The Economists Voice

Faiz, M. Ghaneei, A. Keyhani, A. B. J.

This paper presents the design procedure of a three-phase squirrel-cage induction motor for electric vehicles. The procedure takes into account the requirements imposed by its use in vehicle traction. Also, the design considers the influence of the inverter by current and voltage harmonics. Because most of the requirements are antagonistic, the design procedure uses an objective function to select the optimal design. The objective function is a weighted summation of the motor losses and the motor volume. The objective function is then augmented with the design constraints. The modified Hooke-Jeeves search technique is adapted to solve this constrained nonlinear optimization problem. The influence of the copper weight/iron weight ratio on the motor design is studied and the optimum ratio is chosen in order to improve the performance of the motor.

Faiz, M. B. B. Sharifian, A. Keyhan J.

A significant improvement in motor efficiency is expected when substituting aluminum with copper die-cast rotor in squirrel-cage induction motors due to a reduction in the rotor ohmic losses. Other performance criteria, such as starting torque, power factor, and steady-state current, may also change, and the motor may need to be redesigned to obtain optimal performance. This paper compares the performance of optimally designed squirrel-cage induction motors with aluminum and copper rotor bars with equivalent specifications. The design is optimized using three objective functions.

Benoudjit, A. Guettafi, N. Nait SaÏ A.

This paper deals with the design and performance analysis of a new on-wheel induction motor drive for an experimental electric vehicle. The proposed propulsion system includes two axial flux induction motors, where the rotating part of each motor is fitted into the wheel rim and the stationary part is mounted on the frame structure of the vehicle. Such arrangement eliminates any mechanical transmission, leading to a reduction in the total weight and volume of the whole drive system. The designed motors will be compared with a radial flux induction machine in a short-length rotor topology. In this paper, first the computed machine parameters and motor geometry optimization will be carried out by using a developed design program and by using finite element electromagnetic field analysis. Then, to validate the conceived machines, dynamical performance analysis will be achieved by using SIMULINK toolbox within MATLAB environment. Finally, comparison results will be discussed.

Tsai H.L.

A system with fast torque response is very beneficial in applications where direct torque control (DTC) is highly desirable. The principle of DTC is to control machine torque and flux directly by selecting the switching states of inverter according to the errors of torque and flux; however, the response of direct torque control is slower during start-up and during change in load. Fuzzy control with grey prediction strategy is proposed for the implementation of DTC to improve its slow response. Moreover, it uses less sensors than the conventional method. Experimental implementation of the fuzzy logic controller and the grey predictor is carried out to verify the behavior of the controller. The controller is implemented with a single board computer that uses a TMS320C30 DSP.

Jayabarathi, G. Sadasivam, V. Ramac T.

This paper presents a new approach to include tie line constraints in multiarea economic dispatch problems by using evolutionary programming (EP). The proposed method always finds the global or near global optimum for small and reasonable-sized multiarea economic dispatch (MAED) problems. The inclusion of tie line constraints to MAED does not introduce any complexity in the approach. The applicability and validity of the proposed method is shown by implementing it on three example systems - 2, 4, and 14 areas - and their results are compared with those obtained by classical economic dispatch, network flow programming, and dynamic programming methods, respectively. The results show that the proposed method can serve as a potential tool for solving MAED problems.

Tseng S.C.

This paper reports a new dynamic modeling method for switched reluctance motor (SRM) which can account for neighboring phase coupling effects. The dynamic model is developed using a few sets of finite element method (FEM)-computed phase flux linkage/current/position data under the condition of simultaneous excitation of two neighboring phases. The combination of piecewise Hermite cubic spline and Fourier series techniques is used to represent each FEM-calculated flux linkage data set. The methods to implement and to verify the dynamic model using simulation techniques are presented. The model is ready to be incorporated with various converter topologies and control strategies. It can provide valuable information for motor design evaluation and its controller design evaluation. The modeling method has been implemented using Saber circuit simulator for a 4-phase SRM driven by a split DC-link inverter. Dynamic simulation results have been obtained for two different SRM winding configurations. The significanc...

Joo, Myung-Joong Youn, Hwi-Beom Shi H.

The single-current sensor technique obtaining three phase currents from only a DC-link current sensor has been applied to reduce the cost and increase the reliability for various voltage-fed PWM inverters; however, in this technique there are some practical problems, such as unreliable detection of phase currents andphase shift. A new method that overcomes these problems by employing the estimation scheme with identified system parameters is proposed. The Space Vector PWM (SVPWM) strategy is modified to generate the switching pattern adequate for the proposed method. To obtain a good detection in spite of parameter uncertainties, system parameters used in the estimation method are identified using a recursive least square method (RLSM). Experimental results show that the proposed scheme not only provides a very good detection method for three phase currents without current sensors, but also identifies system parameters very well. This proposed method is very simple, has a small detection error, and gives ...

C. Sahoo, J. X. Xu, S. K. Panda N.

This paper deals with an iterative learning approach for modulating the desired torque profile so as to obtain ripple-free torque in switched reluctance motors. Because of the highly nonlinear relation between torque, current, and rotor position for this motor, it is not possible to obtain a closed-form mathematical expression for current as a function of torque and rotor position. Thus, the current waveforms are conventionally computed by using the linear torque model of the motor, and it is well known that such a scheme results in high torque ripple. In this paper, a novel method is proposed to minimize the ripple. In this new scheme, the current is still computed using the linear torque model, but the value of the torque used for this is not the desired (specified) torque, but rather a modulated-desired torque that is obtained by repeated corrections to the desired torque from iteration to iteration. The conventional rectangular pulse profile is taken as the initial current waveform. The method require...

