Thyristor Voltage Converter Research Articles

Power control of electric heating elements using thyristor voltage and resistance converters

This article investigates the energy efficiency of thyristor converters with an active load. The conversion of electric energy in a “thyristor converter – electric heating element” installation is considered from the standpoint of electromagnetic theory. The energy characteristics of the considered installation was calculated in the MATLAB environment. When thyristor voltage converters are operated under the mode of controlling the power of active load, passive power was found to be generated at the input during the non-conductive state of the converter (thyristor is locked). The use of passive power allows additional thermal energy to be obtained by means of an extensive use of voltage, without increasing the current consumption. An increase in the depth of power control of electric heating elements by thyristor voltage converters leads to a significant increase in passive power. In the active power control range (50–100% of the nominal value), the factor accounting for variations in the total power in the converter due to an incomplete use of the voltage at the input of the considered installation decreases from 1.0 to 0.93. This reduces the power factor of the load converter from 0.97 to 0.925. Despite the high value of the load power factor (in the control interval 0–50% of the rated power value), the factor accounting for variations in the total power was found to be reduced to 0.66. As a result, the power factor of the converter with the load is reduced by ~ 33%. In order to increase the efficiency of converting electrical energy to control the active load power, it is proposed to use thyristor resistance converters that vary the electrical resistance of the load over time. It is shown that unsatisfactory operation of a thyristor voltage converter can be caused by inefficient use of voltage at the input of the “thyristor converter – electric heating element” installation. When using thyristor resistance converters, the current non-sinusoidality factor does not exceed 1.5 % and the voltage non-sinusoidality factor in a 0.38 kV network does not exceed 0.2 %.

Reactive Power of Asynchronous Electric Drives with Semiconductor Converters

The purpose of the article is to obtain a ratio for determining the reactive power of induction motors when powered by semiconductor converters. The task is to determine the dependence of reactive power on control parameters of the converters. The research method used is computer simulation of electric drive based on Fryze power theory for non-sinusoidal currents. The result is the obtained expression for the calculation of reactive power, which takes into account the rated idle current of the motor with sinusoidal power supply and the type of converter, due to introducing special coefficients. Numerical values of the latter, depending on the control parameter, are obtained on computer models with their subsequent approximation. The scientific novelty of the research is in the further development of Fryze power theory in the direction of decomposition of non-sinusoidal current components using computer models. The practical significance is the obtained expression for determining the reactive power of an asynchronous electric drive with a thyristor voltage converter, on the basis of which the capacity of compensating capacitors is calculated to increase its energy efficiency.

Experimental determination of suboptimal parameters for energy-efficient control of an induction motor

The article presents the results of experimental studies of the automatic speed control system (ASCS) of an induction motor (IM). Preliminary experimental studies have shown that the stator current minimum (power factor) is a suboptimal criterion. Optimal in terms of control is the rated power factor. Tests of IM with a thyristor voltage converter (TVC), as a power source, were conducted on an installation created at the department. A mathematical model of ASCS IM corresponding to the experimental setup has been developed. To determine the main functional dependences of IM, such as stator voltage, stator current, power factor, torque on the shaft, a program for approximating experimental data by polynomials was developed. Using the developed mathematical model, the regulatory characteristic of IM that was optimal from an energy point of view was obtained. The necessary indicators of IM and TVC are determined (thyristor control angle, stator voltage, stator current) to change existing settings in order to save electric energy. The results of experimental studies are presented, the graph shows an optimized version of the form of the regulatory characteristic according to the criterion of minimum electric energy consumption.

К вопросу о выборе режима пуска ленточного конвейера

The paper is concerned with the problems associated with starting a long conveyor driven by an induction motor. We consider the dynamic loads in the conveyor belt during start-up, which cause slippage of the conveyor belt on the traction drum leading to its premature deterioration. The relevance of the topic is associated with the high cost of the conveyor belt reaching 60% of the cost of the entire conveyor and more. The aim of the work is to ascertain the possibilities of reducing the wear of the conveyor belt, for example, through the use of the possibilities provided by the system of the electric drive. The possibility of reducing the probability of the conveyor belt slippage by increasing the initial belt tension is noted. However, constant high tension also accelerates belt deterioration. To solve this problem, the use of automatic tension control devices is proposed, as well as the reduction of dynamic loads in the belt by using soft starters based on a thyristor voltage converter, which regulates the amplitude of voltage on the motor stator. The features of an asynchronous motor heating when starting with a constant speed of rotation of the magnetic field are discussed. The interrelation between the start-up duration and the motor stator winding heating with the dynamic loads occurring in the conveyor belt in start-up modes is considered. Modeling results obtained using the Simulink package are presented. Since long start-up modes of asynchronous electric drives based on thyristor voltage converters can lead to motor overheating, especially when the conveyor is restarted under full load and in hot climates, it is advisable to use automatic conveyor belt tension control devices during start-up modes thereby reducing premature belt deterioration together with a decrease in the conveyor motor heating.

