1. Željko V. Despotović, Miloš.D. Jovanović Mihajlo Pupin Institute, University of Belgrade, Serbia zeljko.despotovic@pupin.rszeljko.despotovic@pupin.rs, International Conference Power Plants 2014-Zlatibor 29-31.10.2014

2.  Thermal processing of solid state alloys is carried out in order to improve their technological properties by modifying their structure.  One of the possible ways of heat treatment is indirect electrical resistance heating i.e. annealing.  The main parameters of the heat treatment are the maximal temperature and temperature-time profile heating of the material.  In this paper it is presented the three-phase AC/AC power converter based on thyristor power control and microprocessor control technology.  In this way the traditional modes of heat treatment control (auto- transformers, control transformers, control relay with contactors, etc.) are replaced with contemporary and flexible power electronic units.  AC/AC power converter is pulse controlled in, so called „burst mode“, that achieves efficient and precisely controlled electro resistive annealing of massive metal parts.

3.  Thermal processing of solid state alloys is carried out in order to improve their mechanical, physical and technological properties through modification of their structural composition.  Thermal processing, in this case, means the set of operations of heating, including temperature control and subsequent cooling in order to improve the mechanical and physical properties of treated material, purely through changes to the their structure (due primarily to diffusion processes).

4.  The regime of heating is determined by technological requirements for the specific material that can heat treated, which determinate the temperature-time profile.  Time diagram of heating begins with the ambient temperature and ends up reaching the required temperature on the surface of objects.  Also, the heating operation includes regime preheating or equalization heating temperature of metal parts or alloys, by defining preheating time.  Temperature control during the heating aims to make it possible to fully carry out structural transformation in the material being treated, and it is defined by the control of the heating temperature in corresponding time.  The cooling mode is determined by the start of the cooling temperature, temperature gradient and decreasing time depending from requirements and used coolants.

5.  Annealing is a form of heat treatment where the material is taken to a high temperature, kept there for some period of time and then cooled.  Main task of this type thermal processing is transition the unstable state (performing with the phenomenon of the internal stress, which may be significant) in a structural steady state.  High temperatures allow diffusion processes to occur fast.  The time at the high temperature (soaking time) is long enough to allow the desired transformation to occur.  Cooling is done slowly to avoid the distortion (warping) of the metal piece, or even cracking, caused by stresses induced by differential contraction due to thermal inhomogeneity

6. Basic diagrams of thermal processing of solid state alloys (a) T-C diagram, (b) temperature-time profile for annealing

7. 1.Temperature increase without regulation 2.Controlled temperature rise (typically +100°C/h) 3.Maximal heating temperature with regulation 4.Minimal heating temperature with regulation 5.Controlled cooling (typically -200°C/h) 6.Minimal temperature of controlled cooling 7.Free cooling to the ambient temperature The required power for electro resistive annealing depends on the thermal requirements, but also the size of the metal parts or alloys, that is subjected to heat treatments. In most cases, the active electric power that are required for electro resistive thermal processing (annealing in this case), include the range from 30kW to 100kW. This power requirement greatly determines the designing and apparent power of the power converter that achieves required characteristics is shown in figure.

8.  Today, the standard power modules, used in systems of temperature control and stabilization process heat treatment in thermal processing, is based on solid state relays (SSR) switches.  SSR is actually powerful triac, with optical isolation of control signal and zero cross detection (ZCD) circuit to synchronization with the passage of the supply voltage through zero.  The application of SSR module is limited to a current of 50-100A, so that the application of large power in the heating and heat treatment comprise the use coupling of anti-parallel thyristors, including the driver circuit, which must be provide galvanic isolation from the control circuit  AC/AC power converter and control system which can achieve the required temperature-time diagram of the process of heat treatment (electro resistive annealing)?????

9. By selector switch SW1 it can be easily select mode of operations (1-synchronous mode, 2- asynchronous (random) mode).

10.  This section presents the simulation results for proposed AC / AC power converter achieving to achieve the controlled heat treatment by annealing.  The simulation results are presented for the purely resistive load and anti-parallel thyristors X1, X2.  Two modes operation is provided: random (asynchronous) and synchronous.

11. Simulation results for random (asynchronous) mode of AC/AC power converter (a)simulation scheme, (b)(b) characteristic wave-shape: mains voltage, control excitation pulse, load current, V AB voltage

12. Simulation scheme for synchronous mode of AC/AC power converter

13. Simulation results for synchronous mode of AC/AC power converter; characteristic wave-shapes: mains voltage, control pulse, load current, V AB voltage

14. PRACTICAL REALISATION OF AC/AC POWER CONVERTER FOR ELECTRO RESISTIVE ANNEALING

15. AC/AC POWER CONVERTER – EXPERIMENTAL RESULTS The experimental results of AC/AC power converter for electro resistive annealing - characteristics wave forms; (a) random (asynchronous) mode, (b) synchronous mode

16. AC/AC POWER CONVERTER – EXPERIMENTAL RESULTS The experimental results of AC/AC power converter - characteristics wave forms of voltage and thyristor current; (a) random (asynchronous) mode, (b) synchronous mode

17. The experimental record of temperature profile and control signals for real electro resistive annealing system: I- start of heating, II- controlled rise of temperature, III-temperature maintaining (temperature control), IV, V-free cooling OBTAINED TEMPARATURE PROFIL-electro resistive annealing

18. The temperature profile obtained on the industrial recorder (at the front side of the annealing control unite) for real electro resistive annealing system This temperature value of 680ºC is in fact operating annealing temperature of massive metal part. The tolerance band of temperature is about ±15ºC, i.e. accuracy is <5%. In the sections IV and V is performed free cooling to the value of about 300ºC. Record of achieved temperature profile on the industrial recorder at the front side of the realized annealing control unit.

19.  This presentation was shown the simulation circuits and practical realization of a AC/AC pulse-controlled power converter and the corresponding control system that are used for electro resistive annealing and thermal homogenization of massive metal parts.  In this way it is achieved their transition from unstable state (that is created by the appearance of internal stresses whose values ​​ can be considerable), to a stable structural state.  The confirmation of realized power and thermal requirements for the process of annealing, are presented simulation and experimental results obtained during the few development phases: -development of the proposed AC/AC power converter and corresponding electro resistive annealing control unit, -putting into operation and -adjustment in real operating conditions.

20. OCTOBER 2014 PRESENTER: Željko V. Despotović, PhD.E.E Senior Research Associate Mihajlo Pupin Institute, Belgrade SERBIA THANK YOU IN ATTENTION!!!!