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CONTENTS
INTRODUCTION Scraper conveyors (SC) are the basic means of mountain weight delivery from clearing and preparatory mine faces to the surface. In some cases they are applied in drifts of conveyor-delivery districts to submit a mined rock to other transport means. Productivity of all district depends on work reliability of the scraper conveyor.
Nowadays SC contains in its structure the electric drive including the induction motor with a short-circuited rotor, hydrocouple and an intermediate reducer. However fluid flywheel use, does not allow to carry out dispersal of conveyor traction body with intensity, safe for the personnel. Hydrocouple protects from overloads only the motor and it possesses significant inertance.
Alternatively the electric drive with electromagnetic coupling can be used. It has certain advantages before hydrocouple - it carries out deeper speed regulation, and also does the best protection against dynamic overloads of the conveyor. Main disadvantages of the electromagnetic coupling are rather high complexity and cost.
At present the electric drive with frequency management is considered to be the most perspective. In this case the set diagram of motor dispersal is provided with respective alteration of frequency a supply voltage in an automatic mode.
Application of the frequency converters gives economy of the electric power up to 25-30 % due to optimization of working mechanisms speed rotation and reduction in starting currents of induction motors, dispersal and braking rates assignment, a contactless reverse of the motor, allows to considerably reduce dynamic loadings by a chain. The additional prize is formed due to the increasing of the between-repairs period of electric motors which work in the facilitated mode when supplied through the converter. However high complexity, cost, large dimensions complicate manufacturing of the frequency converter in a miner explosion-proof environment. A plenty of power semi-conductor devices complicates the decision of their effective cooling questions.
Power thyristor commutator has a wide range of functionalities. The choice of the most comprehensible scheme of the PTC for the scraper conveyor electric drive is defined by its functionalities, technical and economic parameters. Basis of the PTC is made by power gate unit, containing thyristor or diode-thyristor pair connected in antiparallel . PTC can be non-reversive and reversive.
In my master’s work will be considered the quasifrequency power supply mode of the induction motor. With reference to the scraper conveyor, it will allow to increase, first of all, a level of work and safety in a long wall, that will reduce a degree of a working personnel traumatism and also will lower dynamic loadings on a chain during start-up, will increase an overall performance and productivity of the conveyor as a whole.
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| Figure 1 – Scraper conveyor |
| 1,2 - head and trailer drive stations; 3 - head transitive section; 4 - linear pan; 5 - the rejecting block; 6 - clearing colter; 7 - tubular directing for a combine; 8 - a linear board; 9 - a board of cable layer; 10 - an arm for elctro -and hydro-communications: 11 - an arm for the lamp. |
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| Figure 2 – Induction motor braked shaft start oscillogramma |
During a start-up on the braked shaft the dynamic twisting moment reaches the maximal value (6,6Ìnom) - rating value for these electric motors. Pulsations of the electric motor dynamic moment at start-up both empty, and at the stopped traction body cause significant dynamic efforts in transmission, traction body.
Thus, rigid demands are made to work of scraper conveyors, one of which is maintenance of the conveyor start-up under loading due to restriction of a dynamic component of the stretching effort at an admissible level.
The urgency of a masters work theme lies in the necessity of the processes researching in the scraper conveyor electric drive to achieve the purpose of improvement the way of its start-up. This gives a demanded level of an attendants work safety by a application substantiation of the automated electric drive with quasi frequency supply.
During the direct start-up of the induction motor, high currents are flowing through its windings. At frequent start-up this can lead to failure of the engine owing to windings isolation destruction. Isolation destruction occurs for two reasons: mechanical destructions and decreasing of the isolation characteristics because of admissible temperature excess. The starting current of the motor in 5 - 7 times exceeds nominal, accordingly, electrodynamic forces acting on windings are increasing in 25 - 49 times. It is obvious, that direct start-up of motors is emergency dangerous mode of operation. The overwhelming majority of engines failures occurs during start-up.
In case if we will give to a thyristor voltage regulator (TVR) a property of the frequency converter (that is achieved by essentially new management algorithms of the TVR), energy losses at start-up will decrease. It is clear, that at classical frequency start-up method energy losses in the engine are same, as in the steady run operating mode on a steady branch of the mechanical characteristic at the equal electromagnetic moment. Calculations show, that by use of the offered quasi frequency system of soft start-up, energy losses in the motor in 2-4 times are less than in system with direct start-up or in 4-7 times are less, than during well-known soft start-up.
Thus, the quasi frequency mode of power supply provides lowered steady frequency of a rotor rotation and can be used for realization of a lowered speed step at start-up that will allow to increase works safety in a long wall. Besides quasi frequency power supply provides start-up of the asynchronous motor at the increased moment.
Hence, one of the drive perfection directions is the replacement of the hydrocouplings by semi-conductor switchboards as a voltage converters to supply induction motors.
Further researches will bring development and detailed studying of the scraper conveyor electric drive mathematical model at quasi frequency power supply of motors. Realization of the mathematical model will be made by means of modern computer maintenance.
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