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An Electronic Switch ?Or Transient Studies

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An Electronic Switch ?Or Transient Studies AN ELECTRONIC SWITCH ?OR TRANSIENT STUDIES A THESIS Presented to the Faculty of the Division of Graduate Studies Georgia Institute of Technology In Partial Fulfillment of the Requirements for the Degree Master of Science in Slectrical Engineering by Marshall Joseph McCann September 1949 107545 ii AN ELECTRONIC SWITCH FOR TRANSIENT STUDIES Approved: zf r ~~v Bate Approved by Chairman A up. d±, f^4f ACKNOWLEDGMENTS I wish to express my sincerest thanks to Prof. M. A. Honnell for his patient guidance and assistance, which were an immense aid in the prosecution of this work. I am also indebted to the Photographic and Repro­ duction Laboratory at the State Engineering Experiment Station of the Georgia Institute of Technology for their splendid cooperation with the photographic work herein. iv TABLE 0? CONTENTS PAGE Acknowledgments iii List of figures vi I- Introduction 1 II- A Survey of the Literature 3 Mechanical Systems.. • 3 Electronic Systems 4 III- Design Considerations for an Electronic Switch 8 General , , 8 Possible Approaches 8 IV- The Switch 12 V- The Square Wave Generator 16 Design Requirement s 16 Symmetrical Multivibrator 16 Improvement of Waveform 22 VI- The Synchronizing Section 37 General 37 Phase Shifting 27 Amplification and V/ave shaping 29 Synchronization of the Multivibrator 39 Summary of the Synchronizing Action 31 VII- Operation 34 VIII- Siamples of Operation 37 Lumped Constant Circuits 37 V P-4GE VIII- Examples of Operation (continued) Transmission Line Transients 41 IX- Summary • 46 appendix I, Analysis of Plate-Inductance Compensation of the Multivibrator 47 appendix II, Scaling of Circuits for A-C Transients 50 appendix III, Parts List 55 Bibliography .... 58 vi LIST OF FIGURES FIGURE PAGE 1 A Gas Tube Switch 9 2 Another Switch 9 3 The Switching Circuit 13 4 Multivibrator, Initial Design 17 5 6SN7 Plate Characteristics 19 6 Multivibrator Grid Voltage, Initial Design 23 Multivibrator Output, Initial Design 23 8 Multivibrator Grid Voltage, Final Design 23 9 Multivibrator Output, Final Design 23 10 Multivibrator, Final Design 25 11 The Synchronizing Circuit 28 12 Synchronizing Section Input Voltage • 30 13 Grid Voltage, Tube VI 30 14 Plate Load Voltage, Tube VI 30 15 Output Voltage, Tube V2 30 16 Multivibrator Grid Voltage with Synchronization. 32 17 Synchronized Multivibrator Output 32 18 Maximum Transient in R-L Circuit 38 19 Minimum Transient in R-L Circuit 38 20 Transient in R-C Circuit 38 21 Transient in a Detuned Resonant Circuit 40 vii FIGURE PAGE 22 Transient in Resonant Coupled Circuit, Over Critical Coupling • 40 23 Transient in Resonant Coupled Circuit, Coupling Greatly Reduced 40 24 Transmission Line: Receiving Snc Current with Load Switched On 42 25 Transmission Line: Sending End Current with Load Switched On 42 26 Transmission Line: Sending 2nd Voltage with Load Switched On 42 27 Transmission Line: Sending End Current with Load Switched Off 44 28 Transmission Line: Receiving ±±nd Voltage with Load Switched Off • 44 29 Transmission Line: Receiving End Current with Inductive Load Switched On 44 30 Schematic Diagram • 57 AN ELECTRONIC SWITCH FOR TRANSIENT STUDIES I INTRODUCTION It is the purpose of this research to develop an electronic switch capable of producing transients in a-c systems. Such a device has two uses: first, as a teaching aid, and secondly, as a means of finding the transient re­ sponse of networks which are so complicated that they are not readily solvable by analytical methods. As a teaching aid, it permits the student to see the waveforms which occur in switching a-c systems, and thus he does not have to rely entirely on his faith in mathe­ matics. To fulfil this purpose the switch need not meet any rigorous requirements, since only qualitative results are necessary. The transient response of networks to alternating voltages becomes laborious to calculate except for the simplest of systems. Whenever long and involved calcula­ tions are made, the risk of error becomes great, and the time required may not be worth the results obtained. Hence a device which would permit us to cause a transient and measure it directly would be useful. Here the results 3 desired are quanitative in nature, and the switch must meet more rigorous requirements. Strictly speaking, an electrical transient is a phenomenon which occurs only once in time, or, to put it another way, it is non-periodic. If, however, we wait un­ til the effects of a transient have subsided, we may cause the transient again. If the initial conditions of the second transient are the same, it will be an exact dupli­ cate of the first. Thus the transient ma;> be made peri­ odic and its voltage and current may he displayed on a cathode-ray oscilloscope. The problem to be considered, then, is