Efficiency and time-optimal control of fuel cell - compressor - electrical drive systems

The proton exchange membrane fuel cell (PEMFC) based power generation sys- tem is regarded as one of the perspective energy supply solutions for a wide variety of applications including distributed power plants and transport. The main compo- nent of the FC system is the FC stack, where the process of electrochemical energy conversion takes place. Additionally, such systems usually contain an auxiliary compression subsystem which supplies the reactant gases to the FC stack as well as maintains certain operation conditions: pressure, temperature, humidity, etc. The proper operation of the compression system signi¯cantly improves the performance characteristics of the total system. On the other hand, it consumes a portion of the electrical energy produced, thus reducing the net e±ciency of the total system. This thesis focuses on an innovative way to improve both the energy e±ciency and the response characteristics of a power generation system with a PEMFC. The approach principally consists of the control of the air compressor powered by the electrical drive. This method could be considered as an alternative to a redesign of the complete system (changing the power level, using an extra energy bu®er, etc). The modern high-speed centrifugal compressor has been regarded as one of the best candidates for the FC system. It has appropriate characteristics with respect to e±ciency, reliability, compact design, etc. However, the presence of a stability margin or so-called "surge line" limits its operation area. With the aim to overcome this constraint, a novel active surge suppression approach has been proposed for application in the system. This control method relies on the high-performance speed control of the electrical drive and accurate measurement and estimation of the thermodynamic quantities, such as air pressure and mass °ow. The choice of an induction motor drive has been justi¯ed by its commonly known advantages: low cost, simple construction, high reliability, etc. These features be- come especially important in high-speed applications. For the detailed investigation and performance prediction of the prime mover, a global electromagnetic design pro- cedure with thermal analysis of a high-speed induction motor has been performed. The obtained analytical results have been veri¯ed numerically by a high-precision Finite Elements Method. A good agreement between the analytical and FEM simu- lation results has been achieved. The mentioned active surge control in combination with the high-performance ¯eld-oriented control of the induction motor has been im- plemented and tested. The test bench comprises the centrifugal compressor with the PVC piping system, the high-speed induction motor drive, the real-time data acquisition and the control system. The experimental results proved the e®ective- ness of the active surge suppression by means of the drive torque actuation: the operation point of the compressor can be moved beyond the surge line while the process remains stable. Using the combined mathematical models of the FC stack, the centrifugal com- pressor and the ¯eld-oriented controlled induction motor drive, the static and dy- namic behavior of the total system have been simulated, allowing to clarify the interaction between the electrochemical processes in the FC stack, the thermody- namic processes in the compression system and the electromechanical performance of the drive. Various system operating regimes have been proposed and analyzed. When the FC electrical load changes frequently and fast, the constant-speed operating regime can be used. In case of a slow variation of the FC electrical load, the variable- speed operating regime is advisable, providing a high energy e±ciency at low FC load. In intermediate cases, the load-following-mass °ow operating regime with the application of the active surge control of the compressor becomes preferable. This operating regime eliminates the relatively long mechanical transient process, keep- ing the energy consumption of the balance of plant (BoP) approximately linearly proportional to the main load. The operating regime with applied linear quadratic Gaussian (LQG) time-optimal control has been proposed as an alternative to the load-following-mass °ow operating regime and the variable-speed operating regime. The transition between two steady-state operating points, where the system e±- ciency is maximum, follows the time-optimal trajectory, keeping the transient re- sponse time small. Finally, recommendations for further research have been formulated concerning the dynamic response and energy-e±ciency of a fuel cell system. Mainly, the recom- mendations concern further improvements of presented control strategies and their more comprehensive experimental veri¯cation using a complete FC system. First of all, the use of a direct induction motor drive for the compressor stabiliza- tion would signi¯cantly improve the e®ectiveness of the surge control. It would allow to control the surge of higher frequency, or to stabilize the compressor operation at larger distance from the surge line. Second, a combination of the electrical drive torque control with a valve position control would result probably in a more e®ective surge control, together with fast transients of the system operating point. Third, the application of the electrical drive for the compressor active surge control in a FC system would require new control algorithms for energy-e±ciency improvement of the induction motor, not compromising its high-performance capa- bilities