Song K.X.

An effective optimal spinning reserve allocation (OSRA) method is proposed in this paper using Optimal Power Flow (OPF). It enables optimal allocation of spinning reserve and load curtailment incorporating full AC network constraints and dynamic restriction on generation such as ramp-rate constraints. A Primal-Dual Interior Point (PDIP) method, which can efficiently handle both equality constraints and inequality constraints, is employed to solve the formulated dynamic OPF problem. In this model, spinning reserve and load curtailment constraints impose an interdependency between the generation output of units which usually are separable in conventional Newton OPF. A decomposition algorithm is therefore derived to handle the interdependency so that the constraint matrix of generation and that of network can be handled separately by slightly changing the entries of Hessian matrix. Therefore, the proposed method is not only still able to utilize the elegant super sparsity technique of Newton method, but also...

Sachchidanand B.D.

In this paper, it is shown that the application of a dynamic brake or thyristorcontrolled braking resistor (TCBR) at the generator terminals enhances the power transfer limit over a transmission line greatly. Two types of dynamic brake configuration have been considered here: a 3-phase, bidirectional, fullwave, Y-connected, phase controlled conventional brake and a 3-phase, fullwave, thyristor-controlled rectifier bridge. A simple rule-based '' ON-OFF'' control law based on the local measurement of generator output power and its derivative is proposed in this paper. Detail digital simulation studies using the PSCAD/EMTDC package have been carried out. It was found that with the insertion of the dynamic brake circuit and its associated control, the system is able to recover following 3-line-to-ground faults even at very heavy power transfer levels, which is not possible otherwise.

M. Harb, L. Mili, A. H. Nayfeh, C-M A.

A bifurcation analysis is performed to investigate the influence of iron saturation on the complex dynamics of a heavily loaded single-machine-infinitebusbar power system modeling the characteristics of the CHOLLA# 4 generator with respect to the SOWARO station. The system has five mechanical and two electrical modes. The results show that as the compensation level increases, the operating condition loses stability via a supercritical Hopf bifurcation. As a result, the power system oscillates subsynchronously with a small limit-cycle attractor. The instability is due to the interaction of the subsynchronous electrical mode with one of the torsional modes. When the machine saturation is considered, again the system loses stability via a supercritical Hopf bifurcation; however, it occurs at a slightly lower level of compensation, indicating that the influence of machine saturation is slightly destabilizing.

K. Hota, R. Chakrabarti, P. K. Chat P.

A novel multiobjective optimization method for economic emission load dispatch (EELD) with nonsmooth fuel cost and emission level functions in power system operation andscheduling phases is presented. In this paper, the problem treats economy, emission, and transmission line security as competing objectives. The load constraints and operating constraints are taken into account. Assuming that the decision maker (DM) has goals for each of the objective functions, the multiobjective problem is converted into a single-objective optimization by the goal-attainment method, which is then handled by the simulated annealing (SA) technique. The solution methodology can offer a global or near-global noninferior (best compromising) solution in a sense close to the DM's requirements. Results for a sample test system have been presented to demonstrate the applicability and validity of the proposed method.

B. Kliman, R. A. Koegl, W. J. Preme G.

The technique of calculating the air gap torque of an induction motor utilizing only the terminal voltages and currents has been known for some time; however implementation of this technique in a commercial instrument that must compete in accuracy with electromechanical instruments raises a number of issues that must be resolved. Because the derivation of the theorem relies on assumptions of symmetry in the motor, questions have been raised as to its accuracy in real machines. Also, the effects of saturation and core losses usually are ignored but are always present, especially in '' commodity''-type motors. There also are issues associated with the way data are processed, especially in the presence of strong harmonics. Finally, there is the question of how the system can be put into the field using only the information available to the user on the motor name plate. To clarify these issues, an experiment was set up using a PC equipped with standard A/D boards and a common 3/4 HP induction motor. The algor...

R. G. Al-Shakarchi, H. D. H. Al-Has M.

To supply the required load, most electric power systems contain a combination of hydro and thermal power plants. The aim of this work is to study the optimal short-term operation of this combination, where the objective function is to minimize the thermal fuel cost and at the same time satisfy the hydro and thermal constraints, such as power balance at every hour, available water resources, reservoir volumes, and generation outputs limits. Two different techniques are used to solve the problem, namely dynamic programming and coordination equations. The mathematical algorithms for the solution of the problem were developed and the results of some computer studies on selected sample systems were obtained. The results show that using coordination equations method leads to lower total cumulative cost than using the dynamic programming method; however, many of the realistic system constraints cannot be properly handled by the coordination equations method.

Lakervi S.C.

This article presents the evaluation of transformer overload capabilities based on variables that affect the functional life of the power and distribution transformers. The combined effect of thermal overloads, mechanical stresses due to transient overvoltage, and other parameters relating to components other than windings must be taken into account. The basic criterion, which limits the transformer load capabilities, is the temperature of the winding and insulation. Several studies to measure the quantitative loss of transformer life due to the effect of thermal aging have been carried out since 1930. Loading beyond nameplate rating and cumulative loss of transformer functional life have been the basic considerations in the well-practiced IEC 354, 1981 ANSI C57.91, and NEMA Guides for loading oil immersed power and distribution transformers with 65°C average winding temperature rise.