the develop­ ment of an electronic switch to produce a-c transients. An ideal switch to show switching transients on an oscillo­ scope would be one which would present an infinite imped­ ance when open, and an impedance of zero when closed. Fur­ ther, the device should open and close periodically at a fixed point in the a-c cycle, the period of opening and closing being under the control of the operator• 3 II A SURVEY OF THE LITERATURE Mechanical Systems. A commutator driven by a syn- i chronous motor has been used by Turner . The commutator switches a voltage from the same source that supplies the motor. Since the commutator will revolve in step with the supply voltage, the commutator will switch at the same point in the cycle. The point in the cycle may be varied by moving the commutator brushes. The chief advantage of a mechanical commutator is that it is easily constructed and can be nude to work well at low frequencies. Its disadvant­ ages are that it is generally limited to CO cycle voltages, and the frequent erratic operation of brushes and commutators. .mother mechanical method has been used by Reich and Marvin . This system uses a relay which is actuated by a thyratron oscillator circuit, which is synchronized in turn to the frequency being switched. The phase at which switch­ ing occurs is varied by varying the grid bias on the thyra­ tron. The disadvantages of this circuit are that it is burner, H. H., "Transient Visualizer,,r Transactions of the American Institute of Jilectrioal Engineers, 43:80^-813, June, 1924. i-ieich, H. J., and G. S. Marvin, "A Combination Sweep Circuit and Periodic Contactor for Studing Circuit and Line Transients with the Cathode Ray Oscillograph,rl Review of Scientific Instruments, 5:7-9, January, 1934. 4 very limited in the range of frequencies it can handle, and that the relay will be a constant source of trouble due to bouncing and other erratic action, Mechanical systems generally are easy to construct and operate. On the other hand they will always be limit­ ed in their speed of operation by mechanical inertia, and are therefore limited to extremely low frequencies, Electronic Systems. A large number of electronic switches have been described in the literature. Their oper­ ation is generally accomplished by Ttgatin&" the signal through a gas tube or a vacuum-tube amplifier. Some have been designed to demonstrate transients; most, however, are for use with double-trace oscillography. A switch described by Sewig3 uses a pair of thyra­ trons for switching. The thyratrons are controlled by one of the signals being switched. The operation is such that alternate half-cycles appear on the oscilloscope screen. Thus this switch is rather inflexible in application. Davidson reports a similar circuit, except that the thyratrons are controlled by the sweep signal of the oscill- °Sewig, Rudolf, "Simultanaufzeichnung mehrerer Vor- gange mit dem Kathodenstrahloszillographen," Zeits. f. tech Physik. 14:152-153, March, 1933. ^Davidson, I. B.f "Double-wave Device for Use with a Cathode Ray Oscillograph," Journal of Scientific. Instruments, 11:359-361, November, 1934. 5 oscope. This feature permits the switching frequency to be varied over a wider range. A circuit discussed by George, He iff, Mayer and Roys5 uses pulses from a thyratron relaxation oscillator to vary the grid bias on two vacuum-tube amplifiers. The amplifiers are thus gated on and off, and therefore act as a switch, Garceau6 discusses a circuit in which two pentode tubes are turned on and off by a signal] on the screen °rrids of the pentodes. This signal is of much higher frequency than the signal being switched.. A circuit due to hughes7 employs a thyratron square- wave generator to modulate the suppressor grid of two type 57 vacuum tubes. This circuit is thus very similar to that of George, Heim, Mayer and Roys. Shumard8 describes circuits in whict the signal being switched is passed through a gas tube. These circuits are 5George, R. H, , H. 0". Helm, H. F. Kuyer, and C. S. Roys, "A Cathode Ray Oscillograph for Observing 2 Waves," Electrical Engineering, 54:1095-1100, October,~~1935. 6Garceau, L., "Dunlex Cathode Ray Oscillograph," Rev. Sci. Inst.t 12:171-172, June, 1935. •n 'Hughes, H. K., "Thyratron Selector for Double Trace Cathode Ray Oscillograph," Rev. Sci. Inst.. 7:89-92, February, 1936. 8Shumard, C. C, "Some Electronic Switching Circuits," Elect. Eng., 57:209, Fay, 1938. 5 primarily employed as counter circuits. Kurokawa and Tanaka9 describe a transient visualizer which uses the output of a phase-shift circuit to operate an impluse generator. The impulse generator either operates a thyratron switch or a relay. The application of this de­ vice is apparently at power frequencies* A switch described by Hughes and Koch1*-1 uses pentode amplifier stages. The pentodes are switched by a square- wave input to the cathode circuit.
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