Vol 89, No 4 (2018)

An algorithm for optimizing the control action in the torque contours of synchronous reluctance and inductor ac drives is described in this paper. It allows one to search for an optimal principle for electric-drive operation that is suitable for any electromechanical converter with magnetic asymmetry of the rotor. The harmonic composition of phase currents serves as the optimization parameter. The optimization criteria are specific electric and magnetic losses. The functional relations presented by a set of equations describing the mathematical model of an object for optimization are given in analytical and numerical forms. The analytical case implies the optimization calculations at the following assumption: the drive is unsaturated, the magnetic conductivity of steel is infinite, and there are no scattering currents. The optimization data are to be refined via the numerical method, in which the electric drive is to be as a system with distributed parameters. An example of synthesis of the best configuration of phase currents through the torque contour of an electric drive with a synchronous independent-excitation reluctance machine is considered as well. The optimum current shape is established to be a trapezoid, while the time of the control signal variation is determined by the time after which the rotor-pole edge overcomes a distance equal to a phase area of the stator winding. The algorithm for control-action optimization proposed in this work enables one to increase the energy efficiency of synchronous reluctance and inductor ac electric drives.

An analysis of independent control actions in different control structures is performed to compare different methods for improving the specific mass and dimensions of electric drives for objects that carry out point-to-point motion or tracking motion. Algorithms for optimizing the form of phase (control) currents are formulated. The physical modeling method has shown that a rectangular oscillogram of the phase current in a series excitation circuit is most convenient for use in overload zones. It is shown that improved mass and dimensions in the low-power range can be achieved in synchronous reluctance electric drives having a complex rotor design (compound design with nonmagnetically conductive inserts). A multiphase circuit with individual sources in each phase is an appropriate circuit for use in high-power electric drives (more than 500 kW). Improved acceleration performance is achieved without complicating the design of the rotor due to the failure of sinusoidal control laws. In this case, the parameters of the electric drive under consideration are comparable with the parameters of a synchronous reluctance electric drive having a compound (complex) rotor. The form of the phase current is rectangular at the initial stage of adjustment, and the ratio between the excitation current and the armature current is determined depending on the saturation coefficient of the magnetic system.

This paper presents a method for improving energy indicators and increasing system reliability by adopting multilevel schemes of power circuits of semiconducting frequency converters of a voltage class of up to 1 kV. An optimum number of phases was selected at the first stage based on the criterion of the minimum cost to solve the problem. Then, this criterion was supplemented by the limitation of the electric losses and, at the final stage, the problem was solved based on the condition of securing the specified indicator of the failure- free operation. Modern semiconducting converters for the large specified capacities are of modular configuration and each of the frequency converter assemblies (rectifier, invertor) includes, as a rule, some parallel connected modules. In these conditions, an increase in the number of phases at which the phase winding of the engine is made multiphase and the previously parallel-connected power modules of self-commutated inverters are galvanically decoupled and connected separately to each of the engine phases, which allows increasing the reliability indicators of the system without any additional capital costs. It is theoretically shown and experimentally confirmed that, due to the development of the modern element base, the use of three-level frequency converters for a voltage up to 1 kV is economically justified. It is determined that, in addition to the known advantages of such a circuit (reduction in the switching losses and overvoltages on the phase winding of the engine), it is possible to increase additionally the coefficient of efficiency of a frequency converter due to a larger number of the degrees of freedom during the switch over to multiphase circuits.

This paper presents a method of control of the metal-transfer mode during electron-beam surfacing with a filler-wire feed based on simultaneous recording of the potential of the wire and article connected with the apparatus frame through the resistors. The main metal-transfer modes are specified that are observed with a change of the energy-input rate. The importance of the solved problem for additive technologies is shown. A scheme of experimental investigations carried out at the ELA-15I apparatus is given, and the obtained oscillograms are presented. The relation of the characteristics of the registered signals with the operation modes of the equipment and transfer parameters is shown. It is noted that, with a change of only the filler-wire feed rate, a changeover from drop metal transfer to the scattering mode can be triggered. The possibility of detecting the mode of appearance of the shot metal, as well as emergency modes related with stopping the filler-wire feed or excessive increase of its feed rate, is substantiated. The influence of the plasma processes on the registered signals in the developed scheme is analyzed, and it is also shown that, to reduce the influence of these processes, it is necessary to decrease the resistance of the measuring circuit. Photographs of the process corresponding to the certain oscillogram intervals are given.

