MODELLING THE EFFECT OF DIVIDED PULSES IN EDM Constantin DIACONESCU, Dan NANU, “Lucian Blaga” University of Sibiu, Romania Abstract: Experimental data published by several researchers indicate a ratio of open circuit pulses in the interval from 10% to 35% of the total number of pulses, during the EDM process. A solution to increase the probability to obtain breakdown in the dielectric liquid, transforming thus an open circuit pulse in a productive one, consists in dividing the voltage pulse in a large number of elementary pulses [2]. By this way an intensification of the thermal phenomena in the dielectric liquid is obtained, favouring the breakdown process. The present paper proposes a model of the active gap supplied with divided pulses, in open circuit conditions. The thermal effect on the dielectric liquid in the gap is qualitatively analysed and quantitatively estimated. Keywords: EDM, divided pulses, dielectric liquid, activation, modelling 1. INTENSIFYING THE HEATING PROCESS IN THE DIELECTRIC LIQUID BY APPLYING DIVIDED VOLTAGE PULSES The presence of a high voltage on the gap has a global stimulating effect on the dielectric liquid, preparing the breakdown conditions. On a microscopic level there are the following phenomena acting in the gap during the delay time [3] : - The electrons extracted from the cathode are accelerated to the anode by the electric force. The electrons transfer there energy to the atoms they collide, generating heat and/or creating electron-ion pairs which, in their turn, will be accelerated in the electric field, realising other collisions. - The polar molecules in the dielectric liquid are submitted to rotation moments, tending to align their electric axes with the electric field force lines. During their rotation movement the molecules interact, generating heat. - The non polar molecules are polarised by deformation due to the electric forces and are submitted, in their turn, to rotation moments, with the same effect. If a voltage pulse, divided in elementary pulses, is applied on the gap in open circuit conditions (Fig.1), the dielectric liquid is submitted to an alternative component of the electric field. This component generates variable forces that agitate the electrons and polar molecules in the liquid. The generation of heat is intensified, the number and volume of vapour bubbles in the dielectric liquid increases. The evolution of the gap towards the state realising an electric discharge is accelerated. Fig.1. Parameters of a divided pulse. t u u(t) o o t 1 2 3 . . . n t i t p t r t f T U a U a non-divided pulse divided pulse elementary pulse u(t) t Nonconventional Technologies Review – no. 2/2009 22