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Variation of Control Characteristics of Power Thyratron Tubes With Variation of control characteristics of power thyratron tubes with frequency by H W Snyder A THESIS Submitted to the Graduate committee In partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering Montana State University © Copyright by H W Snyder (1949) Abstract: A study was made of the variation in the control characteristics of three electrode thyratron tubes operation at frequencies up to 1080 cycles per second. Tests were made on thyratron tubes in half-wave rectification with magnitude d-c grid voltage control. The existence of two control characteristics, a starting control characteristic and an extinguishing control characteristic, at higher-than-power frequencies was confirmed. The starting control characteristic was determined to be dependent uoon frequency of spoiled anode voltage, lnterolectrode capacitance, and tube "dark" currents which are determined by geometry of electrode structures. Extinguishing control characteristic was found to be a tube deionization phenomenon. VA IATION 07 CO-T OL GHA . 4CT2BI3 TIC5 OF 'OmLA THYKATAOw TJDGJ AlTH FRGQJZNCY by H. - . 'JYDaR A THESIS Submitted to the Graduate Committee In partial fulfillm ent of the requirements for the degree of Master of Science in Electrical Engineering a t Montana State College Ap roved: In Charge of Major Work Bozeman, Montana Ju n e9 19^9 V s l ^ l 8 , Cu- i X - 2 - TABLE OF CONTENTS Abstract e ................................................................................................ ....... Introduction ....................................................................................... 4 Historical Survey .............................. 5 D escription of Test Equipment and Procedure . 7 Pest Results, Starting Characteristics . .................... 9 Rxtingulal lng C h a r a c te r is tic s ........................ 20 Conclusions .............................................................................................24 Appendix I .................................................... -39 Apuendix II . ........................................... 42 Ap end Ix III . ......................................................... 43 8 9 3 3 2 -3- ABJTRACT A study wae reads of the variation in the oo itrol characteristics of three electrode thyratron tubes oper­ ation at frequencies up to 1030 cycles per second. Tests were made on thyratron tubes in half-wave rectification with magnitude d-c grid voltage control. The exiotlnco of two control characteristics, a starting control character­ istic and an extinguishing control characteristic, at higher-then-power frequencies was confirmed. The starting control characteristic wae determined to be do mndent upon frequency of a >-Iied anode voltage, Interelectrode capaci­ tance, and tube "dark" currents which are determined by geometry of electrode structures. xtlngilshlng control characteristic was found to be a tube deionization phenomenon _ 4 - VARIATION OF CONTROL OHA ACTStIOTICO OF POAER THYXATRON TUBOS HTH FREQUENCY INTRODUCTION The industrial application of electric X>wer at frequencies other than those normally employed for generation la increasing raoldly, especially for such specialised purposes as heating, brazing, soldering and melting of metals by induction, speed control of motors by frequency variation, servo-mechanism controls, radar modulators, and aircraft rectifiers and Inverters. Power at the desired frequency of utilization is generally obtained from standard frequency power systems by means of frequency converting devices or generating devices which are more and more taking the form of circuits incorporating thyratron tubes. Generally the circuits used in these frequency converting or generating devices are well understood, but until recently the varia­ tions which might occur in the control characteristics of the thymtron tubes when subjected to higher-than-power frequencies were not known, though it was and ia generally accepted that there are frequency limitations in the use of these tubes because of finite ionization and deionization times -5- HISTORICAL SUHVZY Aa early as 1934 Berkey and Haller pointed out that th yratron firin g c h a ra c te ristic s would depend on frequency I of applied voltage. It was Indicated by v.. 9 . Shepherd In 1943 that there were two control characteristics In thyratron control phenomena which he classed as static and 2 dynamic characteristics. H. H. Wittenberg in 1945 Intro­ duced the concept of a starting control characteristic, meaning f i r s t cycle sta rtin g as compared to successive cycle firing, and an extinguishing control characteristic, meaning failure to Ignite on successive cycles, and showed that though these characteristics are generally coincident at low frequencies (60 cycles per second) they may become separate, at least for the control type thyratron tubes. In the range of frequencies. As the frequency is increased both control characteristics exhibit shifts, the extinguishing characteristic generally shifting most. It was uointed out that the starting characteristic would tend to shift In a negative direction because of coupling to the grid through the a node-grid capacitance and also because of increased gas pressure, while the extinguishing c h a ra c te ristic would also tend to shift negatively with Increase In grid resistance, increase of anode current, increase in gas pressure, the presence of and the amount of Inductance in the anode circuit, as well as increasing frequency. It was also noted. mm (> that the shift was more noticeable In those tubes with larger cross-sectional electrode area Vor greater volume). It waa tbe purpose of this Investigation to study the variation In the control characteristics of the larger power thyratron tubes of the single grid, mercury-vapor type because of the trend toward Installations requiring more power than control type thyratro n tubes can supply. One thyratron tube of the Mercury-vapor-1 nert-gas stabil­ ized type was also tested. Tubes tested vers SG-2 7 A, FG-57, FG-67, 3C23, 9 6 7 , UX9 7 3 . - 7 - DESCRIPTION OF TZGT ZQ,UINT AND =XOCZDU .S Figure I Bhoxs a diagram of the circuit used for test­ ing the tubes. The taps on the inverter provided adjustable anode voltage for the tube under test up to ooak values o f 850 volte# Rj was a variable resistor in the anode lead to limit the average anode Current. Filament power was sup­ plied from 1 60 Cycle per second a mroe through a variac in order to keep the filament voltage constant as well as to provide roite measure of control of the condensed mercury temperature of tlie tube under test. A small air blower was also used in this connection. A sine wave of test voltage wgta used because wave shape does affect thyratron performance, and the sine, wave not only gives results easier to analyze but is also most widely used of all wave shapes. Voltage wave shape was monitored by the cathode ray oscilloscope and checked by comparison of the readings of a rectifier type voltmeter and a radial vane type voltmeter. Frecuency of the applied voltage was determined by means of the audio frequency oscillator and the Lissajous pattern on the screen of the cathode ray oscilloscope. Grid voltage was obtained from a 2A vo lt battery and the potent­ iometer. The tube undergoing test was fired at the crest of the anode supply voltage wave by magnitude control of the grid voltage and the firing ?oint was monitored with the _ 8 - cathode ray oscllloacooe. The source of the test voltage was a parallel Inverter using s pair of P1'^67 thyratron tubes. Output power was limited by the input voltage rating of the inverter trans­ former and the current rating of the Inverter tubes. The inverter was chosen as being best able to su nly the re­ quired variable frequency, variable voltage power source of good output wave shape. Other means of obtaining variable 3 , 4, 5 , frequency, variable voltage power were studied but rejected as being too costly or of insufficient frequency range. - 9 - TEST RESULTS STAixTl MO- C H AR AC T ERIGTICS It was a,Dparent as testing advanced that the two con­ trol characteristics found for the smaller tubes would generally be found for the larger tubes also. The starting c h a ra c te ristic s would in general s h ift with the frequency of the applied anode voltage as can be readily aoen in figures 2, 3, and 4 , though the degree of shift might be so slight as to be masked by experimental inaccuracies as evi­ denced by figures 5, 6, and 7 end also by the curve of figure 4 taken at 769 0 . condensed mercury temperature. Figure 3 showing the shift of starting characteristic with frequency in the type 7*3»67 thyratron tube was of especial Interest, since the shift here Is shown in a pos­ itive direction where all other tubes exhibited, where noticeable, a shift in the negative direction. Wittenberg, as was noted previously, indicated that the negative shift could largely be accounted for by tbe voltage induced Into the grid circ ;it through the anode-grid capacitance with other factors such as d-c leakage currents and grid emission also contributing. Consideration of the equivalent circuit for a single grid thyratron tube In half wave rectifier service with a resistance load, source lmoedance negligible, leads to the - 10 - following expression for the voltage Induced Into the grid circuit through the tube interelectrode capacitances in­ cluding tube base and socket; ( See appendix I .) (I) TMs equation Is, of course, valid only during non-conduc­ tion. Figure 3 shows anode-grid cou ling
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