Beatriz Sanchez

Approximate throughput computation of stochastic marked graphs

Abstract Stochastic marked graphs are a concurrent decision free formalism provided with a powerful synchronization mechanism generalizing conventional fork/join queueing networks. Their exact analysis suffers from the classical state explosion problem. Embedded in the divide and conquer paradigm, this paper introduces a new technique for an iterative analysis. The basic idea leads to the notion of a cut to split the original net system into two parts. On a structural level, we compute the workload of the subnets and preserve their marking behavior. For this purpose the results from qualitative theory of marked graphs are taken into account. From the stochastic perspective, the throughput computation on aggregated subsystems uses a response time approximation. Experimental results on several examples usually have an error of less than 3–5%. The state space is usually reduced by more than one or two orders of magnitude; therefore the analysis of otherwise intractable systems is possible. The generalization of the idea leads to multiple cuts, where single cuts can be applied recursively leading to a hierarchical decomposition (i.e., cuts are at different levels).

Use of an AC self-excited switched reluctance generator as a battery charger

We analyzed an AC self-excited switched reluctance generator used as a battery charger, using a second- order nonlinear oscillator model. Generator capacity can be maintained under variable rotor speed by adapting the external capacitance. The AC voltage generated is rectified and adapted with a DC-DC converter to battery voltage. To keep the flux machine under control, the rectified bus voltage should be changed according to oscillating frequency, following a constant voltage/frequency ratio. The simplicity of the system makes it an adequate wind generator for battery charging in isolated locations.

Throughput lower bounds for Markovian Petri nets: transformation techniques

Addresses the computation of lower bounds for the steady-state throughput of stochastic Petri nets with immediate and exponentially distributed service times of transitions. The authors try to deeply bridge stochastic Petri net theory to untimed Petri net and queueing networks theories. Previous results for general service time distributions are improved for the case of Markovian nets by considering some pessimistic transformation rules operating locally on the net structure, its initial marking, and stochastic interpretation. Special interest have the obtained results for the case of live and bounded free choice nets and live marked graphs systems.<<ETX>>

Steady-state behaviour of an ac autonomous switched reluctance generator

The switched reluctance motor can be used in generation mode by selecting the firing angles of the phases. This implies the use of the asymmetrical half bridge Miller converter and position sensors or position estimators. In this paper an alternative current (AC) switched reluctance generator is studied. Generation is obtained by doing resonate each phase inductance with an external AC capacitor. Although the machine only generates periodically in the negative slope of the inductance profile, the set formed with the machine and a resonating capacitor behaves like an AC generator. The energy periodically generated in the negative slope is stored in the capacitor smoothing the pulsed generating process and spread on an AC cycle. So, the set generates when the inductance profile is either positive or negative. Further, position sensors are not used, the magnetic circuit is used in two quadrants and the current profile does not have switching harmonics. The system simplicity suits to battery charge in isolated locations.

Simulation Model of an AC Autonomous Switched Reluctance Generator

Switched Reluctance Machine (SRM) has a nonlinear behavior which difficults its understanding. The electric model shows this nonlinear behavior and it is difficult to derive conclusions but for the linear case. Simulators appear as the ideal tools for studying the machine. Although some papers report simulation models for this machine working in motoring and generating modes, the generating mode is only used for braking but not for generating electricity. In this paper the machine is used for generation purpose and the generated voltage and current are alternative instead of DC as usual. This difference enforces relevant changes demanding a new simulation model. The model has been implemented in Matlab/Simulink and allows analysis of linear and nonlinear cases. It has been used to study the working zones and its steady and transitory state behavior.

Influence of Coil Arrangement and Current Polarity on the Output Torque of a Switched Reluctance Motor

The torque generated by a switched reluctance motor (SRM) is bound to the variation of the flux linkage by each of its phases. Consequently, the arrangement of the coils in the stator and the polarity by which they are excited determines the characteristics of the output torque. A systematic set of 3D-FE simulations and experimental tests demonstrated that the torque is dependent on the number of coils that constitute a phase, its position, and the polarity of its excitation. Those results were confirmed by experimental tests performed on a prototype 24/18-type motor, which was designed for use as a direct drive connected to the shaft of a domestic washing machine

Dynamic behavior analysis of a switched reluctance motor

Within the home appliances sector, the series motor practically shares most applications. However, no further progress in cost reduction or control performances is envisaged. But, the switched reluctance motor, SRM, shows fairly good characteristics such as robustness, easy assembly, and low manufacturing costs. Besides, it allows accomplishment of integrated designs with the product, and an improvement of efficiency up to 90% when compared to about 45% reached by series motors used for home appliances. However, two specific drawbacks of this machine are control complexity and acoustic noise. As a first step, with respect to the acoustic noise, the target of this work is to develop a theoretical modal model of an SRM of four phases with 24 poles in an external stator and 18 poles rotor placed inside. This requires a theoretical simulation by means of an FEM software and an experimental work in a test bench to analyze the motor structure. The comparison of the results obtained by both methods will validate...

Sound power emission of a washing‐machine cabinet

The noise emitted by a washing machine can be divided into airborne noise produced mainly by the electric motor, and structural‐borne noise caused by the cabinet vibration. The aim of this work is to determine the contribution of the structural radiation of the cabinet to the total sound power emitted by a washing machine, and to know which element is the noisiest. The total sound power was measured according to ISO 3744, and in order to know which panel of the cabinet is the noisiest, the standard ISO TR 7849 was applied. This experimental procedure relates the noise radiated by a structure with its vibration velocity, and it was found that the front and lateral left panels were the most relevant. Once the noise transmission paths were characterized, the proposal to reduce noise was focused on reducing the excitation force transmitted by the shock absorbers. As the modification of the force applied by the shock absorber is initially not feasible because it would imply changing the dynamic of the oscillat...