Synthesis of the system for minimizing losses in asynchronous motor with a function for current symmetrization

The operation modes of the TVC-AM electric drive in which the power losses can be reduced were mathematically described. It was defined more precisely that what was at issue is the continued operation of an asynchronous motor at artificial characteristics in the region of nominal slip with an energetics better than at the working section of the natural mechanical characteristic. The system of automatic minimization of power losses of asynchronous motors was developed. It was shown that under conditions of feeding the electric drive from a source with asymmetrical voltage, it is necessary to use phase-by-phase control which requires the use of three control channels and three feedback channels. The logic of the feedback action is that when the load on the AM shaft decreases, it reduces the output voltage of the TVC and the motor currents. As a consequence, it becomes possible to maintain equality of the load angles of all phases to the optimum value. This makes it possible to solve the problem of minimizing the AM losses in a case of equality of the load angles to the optimal value and symmetrization due to equality of load angles by the phases of the motor. The quantitative indices of power loss decrease as well as the symmetrization indices when the electric drive is powered from a source with an asymmetric voltage were shown. The features of the power loss minimization system and the ways of improvement of its efficiency were illustrated. It was shown that the use of the proposed control system led to an improvement in both the power and dynamic parameters of the asynchronous electric drive. A 5 to 45 % reduction of power losses in the range of operating moments of 0£М£М bo relative to losses during operation at the main mechanical characteristic was recorded. The symmetrization effect was characterized by a 1.5 to 6 times reduction of the current asymmetry coefficients. The use of a thyristor voltage converter makes it possible to realize controlled transient start-up and braking modes. Symmetrization of the acting currents of the asynchronous motor results in an 80–150 % reduction of vibrational components of the electromagnetic moment in a steady-state regime

Synthesis of the methods of research of stability of the electric drive “thyristor voltage converter – induction motor

In the open-loop systems of electric drive (ED) «Thyristor voltage converter − induction motor (TVC−IM) on the working section of mechanical characteristics, in some cases, the oscillations of the output values can be recorded. Such unstable modes are considered to be a negative phenomenon, which may limit the use of ED and is subject to detailed study. The transfer function of electromagnetic part of an induction motor was determined, which characterizes the transient function of an electromagnetic torque. It is proved that for different motors the character of the changes in the torque varies, the necessity of selecting specific components of electromagnetic torque was substantiated. Based on the received transfer function, the synthesis of computational−analytical method of the study of the stability of the open-loop system of electric drive of TVC−IM was conducted. The analysis of stability was carried out with different induction motors of one series based on a simplified method by simulations and experiments. The assessment of the impact of different factors on the stability was made, namely: the IM parameters, load torques, the values of the angles of switching valves and initial electromagnetic conditions. It is shown that the study of the ratios of time constants of transient function of electromagnetic torque of IM and electromechanical time constants allows forecasting the possibility of the occurrence of oscillations. The analysis of influence of schematic configurations of pulse-phase systems of control of the TVC on the character of unstable modes was carried out. The results of this work make it possible to qualitatively and quantitatively characterize a mode of oscillations of TVC−IM, while also to determine the area of possible unstable performance, as well as to attempt to eliminate the preconditions to the occurrence of oscillability.

Automated comparison of the tecnical and economic efficiency for crane mechanisms

Various types of the AC electric drive: traditional one with resistor control, with thyristor voltage converter and with frequency converter, the latter with power recovery are considered. The review of static and dynamic characteristics of each of the electric drives is presented, based on which recommendations are made concerning the application of a particular asynchronous electric drive of the proposed options in specific cases. Energy performance, loss in the motor circuits are calculated for specific crane hoist mechanisms. The CAD-system for technical and economic comparison of the crane drives taking into account the main operation features: loads, moments of inertia with/without load, the relative operating time with reduced speed is developed. Due to the unification of approaches to the analysis of the operating conditions of the hoist and swing mechanisms, the program calculates the power consumption in static and dynamic modes. Known capital and depreciation costs and total annual power losses obtained in the CAD-system allow making decisions about the choice or necessity of reconstruction of the electric drive.

Synthesis and research of automatic balancing system of voltage converter fed induction motor currents

Synthesis of methods for investigating unbalanced operating conditions of thyristor voltage converter fed induction motors is performed. The voltage unbalance factors of the power source are calculated, and the formation conditions are determined. Attention is paid to the harmonic analysis of the induction motor currents. The influence of power source voltage unbalance on the induction motor performance is determined on the basis of simulation and experiments. The results of power source unbalance - phase current unbalance, overload capacity reduction, oscillating nature of torque and speed, dynamics deterioration are outlined. The system of automatic balancing of the induction motor stator currents using the thyristor voltage converter is proposed. The synthesis of the elements of control channels and feedback channels of the balancing system, as well as operation principles, are examined. It is determined that balancing reduces the current unbalance factors by more than an order of magnitude, and virtually ensures the equality of the active stator currents in the induction motor phases. The benefits and drawbacks of the automatic balancing system are presented, and recommendations on its application are given.

