement Table 1.2: Transformer short circuit test measurements and calculations. UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 7 Electrical Machines & Applications 1 (DET 204) Laboratory Module 3. Show the calculation during maximum current and voltage to find the circuit equivalent parameters YE, RC,PF, XM, ZSC, Req and Xeq. Draw the transformer equivalent circuit. 4. What is the turn ratio between primary and secondary winding? UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 8 Electrical Machines & Applications 1 (DET 204) Laboratory Module 5. From Table 1.1, Plot a graph Voc against Ioc. What can be concluded from the graph obtain.? UNIVERSITI MALAYSIA PERLIS – Exp.1 (Revision 1) 9