Transformer Waveforms: Observing Voltages and Currents in a Power Transformer, Lab Reports of Electrical and Electronics Engineering

Download Transformer Waveforms: Observing Voltages and Currents in a Power Transformer and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! Transformer Waveforms EXPERIMENT Transformer Waveforms Steady-State Testing and Performance of Single-Phase Transformers Waveforms OBJECTIVE The voltage regulation and efficiency of a distribution system are affected by the electrical and magnetic characteristics of the transformers operating in the network. The design of such a distribution system must consider these effects. This experiment demonstrates the excitation current, magnetization current, AC saturation curve, and core-loss current of transformers. These are investigated at various load conditions. REFERENCES 1. “Electric Machinery”, Fourth Edition, Fitzgerald, Kinglsey, and Umans, McGraw-Hill Book Company, 1983, Chapter 1. 2. “Electromagnetic and Electromechanical Machines”, Matsch, Leander W., Intext Educational Publishers, 1972. 3. “Electromechanical Devices for Energy Conversion and Control Systems”, Del Toro, Vincent, Prentice-Hall, Inc., 1968. BACKGROUND INFORMATION The basic theory of transformer operation is adequately explained in Reference 1. For our purposes here we will concentrate on the test methods and the experimental set- up of Figure 5. Figure 1 shows the traditionally accepted electrical equivalent circuit for a power transformer in steady-state. This particular equivalent circuit’s parameters are referred to side 1. All relevant impedances, voltages, and currents are shown in the figure. Revised: Spring 2005 1 of 11 Transformer Waveforms Figure 1: Steady-state equivalent circuit for power transformer. It is important to note that for a typical power transformer the ratio of the parallel combination of the common leg impedances to the total impedance of either winding will exceed 200. Algebraically, this can be described as jXR XR mc 1 1 // l + > 200 (2.1) Figure 2: Winding configuration of laboratory transformers. Revised: Spring 2005 2 of 11 Transformer Waveforms Figure 4: Voltage source equivalent of Eq. 2.11. NOTE: The voltage signals used above are adequate for waveshape analysis and phase relationships. They are not accurate for magnitude comparisons and should not be used for this purpose. SUGGESTED PROCEDURE The transformers used for this experiment are rated 120V-120V, 0.6kVA. There are three of them on each set of wall shelves. The set of windings connected to the source side of the transformer are called primary windings, and those connected to the load are named secondary windings. To achieve a 600 volt-ampere rating, these two sets of main primary and secondary windings must be in parallel. The other two windings are information (instrumentation) windings and are not designed to support a load. Figure 1 defines the currents that are referred to throughout the experiment. 1. Figure 5 shows the connections that are used to view , , , and i1 ic im φ m . The first transformer is identical to the transformer being tested and is used to help prevent ground loops between the source and the instrumentation. Revised: Spring 2005 5 of 11 Transformer Waveforms The oscilloscope is connected so that many different signals can be displayed. The undesired signals are eliminated by pressing the ground button next to their scope connection. For example, referring to Figure 5, pressing the ground button on the upper input to channel 2 causes the i signal to be displayed. Releasing both ground buttons on channel 2 causes the i signal to be displayed in a differential mode. Obviously, pressing the bottom ground button causes the sum of the i and i signals to be displayed. 1 m 2 c For initial tests, the load is left disconnected and R3 is set to zero. Slowly increase the voltage from the Single-Phase AC Source until 120 V AC is registered on the voltmeter. The signals across R1 and R2 are proportional to and i , respectively. From Figure 1, it is seen that . Adjust i1 c iiii cm 21 −−= R2 to eliminate the i component from the display. Perhaps the easiest way to minimize is to display c ic φ m versus on the scope and adjust im R2 until the hysteresis goes away. When R2 is properly adjusted, i is observed when both differential amplifier inputs for channel 2 are used, i when only the upper input is used, and the total when only the lower input is used. m c i1 At the transformer’s rated voltage, record the waveforms , , and , 1i ic im φ m versus time waveforms, the hysteresis loop(φ m versus ) and the AC saturation curve( , 1i φ m versus i Note that i is very symmetrical when correct cancellation of i is obtained. Now, vary the primary voltage from zero to 120 V and describe in the report the changes in each i , i , i , m ). m c 1 c m φ m and saturation. Revised: Spring 2005 6 of 11 Transformer Waveforms NOTE: The signal being observed as is a voltage from the 6V winding, but is a signal that has the same waveform as i ic c. Also, the φ m signal is an integrated voltage signal having the waveform of φ m . 2. Add a secondary resistive load by connecting the circuit of Figure 6 to the transformer load connection of Figure 5 and set input voltage to 120 V AC maintain constant. Adjust R3 to remove the load current i2 from the oscilloscope display. R2 may also need to be readjusted slightly. Record the AC saturation curve as the load current is varied between 0, 1.0, 1.5, 2.0 and 2.5 AMPS. Describe in the report the change in the peak-to-peak magnitude of flux as the load resistance is changed. For different primary voltages, describe the changes in the waveforms and curves of , , , i1 ic im φ m and saturation as the load is varied. From this information, comment on the transformer’s performance at different voltages. 3. Bypass the integrator by reconnecting the oscilloscope leads directly to terminals and Y . Turn the calibration knob of the corresponding differential amplifier fully counter-clockwise. This will prevent the amplifier from running over the maximum allowed range. Observe the relative magnitudes and phase angles of the terminal voltage and as the load resistance is varied. Magnitudes can be obtained directly from the meters on the panel. To obtain phase angle between signals displayed, use the MEASURE capability of the scope with selections of PERIOD and DELAY from the vertical menu. Determine the period of the voltage waveform because this signal is harmonic free. Y 3 4 i1 Revised: Spring 2005 7 of 11 Transformer Waveforms Figure 5 Revised: Spring 2005 10 of 11 Transformer Waveforms Figure 6 Revised: Spring 2005 11 of 11