Observer based identification of unbalanced magnetic pull in asynchronous motors

This paper presents an iterative approach to identification of sources and characteristics of rotor and airgap eccentricity and, consequently, unbalanced magnetic pull (UMP) in asynchronous motors. The presented method is based on the number of mathematical modeling techniques, namely: finite elements method (FEM), Lagrangian dynamics and optimal estimation in combination with experimental measurements. © 2010 IEEE

A comparative study between inner and outer rotor variable flux reluctance machine topologies for heavy-duty electric vehicles

This article presents a comprehensive comparative study between two variable flux reluctance machine topologies with inner and outer rotors. Electrical and geometrical parameters of both topologies are optimized for maximum torque density, minimum torque ripple, and maximum efficiency to be used in a heavy-duty electric vehicle which requires 500 Nm electromagnetic torque at 1200 rpm base speed. Then, both optimal designs are compared using the 2-D and 3-D finite element and analytical models. The 2-D electromagnetic finite element model coupled with a 3-D analytical thermal model shows that the outer rotor VFRM is able to reach higher torque density with 21 Nm/L than inner rotor topology when the water cooling is also included in the volume of the electrical machine. Although the optimal variable flux reluctance machine with the inner rotor exhibits slightly lower efficiency and higher torque ripple than the optimal design with the outer rotor, it is possible to apply higher current densities with the outer rotor topology due to its improved thermal characteristics. However, the 3-D mechanical finite element model coupled with the electromagnetic model shows that the maximum von Mises stress and deformation in the electrical steel is more than one order of magnitude higher in the outer rotor topology than the inner rotor topology

Comparative analysis of the switched reluctance motor as an alternative to the permanent magnet motor for automotive applications

This paper presents comparative analysis of several configurations of the switched reluctance motor (SRM) for In-wheel drive of the automotive series hybrid system. The SRM motor is regarded as the primary candidate for possible replacement of the permanent magnet (PM) motor. The analysis is performed using magnetostatic FEM and transient modelling techniques

High-speed slotless permanent magnet machines: modelling and design frameworks

This paper presents a design framework for high-speed slotless permanent magnet machines based on extended harmonic modeling (HM) technique to predict various electromagnetic properties and torque distribution. The developed models for generic design framework are able to evaluate slotless PM machines' topologies with a wide range of 3D slotless windings, (including those with skewing), and can be also used for future design optimization routines

Comparative analysis of the switched reluctance motor as an alternative to the permanent magnet motor for automotive applications

This paper presents comparative analysis of several configurations of the switched reluctance motor (SRM) for In-wheel drive of the automotive series hybrid system. The SRM motor is regarded as the primary candidate for possible replacement of the permanent magnet (PM) motor. The analysis is performed using magnetostatic FEM and transient modelling techniques

Comparison of two anisotropic layer models applied to induction motors

A general description of the Anisotropic Layer Theory, derived in the polar coordinate system, and applied to the analysis of squirrel-cage induction motors (IMs), is presented. The theory considers non-conductive layers, layer with predefined current density and layers with induced current density. The electromagnetic field equations are solved by means of Fourier analysis. Further, we propose two different magnetic models for IMs, namely the Direct Rotor Current (DRC) model and the Indirect Rotor Current (IRC) model. The magnetic models are coupled to the single phase equivalent circuit by means of an iterative algorithm, which also accounts for saturation of the main flux path. Finally, motor parameters are given for a benchmark motor and the DRC model and IRC model are validated against Finite Element Analysis predictions. Comparison of the validation results shows that the IRC model is the most promising one

Electrical and magnetic model coupling of permanent magnet machines based on the harmonic analysis

A widely used method for the magnetic field calculation in permanent magnet (PM) machines is the harmonic modeling (HM) method. Despite its many advantages, the application of this method is limited to machines in which armature currents, as a source of the magnetic field, are known. Since most of PM machines are supplied with three phase voltages, any variation of parameters in the armature electric circuit could lead to the uncertainty in knowing the armature currents and therefore limit use of the harmonic modeling method. In this paper an approach for overcoming this limitation is presented which enables the calculation of the magnetic field in PM machines without a priori knowledge of the armature currents

Electrical and magnetic model coupling of permanent magnet machines based on harmonic analysis

This paper presents a new hybrid analytical approach for obtaining a complete solution of the coupled electro-magnetic model for permanent magnet machines. The main advantage of this approach in comparison with existing methods is the use of the voltage as a main input for the modeling. This allows analysis of PM machines taking into account the influence of voltage harmonics from power electronic converters