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Phy 213: General Physics III Lab Instructor: Tony Zable 1 Experiment: Transformers A transformer is a device that changes the AC voltage and current. It works based on the electromagnetic induction principles, and consists of 2 coils: the primary coil and the secondary coil. Primary coil is connected to an AC power supply. The coil has a ferromagnetic core that enhances the magnetic field generated by the AC current. Since the AC current periodically changes its direction, the magnetic field will also change its direction following a sinusoidal equation. The changing magnetic flux generates induced emfs in the secondary and the primary coils Secondary coil is also wound around a ferromagnetic core and it is placed inside the magnetic field generated by the primary coil (usually on top of the primary, on the same ferromagnetic coil). The changing magnetic field of the primary coil induces an electromotive force in the secondary coil, according to Faraday’s Law: Esec ondary N sec ondary B t where Nsecondary is the number of loops in the secondary coil (the changing magnetic field is produced by the primary coil). The magnetic flux is the same for both coils, so the emf induced in the primary is E primary N primary B t . Since the resistance of the coils is almost zero, the equation of the transformer becomes E primary Esec ondary N sec ondary N primary If Nsecondary>Nprimary, then Esecondary>Eprimary, this is called a step-up transformer. If Nsecondary<Nprimary, then Esecondary<Eprimary, this is called a step-down transformer. For high quality transformers, the resistance is small and the induced emf is approximately equal to the terminal voltage V (Eprimary ~ Vprimary & Esecondary ~ Vsecondary), or Vprimary Vsec ondary Nsec ondary N primary Since the total energy is conserved, the electrical energy in the secondary has to be equal to the electrical energy in the primary. In the ideal case, Pprimary=Psecondary or, VsecondaryIsecondary=VprimaryIprimary. Therefore, Vsecondary/Vprimary=Iprimary/Isecondary. Note: the ratio of the currents is inverse to the ratio of the voltages. PCC-Cascade Campus Department of Medical Technology & Science Phy 213: General Physics III Lab Instructor: Tony Zable 2 It also should be noted that some energy is lost as heat in the coils and inside the ferromagnetic core. The efficiency of the transformer shows how much of the input energy (primary) is transferred to the secondary coil Psec ondary Pprimary Vsec ondaryI sec ondary V primaryI primary OBJECTIVES Build a step-down transformer Measure experimental values for current and voltage in the primary and secondary coil Find the transformer ratio using the slope of the Vsecomdary versus Vprimary graph Calculate and compare the efficiencies of the transformer with the ferromagnetic core and without the ferromagnetic core. MATERIALS AC adjustable power supply Digital multimeter Magnetic wire switch Ferromagnetic core switch connecting wires PRELIMINARY QUESTIONS 1. A step-down transformer is labeled “Primary 120V, Secondary 12.6 V”. What is the “turns ratio” for this transformer? 2. Sketch below the graph Vprimary vs. Vsecondary obtained by applying to the primary an increasing AC voltage (4,6,8,10,12 V respectively). What is the significance of the slope of this graph? PROCEDURE 1. Build the transformer primary coil by winding 150 turns of wire around the ferromagnetic core; cover the primary coil with tape (insulator), leaving the ends accessible for connecting to the power supply. On the same core, on top of the primary coil, build the secondary coil by winding 50 turns of wire. PCC-Cascade Campus Department of Medical Technology & Science Phy 213: General Physics III Lab Instructor: Tony Zable 3 2. Without plugging in the power supply, connect the primary coil to the AC Power supply. 3. Prepare the Digital Multimeter: plug the black lead into the COM terminal of the Multimeter and the red lead into the V terminal. Put the switch in the “AC” position. Turn the multimeter on by setting it to the 20V scale 4. Have your instructor check your circuit and then turn on the power supply. Adjust the power supply to 5V and measure the terminal voltages across the primary and secondary coils. Record your data in the table below. 5. Disconnect the multimeter and turn it off. To measure the currents, the multimeter has to be connected in series in the primary circuit and then in the secondary circuit. Move the red lead to the 10A terminal of the multimeter. Switch to the 10A scale. Connect the multimeter in series in the primary circuit and measure the current Iprimary. Connect the multimeter to the secondary coil and measure the current Isecondary. Record the values in the table below. 6. Repeat the voltage measurements (not the current measurements!) for 4 more values of the primary voltage. Record values in Table 2. 7. Using Graphical Analysis, plot the graph Vsecondary vs. Vprimary. Is the graph linear? Using the Linear fit feature, find the linear fit equation. Slope = ________±_______ Y-intercept = ________±_______ 8. Cut and paste the graph into Word and print the graph. 9. Remove the ferromagnetic core and repeat step 5 (the measurements of voltage and current for the primary voltage V=5V). Record your data below in Table 1. DATA TABLE 1: (For Vprimary = 5V, record the measurements with and without the ferromagnetic core) With ferromagnetic core : Without ferromagnetic core : Vprimary Vprimary 5V 5V Vsecondary Vsecondary Iprimary Iprimary Isecondary Isecondary Pprimary Pprimary Psecondary Psecondary Efficiency () Efficiency () PCC-Cascade Campus Department of Medical Technology & Science Phy 213: General Physics III Lab Instructor: Tony Zable 4 DATA TABLE 2: Trial Vprimary (V) Vsecondary (V) ANALYSIS 1. What is the efficiency of the transformer with and without the ferromagnetic core? Record your results in Data Table 1 (above). 2. What can you conclude from the efficiency values in (1)? 3. What is the significance of the slope of the Vsecondary vs Vprimary graph? Compare to the theoretical value Nprimary/Nsecondary. 4. The step-down transformer you constructed lowered the voltage. How does the secondary current compare to the primary current? Is this consistent with the theory? Explain. PCC-Cascade Campus Department of Medical Technology & Science