Vol 88, No 4 (2017)

Problems of expanding the range of speed regulation in electric transport vehicles by replacing gear-shift transmission with a two-channel electric drive comprising two electric motors, each coupled with its own reducing gear units possessing different gear ratios. The output shafts of these reducing gear units are coupled to one differential speed-reduction device. To solve this problem, a mathematical model of the electric-motor drive has been developed and the static characteristics of the individual motors and characteristics of the motors with their output shafts driving one differential speed-reduction device have been determined. Steady conditions of the operation of a two-channel electric drive are considered. To describe real changes in the speed variables, the electromechanical transducer is approximated in the mathematical model with nonperiodic first-order links. The functions of the controlling effects, which are formed on the basis of the criterion of fullest utilization of electric machines with respect to their electromagnetic torque in stable conditions and in transient processes, were determined. Recommendations are given on application of the proposed solution to traction mechanisms, which should meet increased requirements to acceleration of electric-motor vehicles.

This article analyzes the existing methods of increasing reliability through multiple overstatement of the installed power of electric equipment according to the criterion of minimum total yearly costs. The proposed method of optimization of power circuits of semiconductor converters includes stages of choosing the configuration of power circuits on the basis of the limit of semiconductor switches and choosing the optimal number of phases and redundant units. The distinguishing characteristic of the method is the criterion used, namely, the probability of failure-free operation. This new method reduces the cost of installed power of a semiconductor converter compared to the known synthetic method on the level of total annual costs. It is shown that this method has some degree of flexibility; e.g., the development of the element base can be considered on the basis of the corresponding coefficients that depend on the operating temperature of power semiconductor valves. The proposed method can be successfully used in the design of semiconductor converters for supplying powerful electric drives, where, in the general case, the number of phases of a power circuit is not three. In this case, multiphase windings of an electric machine are connected according to a scheme with potential phase separation, which can be considered as an equivalent multiple redundancy system.

A survey of methods of calculating losses has been proposed for a class of controlled ac drivers that work under extreme conditions with arbitrary control laws of currents and configurations of magnetic systems for electrical machines. It is shown that the existing methods have low calculation accuracy for the considered class of electric drivers (error up to 30%). The results of calculations by the finite-element method and the experimental data of simulations of thermal conditions and losses using 3D models, in which an electromechanical converter is regarded as a system with distributed parameters, have been considered; moreover, the parallel calculating algorithms have been used. For example, the calculations of losses in the 3D models of a field-regulated reluctance machine have shown that the discrepancy with the experimental data is not higher than 2%. Note that the main share of losses in steel is eddy currents. The results of calculations by numerical methods using 3D models allow one to determine the losses in controlled ac drivers for nonsinusoidal control laws of phase current of the stators with complex configurations of the magnet system of the electromechanical converter. For the considered class of electric drivers, the largest share of losses (from 60 to 80%) is eddycurrent losses.

A list of recommendations to increase the energy efficiency of an electric drive by decreasing the total losses has been given. Three ways in which an energy-saving drive can be developed have been differentiated. The first approach is improvement of conventional solutions. This can be achieved by means of using new high-quality active materials in electric machines and by optimization of their power circuits. The second approach is a systematic approach to designing and taking into account the combined work of all the components of an electric drive. In this case, improvement entails development of new types of electric drives, such as switched reluctance drives, electric drives with a field-regulated reluctance machine, and others. New solutions actively use the advantages of control and power electronics. The third approach provides improvement of both the existing solutions and taking into account the combined work of a semiconductor converter and motor. Using the example of an electric drive with a field-regulated reluctance machine, calculations of total losses have been carried out and ways to decrease losses by means of choosing efficient control laws for stator currents and power transfer circuits have been demonstrated. Recommendations that allow improvement of the electrical safety of technical personnel who service electrical systems have also been suggested.

A circuit design of an ac/dc three-phase converter with an isolated output is proposed. The converter consists of identical phase modules with forward pulse transformers, the primary windings of which are fed with phase voltage through power switches. The secondary windings of the transformers are connected to rectifiers, the outputs of which are connected in series. The energy of magnetizing inductance is also transferred to the load through an auxiliary low-power converter. The presented circuit design provides control of the converter currents that makes it possible to perform an active power-factor correction. The design is characterized by a small number of semiconductor switches in the power circuit due to the use of a direct-conversion principle. In addition, the proposed circuit design makes it possible to use power switches with a reduced permissible voltage. High-frequency energy conversion makes it possible to reduce the mass and dimensions of the device. The converter can be used as a power supply for an electric arc for welding and plasma processes. It is possible to generate output characteristics of any type, from direct-voltage to direct-current regulation. If necessary, it is possible to provide increased open-circuit voltage to simplify electric-arc ignition.

Inverter converters used for dc-arc electric welding have been manufactured by many foreign and domestic companies and showed good results for decades. Until recently, ac welding was carried out only at an industrial frequency of 50 Hz. Earlier, we suggested carrying out arc welding at an increased frequency within the range of 25–75 kHz, proving it to be efficient taking into account welding physical processes. Results of investigations of “slow” electrical processes in a high-frequency inverter for ac welding at different stages of the welding cycle are given in this article. In particular, the transition processes in the inverter are studied in detail taking into account a whole variety of factors. We conducted a complex of investigations concerning the study of the character and duration of the transient processes in an alternative sign welding inverter with increased frequency, as well as the level of influence of the transition coefficient and time constant of the inverter-control system on these processes. The influence of design and parasitic data of the scheme on the nature of the “fast” and “slow” transient processes and changes in the parameters of the electric arc (loading) and parameters of the actuating signals were assessed. All this allowed determining the effect on the stability of the welding current and, therefore, on the welding quality.

Since the extension of voltage source converter (VSC) based high voltage direct current (HVDC) transmission system to multi-terminal direct current (MTDC) system is easy, the application of MTDC system is more attractive than before. But the VSC based HVDC system is more vulnerable to DC line faults due to the high discharge current from the DC link capacitance. In this aspect, a super conducting fault current limiter (SCFCL) is a better choice to reduce the consequences of DC line faults. The main objective of this paper is to improve the dynamic performance of an offshore wind farm connected multi-terminal VSC-HVDC transmission system by incorporating SCFCL at different locations. Here, the SCFCL integrated MTDC system is modelled in MATLAB/Simulink and the results are presented for various AC/DC fault conditions.