           FLORIN DUMITRU POPESCU, ILIE MITRAN The Machine and Installation Department, The Mathematical Department University of Petro#ani Str. UniversităŃii, Nr.20, Petro#ani ROMÂNIA fpopescu@gmail.com , iliemitran@gmail.com http://ime.upet.ro/mi/html/conf_univ_dr_ing__florin_popes.html http://www.upet.ro/facultati/stiinte/catedre/mate/CVISTORIC/mitran.html Abstract: The operation of vertical transport installations are based on cable systems with electrical adjustable operation, which ensures the technical conditions regarding time variation in speed, the current in the main actuation engines and the acceleration during start4up and breaking. The actuation electrical systems are part of the category of the fast process of the automation equipment. For the vertical transport installation the size which has to be controlled from a command is the speed, the dependent factor being the tahogram. Adjusting determines the dependence of the sizes in the process, through a default law, both in relation to the independent sizes and in relation to the dependent ones from the process, assuring the reduction of the influence of harmful sizes to the process. The stability of an automatic system can be defined in different ways. We will consider stability of an automatic system, that feature which is that when it is subjected to the action of one harmful size of the moment the system will return in the end at the stationary phase. KeyWords:  Nyquist stability criterion, place of transfer, transfer function, speed adjustment      This stability criterion is the most frequently used in the study of automatic linear systems, because it is based on the theory of functions of complex variable and it establishes the stability conditions of the closed system based on the analysis of the transfer of open system. If to an automation element, linear or non linear, or to a system a step signal is applied at the entrance, the output size will have a transitional regime. As a consequence an element or a linear system is called stable if the transitional process of the output size fades in time, this one going to a final constant value if at the entry is applied a unit step signal. The Nyquist stability criterion allows the deduction of behavior of the closed adjustment circuit from the analysis of the behavior of the open adjustment circuit. The answer to frequency of the open adjustment circuit can be presented by the following relation: Z1 Z2 d a N1 N2 (1 j T)(1 j T ) 1 Y (j ) k (1 j T ) (1 j T ) Z( j ) N(j ) +ω ⋅ +ω ⋅⋅⋅ ω= ⋅ ⋅ α +ω ⋅ +ω ⋅⋅⋅ ω = ω (1) The stability criterion is applied in the allowing hypothesis: 4 the degree of the polynomial Z has to be smaller than that of the polynomial N; 4 the open circuit is stable ( α =1) or they have an integral behavior ( α =j ω T); From those mentioned above, the Nyquist stability criterion can be state as: a closed adjustment circuit is table if the place to transfer the answer to frequency d Y (j ) ω of the open circuit does not surround the coordinate point (41,j0). The Nyquist stability criterion has the advantage that in can be applied in:  If not all the blocks of a closed circuit are known, still the answer to the frequency can be measured;  Allows observation on stability as well as on the amortization of the transitional process. Figure 1 represents the transfer place of a stable system. The phase D ϕ to which the transfer place overlaps the radius of a circle the unity gives the information on the degree of amortization of the system, so that for values smaller than D ϕ the amortization is stronger. The stability of an adjustment circuit can be Proceedings of the 8th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING ISSN: 1790-2769 125 ISBN: 978-960-474-131-1