Development of selection criteria for electric drive crane mechanisms

The paper considers several options of AC drive: traditional with the resistor control, with thyristor voltage converter and with frequency converter, the latter with a resistor braking or energy recovery possibility. An overview of the static and dynamic characteristics of each of the electric drives is proposed, based on which recommendations are made for using an asynchronous electric drive from the proposed options in specific cases. Particular attention should be paid to the energy performance, losses in the motor circuits that need to be calculated for each individual crane mechanism. The proposed recommendations allow the designers of crane mechanisms (lifting, travel, slewing), the personnel operatig them, to consciously choose the electric drive for the designed or reequipped actual cranes operating in specific, well-known conditions. Thus, the cranes will be equipped with the asynchronous electric drive, best for the given conditions.

A soft-startup device for an induction motor with supply-network voltage control

The use of a thyristor voltage converter to limit voltage dips during the induction motor startup is considered. It is shown that the traditional solution—the current-cutoff startup mode—does not solve the problem of maintaining the residual voltage at supply-network bars during the motor startup at a level, at which the uninterrupted operation of other equipment connected to this supply network is provided. To limit the voltage dips during the startup, it is necessary to control the supply voltage during the startup. A control-system structure of the thyristor soft starter, which implements a closed loop of supporting the supply-network voltage during the startup, and the synthesis procedure of a controller in this structure are proposed. It is shown that the controller, which is created based on a simplified object model, gives acceptable results in practical use. Results of modeling and experimental data that show the operability of the proposed approach are presented. The desired supply-network voltage is directly set to the input in the system with the voltage control, and the possibility of maintaining this voltage during the induction-motor startup with the structural and parametric variations in the supply network and load is provided with high accuracy and adequate quality of transient processes.

Совершенствование автоматизированных электроприводов агрегатов прокатного производства

Although the majority of domestic metallurgical enterprises purchase and install processing lines from foreign suppliers, development, implementation and promotion of domestic equipment is still an urgent and challenging task. Technological achievements of the joint research teams from Magnitogorsk State Technical University, the South Ural State University and the OJSC “Magnitogorsk Iron and Steel Works” (OJSC “MMK”) are described in the article. One of the priorities in the research work is energy saving both in electric drives and in automation equipment of electric drives. The authors describe the approach and the technical solutions providing reduction of electric power losses due to the reduction of reactive power consumption by thyristor electric drives of rolling mills. Technical efficiency is achieved thanks to the reduction of the thyristor voltage converter storage without any compensating devices. Some algorithms and software providing energy and resource saving due to improvement of the speed and load modes of electric drives in the process of plate production from the difficult-to-form/hard steel grades are given. The structure of the automation system for 5000 plate mill rolling stand is presented. Local systems of the automatic end-point control providing adjusting of certain technological variables are described. Automation of level 2 ensuring the production process control according to the schedule predetermined by the operator is considered. Great attention is paid to the developed resource saving electrical systems of the wide strip hot rolling mill. The authors introduced a complex of scientific developments aimed at improvement of automatic electric drives and process variables control systems. The resource saving is provided by reduction of specific consumption of materials as a result of defective product reduction and the decrease of wastes caused by strip end crops. The authors studied the system of automatic tension control with an extra fast acting channel adjusting the hydraulic screw-down structure of the rolling stand. They intro-duced the method and the algorithm of tension control in the intestand gaps of the hot rolling mill roughing train. All the de-veloped control methods and algorithms have been implemented into the operating rolling mills of the OJSC “MMK”. The main advantages of the developed solutions are high efficiency and relative simplicity of implementation/putting into practice. The introduction of the algorithms will improve the production capabilities of the operating and new rolling machinery, contribute to production efficiency improvement, resource and energy saving and make the final product more competitive in the market.

Generalized Mathematical Model of Controlled Linear Oscillating Mechatronic Device

This paper presents the general mathematical model of double-sided splingless linear oscillating mechatronic device – oscillating motor- ompressor which is supplied by thyristor voltage converter. The model presents possibilities to evaluate losses of the windings and losses in the magnetic circuit, nonlinear inductances, mutual inductances, special current feedback using harmonic analysis for the control, oscillations not only of the moving part, but also the “stator” oscillations. Also using this model such parameters can be calculated – oscillation centre drift, oscillating coordinates and velocities of mover and stator, all currents and voltages, electromagnetic force, harmonics of each time-dependent parameter. Ill. 7, bibl. 13 (in English; abstracts in English and Lithuanian).http://dx.doi.org/10.5755/j01.eee.108.2.145