Volume 87, Nº 12 (2016)

The disadvantages of three-phase asynchronous motors with short-circuited rotors, which are widely used in practice, are discussed. It is determined that the most significant disadvantage is consumption of two types of electric energy from a network: active energy for conversion into mechanical energy accompanied by inevitable heat losses and reactive energy that is not converted into other types of energy, but is expended on generation of the magnetic field required for electromechanical conversion of energy in an electrical machine. It is suggested to use internal capacitive compensation of reactive power to improve the technical and economic indices of asynchronous machines of both motors and generators. In addition, each phase winding of the asynchronous machine stator is divided into two parts equal in number of turns spatially shifted between each other in the core slots by an angle of 30°. These parts are connected according to the scheme of the rotary autotransformer to the electrical capacity at its output. The change of the spatial position of the stator windings leads to a change in time of the action of the phase angles of their electromotive force and, therefore, of currents and voltages without changing the introduced reactive impedances. Thus, the spatial coordinate of the electrical value is converted into the time coordinate in the electromagnetic circuit with a rotating magnetic field. The combination of this property of the stator windings in accordance with the rotary autotransformer with the action of the electrical capacity at its output creates the effect of internal capacitive compensation of reactive power in an asynchronous machine. The ability to use internal capacitive compensation of reactive power in single-phase asynchronous machines is considered. A method of calculation of the characteristics of compensated asynchronous machines taking into account the change of the magnetizing contour resistance is presented. The advantages of compensated asynchronous motors and compensated asynchronous generators are determined. Recommendations as to their effective practical use are given.

In a correctly designed magnetic reducer, the electromagnetic moments depend on the torque angles, with the dependence being near-sinusoidal. If the active zones have a nonoptimal geometry, the electromagnetic moments have high-frequency oscillations caused by the reaction moments, resulting in discrete locking or sticking of the fixed rotors. Under unfavorable conditions, the amplitudes of the pulsations may be comparable with the maximum or tilting electromagnetic moment value. The electromagnetic moments of a magnetoelectric gear reducer (MER) are calculated by the energy method based on trigonometric-series expansion of the specific magnetic conductance of the air gaps and the MMF of the magnets. For this purpose, integral expressions are found for the magnetic field energies in the air gaps which depend on the rotational positions of the rotors with respect to the fixed stator magnets. For arbitrary selected harmonics of the magnetic conductance of the air gaps and the magnets’ MMF, the conditions are determined for the orders of the above harmonics and the number of the cogs and poles at which the integrals for the magnetic field energy will be different from zero. Such an approach allowed deriving equations for the basic electromagnetic moments and the concomitant stray moments. If the found conditions are not met, the extraneous harmonics of the electromagnetic moments are eliminated. Selection of a number of modulator rods equal to a prime number is a radical means of eliminating higher harmonics of the electromagnetic moments. If a prime number of the rods cannot be selected, such numbers of the stator pole pairs should be selected at which the presence of higher harmonics of the electromagnetic moments is possible at the highest order values of the harmonics of the magnetic conductance of the air gaps and the MMF of the stator and rotor magnets.

The energy properties of an alternating-current drive are analyzed. The suggested method allows us to compare in an analytical form the energy quality indices of various drives under different means of electromagnetic- torque formation in an electrical machine. Using the relation between power and energy characteristics as functions of the energy state, the energy efficiency, and intensity of energy-conversion processes, as well as the usage efficiency of voltage and power applied to motor windings, are estimated. By means of the constructed relations between quality criteria, the developed method allows one to determine at the stage of design the main properties of an electric drive under static and dynamic conditions. Using energy-state functions and arguments based on means of electromagnetic-torque formation, the method allows us to determine the thermal state at selection or testing of an electrical machine on the basis of heating or overload conditions.

Results of theoretical and experimental studies of an asynchronized converter-fed motor (ACFM) are presented. A proposed version of a converter-fed motor is designed on the basis of an asynchronous motor with a phase-wound rotor based on a voltage inverter with IGBT modules as power switches. A signal from the current sensor to the microintegrated control system further shifts the driving pulse toward an ordered constant angle. These modifications improve energy performance and increase the switching capacity of the inverter by adjusting the delay angle of the power modules. An ACFM model has been developed on the basis of presented equations. It is useful to obtain the ACFM’s stator and rotor transfer function and to determine the controlled variables and control and disturbance influences. The proposed mathematical model makes it possible to investigate transient processes in an electric machine during starting up to speed, as well as under changes in shaft load.