RELIABILITY OF AN INDUCTION HEATING INSTALLATION IN RELA- TION TO PRODUCTION SCHEDULE Prodan Prodanov Mintcho Simeonov and Dobroslav Dankov Technical University of Gabrovo Dep. Electronics +359 66 2564 e-mail: symeon@tugab.bg 7 Hadji Dimitar str. 53 Gabrovo BULGARIA Abstract Reliability prediction for a system of elements functioning jointly concerns determining the causes for failures in this system and also detecting the elements that can cause failures. This type of prediction is known as a fault tree analysis and consists of determining the failure rate at a lower level of the system examined. Such an analysis will be applied in the reliability study of a thyristor converter for induction heating. Key Words: Reliability Failure rate Fault tree Analysis Thyristor converter induction heating. Introduction To determine the quantitative indicators of the reliability of a complex system such as a volumetric heating induction installation is a complicated task. The authenticity of results is connected with certain knowledge of the reliability theory and consideration of different factors which are often contradictory. The reliability indicators set for an induction system put into operation are bound to the rated duties of both its constituent elements and the converter [ 2]. The subject in the present case is thyristors in an Induction heating installation used in a forge shop for heating at least 2 C temperature of steel work. Deviations from the conditions under which their reliability have been determined are asserted. These changes are mainly linked to productivity acceleration and heating of of a wide external diameter range. Paper aim: to investigate the reliability indicators of rectifier and inverter thyristors in an Induction heating installation for heating steel at operating modes. The basic blocks of the Induction heating installation are: thyristor converter - SMK UB 2F2 63/.5/.6-R and a mass heating machine for steel parts - ITO 63/-A-L. The main parameters are: Operating voltage across the load 6 ; Operating frequency within the range (75 5) Hz; Rated power 63 ka; The thyristor converter SMK UB 2F2 63/.5/.6-R consists: The three-phase rectifiers working according to a Larionov circuit. The using thyristor in rectifier are: Th Th6 T967-4-2; Input choke Starting circuit. The basic elements in the starting circuit are: Diodes D D4 DR856-25-2; Thyristors Th2 Th22 TR967-26- 2; Capacitors battery C2 C4 POAJN- 75/44 The bridge current inverters. The using thyristor in inverters are: Th7 Th TR967-4F-2. The mass heating machine for steel parts ITO 63/-A-L consists: Depending on the crosssectional dimensions of the work used one of the following types of inductors can be used - Table : R-ITO 6- R-ITO 6- R-ITO - R-ITO - R-ITO -2 Table Load capacitors battery which is from 24 capacitors. The rated capacity is 23 µf. A database has been created which contains operating modes of all elements in the power circuit of the rectifier and inverter in the Induction heating installation. Table 2 presents the main
parameters of the operating modes for five of different diameters and for preassigned productivity. Table 2 Operating Parameters for Inverter Diameters Long for thy ristor s I A A ø7/3 R-ITO 6- ø/3 R-ITO 6- ø9/27 R-ITO - ø/27 R-ITO - ø/47 R-ITO - 2 Prod u ctivity Kg/h U DC I DC A U LOAD f Hz U BA t q µs 6 3 65 46 3 46 65 5 7 4 5 85 5 6 6 42 85 55 2 75 44 82 6 35 65 42 95 62 5 55 85 6 6 82 7 62 95 6 Fig. 3 oltage and current of an inverter thyristor for a 7 piece Fig. and Fig. 2 show oscilograms of the rectifier thyristors for two work. Fig. 3 and Fig. 4 show oscilograms of the inverter thyristors for the same. Fig. oltage and current of a rectifier thyristor for a 7 piece Fig. 2 oltage and current of a rectifier thyristor for a piece Fig. 4 oltage and current of an inverter thyristor for a piece The mathematical calculation of the failure rate FR for the all thyristors in the power circuit of the induction installation have been calculated [34] as follows: assumes that the objects consist of electronic elements in an operational period for which an exponential law of distribution of random variables applies. At nominal operating and temperature modes and at ambient conditions EL i of these elements have been calculated as follows: Th b. π T R. π S Q E is base Failure rate; b πt - Temperature factor; e T J 382 T J + + 273 298 - Junction temperature. The standard junction temperature is calculated using the following relationship: TJ TC +θ JC. P T - Junction temperature in degrees centigrade; J
