Maged Ibrahim

Design of Variable-Flux Permanent-Magnet Machines Using Alnico Magnets

This paper proposes a novel design for variable-flux machines with Alnico magnets. The proposed design uses tangentially magnetized magnets to achieve high air-gap flux density and to avoid demagnetization by the armature field. Barriers are also inserted in the rotor to limit the armature flux and to allow the machine to utilize both reluctance and magnet torque components. An analytical procedure is first applied to obtain the initial machine design parameters. Then, several modifications are applied to the stator and rotor designs through finite-element analysis (FEA) simulations to improve machine efficiency and torque density. A prototype of the proposed design is built, and the experimental results are in good correlation with the FEA simulations, confirming the validity of the proposed machine design concept.

Core Loss Prediction in Electrical Machine Laminations Considering Skin Effect and Minor Hysteresis Loops

This paper presents a method for the estimation of core losses in electrical machine laminations exposed to high-frequency and nonsinusoidal excitations by using only low-frequency measurements. The developed model takes into account the nonuniform distribution of the magnetic field inside the lamination. Accurate core loss prediction in the presence of minor loops is achieved using the energetic model to calculate the quasi-static hysteresis loops. The results are verified experimentally by comparing to the measured core losses in laminations exposed to the flux waveforms in different sections of an inset permanent magnet machine. The comparison between measured and calculated core losses shows excellent agreement, confirming the validity of the model.

Magnetic Characteristics and Core Losses in Machine Laminations: High-Frequency Loss Prediction From Low-Frequency Measurements

To study the fundamental essence of core losses and to achieve an accurate core loss separation formula, a dynamic finite-element model for the nonlinear hysteresis loop of laminations has been established. In the model, Maxwells equations are solved for the hysteresis character in the magnetic lamination, using the Galerkin finite-element method, where the hysteresis is represented by an energetic hysteresis model. Based on the simulation results, the magnetic characteristics, skin effect, time delay, and magnetic field distribution are discussed. Then, core losses, particularly excess losses, affected by the magnetic characteristics are carefully examined. It is concluded that excess current loss formula is only applicable for the cases where skin effect is negligible and the sum of hysteresis losses and eddy current losses can more generally represent total losses.

Advanced Testing and Modeling of Magnetic Materials Including a New Method of Core Loss Separation for Electrical Machines

This paper presents a new method for the separation of core loss components (hysteresis and eddy current) in laminations exposed to high frequency excitations. Accurate separation of core losses is achieved by calculating the hysteresis losses at each frequency taking into account the non-uniform flux distribution inside the lamination. The results highlight that the assumption of constant hysteresis energy loss per cycle is only valid at low frequencies, where skin effect is negligible. The developed model is then used to study the effect of the annealing process on core loss components in laminations exposed to high frequency excitations. Core loss measurements are performed on different laminations at several frequencies in the range of 20 Hz − 4000 Hz. A comparison of the separated core loss components shows that a huge reduction in the hysteresis loss is achieved by annealing, while the annealing process increases the eddy current loss component at high frequencies and high flux densities. The results are then analyzed by comparing the separated eddy current loss with an analytical eddy current loss model that accounts for the non-uniform distribution of the magnetic field.

Novel equipment for the measurement of core losses in laminations for advanced machines

This paper presents a novel state of the art equipment for core loss measurements under high frequency and non-sinusoidal excitations. Sinusoidal high frequency measurements are performed using a commercial test system especially designed for high frequency testing. Comparative measurements are performed over a wide range of frequencies using the three standardized testers; Epstein frame, toroid tester and single sheet tester. Discrepancies in the results obtained from the three testers are shown and analyzed. A test bench is also developed to carry out core loss measurements under non-sinusoidal excitations. Specific core losses at different parts of a permanent magnet motor are predicted by generating the motor flux waveforms in motor laminations using the Epstein frame. The results show a significant increase in the specific core loss in the stator tooth of the machine.

