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Anna Postolnik |
Faculty: Electrotechnikal: Specialty: Electromechanics systems of automation and electrical drive Head: Borisenko V.F. e-mail: Engel3000@yandex.ru |
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THE DEVICE OF A START-UP WITH THE CONTROL OF TECHNOLOGICAL PARAMETERS As against functions of redistribution of the loadings demanding regulation of speed within the limits of several percents from working speed, a control of start-up and technological regulation require more significant change of speed. Start-up of heavy conveyors is made rather seldom. In this connection an application of electric drives with the limited range of regulation that allows to lower the charge of the electric power, to lower capital expenses for the electric drive, and also to provide the modes of operation close to optimum, becomes expedient. One of the basic requirements of a control system of the conveyors is smoothness of start-up with the limited acceleration. Especially it is important for conveyors of the big extent that have danger of the slipping and danger of occurrence of oscillatory transient of the excurrent branch of the band. At start-up of band conveyors of the big extent breakaway of an excurrent branch from a place may begin after the motor, drive reel and incident branch have achieved the established speed because of elastic suppression of a band. Thus dilatational oscillations of speed begin in a dilatational branch that results to premature deterioration of branch and even its break. Thus danger of slipping, and also of occurrence of inadmissible fluctuations of speed because of elastic deformation of band requires that the drive and a control system provided strong limitation of quantity of acceleration at start-up of conveyor. The device for start-up of conveyors with control technological parameters at start-up allows to avoid of adverse situations for mechanism. For limiting of peaks of the moment arising at switching of a starting rheostat, protection from slipping, protection against emergency slipping and deterioration of band during start-up and operation speed, the system is supplied with blocks – device of the control of deterioration of a band and of lining DCD, device of a control of temperature DCT, device of measurement of power of friction DMPF, device of division of modes of operations DDM, block of suppression of picks BSP, block of suppression of slipping BSS. Block of suppression of picks consist of next units: unit of formation of pulse UFP, unit of formation of duration of pulse UFDP, unit of comparison UC and of switching element S. The analysis of processes occurring during start-up at control DSTF showed that signals on shunting of next sections of Rd is formed at completely open controlled rectifier CR. For this reason unique factor limiting size of a current of a rotor is resistance of starting rheostat, that change spasmodically, that results to burst of current. In a next, increase of control corner CC occurs because of dispersal of motor that limit a current on the necessary level. For this purpose at the moment of formation of a signal about next shunting Rd it is necessary spasmodically to lower level Uzi that will lock and will remove burst of a current. To provide switching steps Rd only after termination of shunting of integrant size, in other input UC the output of null-detector of the block of logic is connected. At concurrence of signals on an input of UC, a signal is formed on its output to the allowing of shunting of Rd. The analysis of processes and the phenomenon accompanying a mode of slipping of the conveyor allowed to formulate requirements to the device of protection against emergency slipping and to its principles of build-up. DMPF consists of the adder and the first block of multiplication with which help the voltage proportional to value: P=Mò ( wá - wë ) is formatted.Where P is the power depositing in the center of slipping. On an output of the block of multiplication the signal proportional to power of forces of friction the center of slipping is formed. Unit UCT includes the second block of multiplication x2, the integrator and comparator. Input signals of UCT are P and wë. The signal t proportional to temperature of overheating of a branch is formed on an output. This signal goes on the comparator and causes switching-off of the conveyor in a case of excess of an allowable level. The output of the integrator is moved on its inverting input with factor B/c that allows to take into account feedback of heat in an environment: ( B / c ) t where ñ - is thermal capacity of the center of slipping B - is factor of heat dissipation As currying out of heat from the center of slipping by a moving band is proportional both value t, and of speed of movement of a band vë, the signals proportional to these values are moved on inputs of multiplication x2 , the voltage proportional wë t is formed on the output. This voltage also goes on an inverting input of the integrator, than the account of influence on process of heating of the second component of losses of heat is achieved wë t = ( y / c ) vë t Where vë - is the factor which is taking into account carrying out of heat by moving band. The Attribute of end of start-up is equality of input signals Uzs and Uzi and a signal Mm becomes equal to zero. When acceleration is completed completely, Up becomes equal to zero. Due to the scope of this comparator by a positive feedback it is self-fixed and in this condition becomes nonsensitive to a level of a signal ( wá - wë ) Now the most perspective system of electric drive for conveyors is necessary to count CF – AM, which realizes required rate of acceleration, stepless moves to the lower frequency of rotation and on the contrary, realizes work of system on the minimum of power consumption. |
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