JC T C - Case temperature in degrees centigrade; θ - Junction to case thermal resistance in degrees centigrade per watt; - Power dissipated in watts; 4 π R ( I f _ RMS ) - Current rating factor; I f _ RMS - RMS rated forward current; 9 BA π - oltage stress factor; S BR - Blocking voltage applied - blocking BA BR voltage rated; - Quality factor Select the quality π Q value from [3]. π E - Environment factor - find the environmental factor from [3]. For the inverter thyristor calculations are: - b is base failure rate: b 22 2 - Temperature factor calculate - π T Изчислява се според температурата на кристала в зависимост от загубите на мощност. За определянето на температурата на кристала трябва да бъдат определени загубите на мощност и да се знае топлинното съпротивление на корпуса на елемента: 3 πt exp 382 Tj + 273 298 T T + R. P 5 + 4.342 59 C 4 BR j c jc tot πt exp 382 Tj + 273 298 exp 382 288 59 273 298 + - Current rating factor calculate 4 π R ( I A ) 4 85 Where I A 85A. - oltage stress factor calculate: 9 ( S ) 458 9 23 π S U U S BA BR BA 55 458 2 5 6 - Blocking voltage applied; - blocking voltage rated on the thyristor TR967-4F-2. - Quality factor and Environment factor calculate π Q - Since the product is a coercial device with an unknown screening level the quality factor has a value of π Q 5.5 as shown in [3]. π E - For ground fixed conditions the value is π E 6 - moderately controlled environments such as: installation in permanent equipment racks with cooling air installation in unheated buildings permanent installation of air traffic control radar and counications facilities. Total generic failure rate for the one used thyristor is [3 4]: ThInv b T R S Q E 22.288.25.23.55.6 54 7 The data from the failure rates calculated for different operating modes of the inverter thyristors are presented in Table 3: b ø7 R-ITO 6- ø R-ITO 6- ø9 R-ITO - ø R-ITO - ø R-ITO -2 Table 3 π T π R π S π Q π E ThInv 22 36 23 55 6 2294 22 97 22 55 6 346 22 288 25 23 55 6 54 22 35 9 27 55 6 768 22 5 5 28 55 6 628 An almost five-fold increase in failure rate is observed when operating with from the least to the greatest diameter. The considerable difference in the reliability indicators is a result of the change in operating frequency and the greater losses in the inverter thyristor. For one rectifier thyristor following the same sequence of calculations the following results have been obtained - Table 4: b ø7 R-ITO 6- ø R-ITO 6- ø9 R-ITO - ø R-ITO - ø R-ITO -2 Table 4 π T π R π S π Q π E ThRect 22 2 86 55 6 824 22 3 934 55 6 969 22 35 957 55 6 32 22 3 943 55 6 986 22 4 979 55 6 95 In the case of rectifier thyristors the dependence of failure rates on the operating mode
is mainly related to the current across the element and the power losses in it. The results for the failure rate of rectifier thyristors are directly dependent on the magnitude of the current flowing across that element and the power losses in the element. The slight difference in the failure rate values for these operating modes is a result of the approximately identical thermal and electric modes. 4 ThI Th _ Inv 9 i The mathematical calculations of the FR for the all thyristors in the power circuit of the induction installation have been calculated as follows and given in Table 5: Th ThR + ThI Table 5 th 2 9 7 6 5 4 3 2 6 7 9 2 Th_inv D Th_rect Fig. 5 Dependence of failure rate on piece diameter for inverter and rectifier thyristors The results are shown in Fig. 5 with a graphic representation of the dependence of failure rate on the piece diameter. The comparison of the two graphs gives rise to two conclusions: Conclusion the inverter thyristor has a higher failure rate than the rectifier thyristor and its operating modes determine largely the overall reliability of the Induction heating installation. Conclusion 