Vladan Vučković

FIELD-ORIENTED CONTROL OF INDUCTION MACHINES IN THE PRESENCE OF MAGNETIC SATURATION

ABSTRACT This paper presents derivation of the equations which describe field-oriented control of saturated induction machines. Field-oriented control principles are applied on the saturated model of induction machine and new equations are obtained, which differ significantly from the well-known model valid when the main flux saturation is absent or neglected. Rotor flux oriented control of voltage-fed and current-fed saturated induction machines is discussed. New decoupling circuits which are needed under saturated conditions are presented. Simulation results, which show the influence of main flux saturation on transient behaviour of the drive and which verify the theoretical results are included.

The effects of a river valley on an isolated cumulonimbus cloud development

Abstract The mechanism of development, propagation and front-side cell regeneration of a three-dimensional isolated cumulonimbus (Cb) cloud is investigated by a cloud-resolving mesoscale model. Interactions of the simulated storm with orography including a river valley were studied. According to observational evidence, the mountainous environment of the Western Morava valley (Serbia) is an important place for the formation of isolated Cb clouds. Once formed, they move down into the valley and continue to propagate along it. The effects of orography on the development, propagation and regeneration of the model Cb cloud are recognized by comparison of its development with that simulated over flat terrain under the same conditions. In our study, two cases are considered: complex terrain case (referred to CT case) and flat terrain case (referred to FT case). It is found that: – Orography effects (CT case) play an important role on Cb cloud life; the cloud propagation and development are inhibited in lateral direction and its form is more compact. Cold air outflow near the ground remains in the valley with an increased depth compared to the FT case. Warm environmental air approaching the cold air nose from the opposite direction is forced aloft more frequently than in the FT case. As a consequence, the simulated cloud propagates faster in this case. – Warm environmental air forcing over cold air nose in the FT case is stronger initially than in the CT one, since the cloud development is not prevented by orography from the lateral direction. Consequently, the cold air outflow is more intense. In contrast, the cloud regenerates more slowly, since the cold air diverges in all directions, which in turn, makes the cold air nose thinner. The alternate reform and collapse of the cold air nose are more expressed in the CT case.

On the sensitivity of cloud microphysics under influence of cloud drop size distribution

Abstract The unique Khrgian–Mazin size distribution is suggested as an alternative approximation of the entire drop spectra besides the conventional approximation in which the monodisperse and the Marshall–Palmer size distributions for cloud droplets and raindrops are used, respectively. This approach is employed in bulk microphysics. It is shown that the effects of the changed drop size distribution are particularly pronounced in the case of the microphysical production terms with rain. This is a consequence of the fact that the Khrgian–Mazin size distribution produces more small raindrops and less large ones compared to the Marshall–Palmer one, where both have the same rain water mixing ratio. The new production terms with cloud water do not differ from those with the monodisperse size distribution used. In order to show the sensitivity of cloud microphysics with respect to the change of drop size distribution function, the experiments are perform with the two versions of a forced 1-D time-dependent model. The first one involves the microphysical production terms with the Khrgian–Mazin size distribution, while the other one uses the conventional approach. Experiments clearly show that cloud microphysics essentially depends on cloud drop size distribution. Some microphysical aspects of a model cloud with the new drop size distribution are in fair agreement with observations.

ROTOR FLUX COMPUTATION IN SATURATED FIELD-ORIENTED INDUCTION MACHINES

ABSTRACT The paper discusses effects of main flux saturation in field–oriented induction machines and proposes improved schemes for rotor flux space vector calculation. Field–oriented control with rotor flux computation out of the measured quantities is elaborated. Two rotor flux computer structures are analyzed. The first one is based on stator voltages and currents as measured variables, while the second scheme carries out calculations on the basis of sensed stator currents and rotor speed (position). The sensitivity of the constant parameter rotor flux computers to saturation degree variation is investigated in detail. Modified structures of the computing schemes are then proposed, which account for change in saturation level in the machine. The modified rotor flux calculators are derived from flux state–space model of a saturated induction machine. Accurate computation of rotor flux space vector is enabled in this way, irrespective of the actual saturation level in the machine. Verification of the dev...

The impact of the choice of the entire drop size distribution function on Cumulonimbus characteristics

The study compares two different ways to represent the size distributions of cloud droplets and raindrops in bulk microphysical schemes in the scope of cloud-resolving mesoscale modeling of cumulonimbus clouds. A unified Khrgian-Mazin size distribution for the entire liquid water is systematically compared to a widely applied approach using a Marshall-Palmer size distribution for rain and a fitted monodisperse distribution for cloud droplets. The impact of the distribution function on cloud microphysics, precipitation characteristics, cloud appearance and dynamics of one simulated cumulonimbus case is investigated. The agreement of the model values with typical observed cloud characteristics is discussed. The results of our study reveal that there are considerable differences between the two approaches, both with respect to the microphysical production terms and with respect to the cloud appearance while the differences are less pronounced in cloud dynamics. An important result is that the Khrgian-Mazin size distribution leads to more cumulative rain compared to the one composed of a monodisperse and a Marshall-Palmer distribution. There is observational evidence that a storm splitting, hail field characteristics and cumulative total precipitations are simulated more accurately by the Khrgian-Mazin distribution function.

Magnetizing curve representation methods in digital simulation of induction machine dynamics

Two nonlinear functions are present in the modified saturated induction machine model, namely, magnetizing flux and dynamic inductance. Different interpolation and approximation methods for these functions are considered. The methods are compared with regard to the accuracy of digital simulation results and computer CPU time requirements