The problem of simulation of the arc-suppression process in the case of switching off high-current dc circuits by devices with a liquid–metal working medium is considered. Since devices with a liquid–metal contact are designed to switch specific power circuits, the possibility of joint consideration of the electrical circuit and the switch is used to optimize the shutdown process. The obtained mathematical model allows with the use of the known temperature profile of the dynamic arc to calculate the main dynamic characteristics of the arc discharge, such as current, voltage, conductivity, temperature, and the trunk conductor radius. It is shown that, if the real temperature profile of the arc is not known, calculation of such integral characteristics of the arc-suppression process as current, resistance and intensity of the arc trunk can be carried out with the use of an equivalent temperature profile. On the basis of the obtained mathematical model, a calculation methodology is developed for the process of circuitry cutout by an apparatus with liquid–metal contacts with a change in the topological structure of the circuit being switched off during the commutation process.

Electric drives based on traction linear-induction motors are used in industry and city transport. The main advantages of this electric drive are unrestricted acceleration and vehicle climbing angles, as well as ecological cleanliness. The development of high-speed design and analysis software tools for linear electric motors is thus becoming quite timely. The use of the finite-element method for solving field problems in a three-dimensional setting and dynamic vehicle-operation regimes is time-consuming. It is proposed to use a detailed magnetic equivalent circuit of a linear-induction-motor longitudinal section with the introduction of the Bolton coefficient to correct for the transverse-edge effect in a solid secondary element. Model features based on multilayer magnetic equivalent circuits are shown, and the effect of the number of layers on the designed motor-traction force is evaluated. When selecting an appropriate pole pitch-to motor gap ratio, a two-level equivalent circuit turns out to be suitable. The secondary element is represented in the model as a set of rectangular rods connected in parallel. The influence of the calculation method of the secondary rods’ motional EMF on the motor-traction-force accuracy is shown. The greatest error is observed in a low-slip region (when the motor passes into a generative braking regime) and the simplest spatial derivative of the magnetic flux. When the derivative calculation formula becomes more complicated, the indicated error sharply decreases. Another way to reduce the calculation error of the motional EMF in the secondary element circuit is to increase the number of these circuits by decreasing the width of the elementary rod. The prominence of the detailed magnetic equivalent-circuit method and convenience of online viewing should be noted.

The usability of a thyristor frequency converter with a supplementary controlled series compensator for soft starting of induction motors with high torque of inertia of actuating mechanisms is considered as an alternative to transistor converters. It is shown that the supplemental device, in the form of a transistor–capacitor convertor, can be considered as an adjustable capacitor, the equivalent capacitance of which can be varied from very large values (to a maximum of infinitely large) to some fixed minimum value. The voltage of an adjustable series capacitor provides the desired switching conditions of thyristor inverter, and the capacitive reactance ensures the compensation of reactive power consumed by the induction motor and thyristor inverter, reducing the current load of the motor and converter. The addition of a thyristor current converter with a controlled series compensator of reactive power in the form of a transistor–capacitor unit makes it possible to carry out a soft start of induction motors similar to the starting modes of synchronous machines. Simulation results confirming the achievement of the desired effect are presented. It is shown that the supplemental device in the form of transistor–capacitor converter is a robust system well adaptable to various conditions of electric-drive operation. The functional equivalence of a transistor frequency converter and thyristor frequency converter with a supplemental transistor–capacitor unit with a substantially lower cost of the latter makes possible a new mechanism for soft starting of powerful electric drives with induction motors under conditions of high torques of inertia of actuating mechanisms.

The list of author’s affiliations should read as follows:

^aSamara State Technical University, Samara, 443100 Russia

^bNational Research University Moscow Power Engineering Institute, Moscow, 111250 Russia