2 heating work of diameters 9 to 2 C in the production schedule changes the current load of the inverter thyristor by %. It can be seen in Fig.5 however that the failure rate of inverter thyristors increases by 46.5% for having diameters above 9. The next calculation failure rates are for all rectifier and inverter thyristors. The FR of these components is obtained simply by suing the individual failure rates. Also FR calculations are based on many assumptions about their operating conditions which should not differ. Note that for the same operating mode the Failure Rates (FR) of the thyristors in the rectifiers and the inverters have been calculated [345] as follows: - For the thyristors used in the rectifiers: 6 ThR Th _ Rе 8 i - For the thyristors used in the inverters: ø7 R-ITO 6- ø R-ITO 6- ø9 R-ITO - ø R-ITO - ø R-ITO -2 Th _ Rе Th _ Inv ThR ThI Th 824 2294 4944 976 42 969 346 584 3844 9658 32 54 692 264 27832 986 768 596 3432 36348 95 628 657 4652 5382 The reliability function P( t ) exp(.t ) can be used to determine the Probability of the operation R Th (t) for the thyristors and have been calculated as follows: R Th 6 4 ( t ) exp Th Re ct + Th Inv. t On the basis of the above calculations for thyristor failure rates the so-called production schedule can be made out - maintaining Probability of the operation in the given operation period at a level R Th (t) 85 and calculating the operation time according to the values obtained for the failure rate. It is possible to determine the operation time for each type of inductor in case of constant Probability of the operation for certain values of thyristor failure rates: - For an inductor of the type R-ITO 6 - and a piece of diameter 7 : ln P( t) ln 85 t 5h 9 th 42. 2 - For an inductor of the type R-ITO - 2 and a piece of diameter : P t t ln ( ) ln 85 36h 9 th 5382. 3 The results have been systematized in Table 6.
diameters ø7 R-ITO 6- ø R-ITO 6- ø9 R-ITO - ø R-ITO - ø R-ITO -2 ThR ThI Th Table 6 R(t) t hours 4944 976 42 85 5 584 3844 9658 85 8267 692 264 27832 85 584 596 3432 36348 85 447 657 4652 5382 85 36 The mathematical calculations of the Probability of the operation R Th (t) for the all thyristors in the power circuit have been calculated depending on the diameter of the heated and the required operation time. The results have been systematized in Table 7. OPERATION TIME t hours Table 7 5 5 5 R Th ø7 998 992 985 93 868 493 R Th 7 ø 998 99 98 96 82 374 R Th ø9 997 986 972 87 757 248 R Th 9 ø 996 98 964 833 695 62 R Th ø 994 973 948 766 588 7 R Th Fig. 6 shows the graphical solutions for. R(t) _ 9 8 7 6 5 4 3 2 t [h] the Induction heating installation can be used for nearly 9 hours free of failures. When a mixed production schedule is applied this period is approximately 4 hours or one year. 3 Literature [] Simeonov M. P. Prodanov Reliability Analysis on the Induction Heating Installation using the Fault Tree Method. X-th. International Symposium on Electrical Apparatus and Technologies. Siela 27 3 May- June 27 Plovdiv. Proceedings olume pp. 67-73. [2] Simeonov M. P. Prodanov Reliability functional relationship analysis on the power circuit in Induction Installation using fault tree method. 4th International Symposium on Power Electronics Ee27 Novi Sad Republic of Serbia. November 7th 9th 27. [3] MIL-HDBK-27F Reliability Prediction of Electronic Equipment Notice July 992 Notice 2 28 February 995. [4] Pham H Handbook of Reliability Engineering. Part ІІ Statistical Reliability Theory Springer -erlag London Limited 23 ISBN-85233-453-3. [5] The Rome Laboratory. Reliability engineers toolkit. April 23. R(t)_7 R(t)_ R(t)_9 R(t)_ R(t)_ Fig. 6 The graphic design on the Probability of the operation R Th (t) 2 Conclusion The results obtained related to maintaining probability of operation R Th (t).9 for the entire range of work in the production schedule can be used to determine one of the reliability characteristics of the Induction heating installation namely MAINTAINABILITY. For instance in the case of a piece of diameter 7 it can be utilized failure-free for 75 hours and need not be stopped for preventive maintenance. In the case of a piece having a diameter of