Design of high torque density variable flux permanent magnet machine using Alnico magnets

The paper proposes a high torque density design for variable flux machines with Alnico magnets. The proposed design uses tangentially magnetized magnets in order to achieve high air gap flux density and to avoid demagnetization by the armature field. Barriers are also inserted in the rotor to limit the armature flux and to allow the machine to utilize both reluctance and magnet torque components. An analytical procedure is first applied to obtain the initial machine design parameters. Then several modifications are applied to the stator and rotor designs through finite element simulations (FEA) in order to improve the machine efficiency and torque density.

Control strategy of a variable flux machine using AlNiCo permanent magnets

This paper is concerned with the development of a vector controller for a variable flux inset permanent magnet synchronous machine (VF-IPMSM). This variable flux machine (VFM) uses low cost aluminum-nickel-cobalt (AlNiCo) permanent magnets (PMs). The demagnetization of the permanent magnets (PMs) is conducted on the stator winding of the machine hence there is no separate winding for the excitation of the PMs. The proposed control method incorporates field weakening for smooth transitions between torque and speed from a higher magnetization state to a lower one. The benefits of the variable flux over a normal insert permanent magnet synchronous machine during field weakening operation are demonstrated. Due to the use of AlNiCo PMs the size of the converter is reduced and hence converter costs reduction.

Advanced testing and modeling of magnetic materials including a new method of core loss separation for electrical machines

This paper presents a new method for the separation of core loss components (hysteresis and eddy current) in laminations exposed to high-frequency excitations. Accurate separation of core losses is achieved by calculating the hysteresis losses at each frequency taking into account the nonuniform flux distribution inside the lamination. The results highlight that the assumption of constant hysteresis energy loss per cycle is only valid at low frequencies, where skin effect is negligible. The developed model is then used to study the effect of the annealing process on core loss components in laminations exposed to high-frequency excitations. Core loss measurements are performed on different laminations at several frequencies in the range of 20-4000 Hz. A comparison of the separated core loss components shows that a huge reduction in the hysteresis losses is achieved by annealing, while the annealing process increases the eddy-current loss component at high frequencies and high flux densities. The results are then analyzed by comparing the separated eddy-current loss with an analytical eddy-current loss model that accounts for the nonuniform distribution of the magnetic field.

The Effect of Two- and Three-Level Inverters on the Core Loss of a Synchronous Reluctance Machine (SynRM)

The paper shows the reduction of core losses by using a three-level inverter over a two-level inverter for the same dc-bus voltage and switching frequency. A synchronous reluctance machine stator-core toroid was used in the analysis. Hence, the analysis is realistic as it accounts for mechanical effects through the stator core, and the distorted supply of the line-to-line inverter voltage supplies. The paper also shows the limitations of using finite element-derived excitation; hence, the toroid was directly supplied by the inverter. The reduction of core losses by use of a three-level inverter is significant at very high flux densities and frequencies; approximately 60% lower core losses. Therefore, it can reduce the cooling burden especially in the hard to cool teeth, increase the service life, and allow increased output. The latter is because the output of the three-level fundamental voltage is higher for the same dc-bus and switching frequency at lower machine losses.

Design of variable flux permanent magnet machine for reduced inverter rating

This paper presents a design procedure for tangentially magnetized variable flux machines that aims to reduce the machine inverter rating. The proposed design can achieve high torque density at low speeds, and high efficiency at an extended speed range, as armature d-axis current pulses are applied at high speeds to reduce the magnet flux. In order to regain the full torque capability when the motor slows down, a magnetizing current pulse has to be applied. The amplitude of the magnetizing current is usually larger than the machine rated current. Therefore, the machine may require an oversized inverter in order to be able to remagnetize the magnets. The impact of different machine design parameters on the magnetization current requirement is investigated in order to reduce the inverter cost. The proposed machine design procedure is validated by experimental measurements on a variable flux machine prototype.