Lesedi Masisi

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.

The use of entropy to measure structural diversity

In this paper entropy based methods are compared and used to measure structural diversity of an ensemble of 21 classifiers. This measure is mostly applied in ecology, whereby species counts are used as a measure of diversity. The measures used were Shannon entropy, Simpsons and the Berger Parker diversity indexes. As the diversity indexes increased so did the accuracy of the ensemble. An ensemble dominated by classifiers with the same structure produced poor accuracy. Uncertainty rule from information theory was also used to further define diversity. Genetic algorithms were used to find the optimal ensemble by using the diversity indices as the cost function. The method of voting was used to aggregate the decisions.

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.

A Modulation Strategy for a Three-Level Inverter Synchronous Reluctance Motor (SynRM) Drive

In this paper, two modulation strategies called Mod-1 and Mod-2 are compared for a three-level diode neutral point-clamped (NPC) inverter synchronous reluctance machine (SynRM) drive. These modulation strategies do not operate at a fixed switching frequency. A new modulation strategy which operates at a fixed switching frequency is proposed called Mod3. The SynRM suffers from a poor power factor which makes it harder to archive capacitor voltage balancing for a three-level NPC inverter. These modulation strategies are of the nearest three vector (NTV) family. Mod-1 and Mod-3 choose certain vector states to achieve capacitor voltage balancing, whereas Mod-2 strategy makes use of the inverter vector states dwell time. The parameters of interest are the torque ripple, the neutral point (NP) voltage ripple, the inverter efficiency, and the d-axis current ripple. Though lower torque ripple and better power quality are registered on Mod-2 strategy. Mod-1 strategy has 14% improvement in inverter efficiency at lower modulation index (mi = 0.4). Mod-2 strategy has 11% more NP voltage ripple at a modulation index of 0.4. One of the disadvantages of Mod-1 strategy is the varying switching frequency. Mod-3 scheme produced higher inverter efficiency, better balancing of the capacitor voltages, and guaranteed fixed inverter switching frequency. The disadvantage with Mod-3 strategy is that it generated higher torque ripple, daxis current ripple, and current total harmonic distortion when compared to Mod-1 and Mod-2 at the same load conditions.

A Three-Level Neutral-Point-Clamped Inverter Synchronous Reluctance Machine Drive

This paper is concerned with the use of a three-level inverter for a synchronous reluctance machine (SynRM). The SynRM suffers from a poor power factor due to poor machine saliency. This makes it harder to operate the three-level neutral-point-clamped inverter due to a strict requirement to have the two dc-link capacitor voltages balanced. This paper proposes a new nearest three vector modulation algorithm which can balance the two dc-link capacitors even at poor machine saliency ratio (poor power factor). In this paper, a method of sizing the two dc-link capacitors based on the machine power is also proposed. A comparison between two- and three-level inverters was conducted. The SynRM registered lower d-axis current ripple with a three-level inverter, indicating lower core losses in the machine.

Three level NPC inverter DC capacitor sizing for a synchronous reluctance machine drive

This paper introduces the use of machine saliency as one of the parameters for the sizing of a three level inverter dc-link capacitor. A new nearest three vector (NTV) balancing algorithm is also introduced for the dc-link capacitors. A comparison between a two and three level inverter was conducted whereby the inverter input dc current for the two level inverter registered higher ripples than that of a three level inverter. This promises longer lifespan of the battery on a three level inverter driven SynRM. The SynRM also registered lower d-axis current ripple with a three level inverter which promises lower core losses in the machine. The benefits are even more significant at higher switching frequencies.

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.

Performance comparison of a two-level and three-level inverter permanent magnet synchronous machine drives for HEV application

This paper investigates the performance of a two level and a three level inverter drives for a permanent magnet synchronous machine (PMSM) under different switching frequencies. The experiments were conducted via a real time based system called Opal-RT. The inverters were operated at 1 kHz and 5 kHz switching frequencies. The torque was varied between 2 N.m and 4 N.m at a constant speed of 800 rpm. However the torque ripples and the copper losses between the two inverter drives were similar and the copper losses were independant of the swicthing frequency. At 5 kHz inverter switching frequency the two level inverter performed better at 4 N.m with a total current harmonic distortion of 2.55% and a torque ripple of 10%. However overall the three level inverter had low voltage and current harmonic distortion which translates to lower iron losses. Swicthing at 5 kHz frequency showed to have lower overall harmonic distortion than at 1 kHz for both inverters. The greater the capcitor voltage error the more the torque ripples even though the machine registered lower current total harmonic distortion for the three level inverter.

A comparison between A 2-level and 3-level inverter for a permanent magnet synchronous motor drive under different inverter switching frequencies

This paper is concerned with the comparison between two motor drives, one driven from a 2 level inverter and the other from a 3-level inverter. The inverters were operated at different switching frequencies, 5kHz, 10kHz and 20kHz. The motor copper losses and torque ripples were parameters of interest. Field oriented space vector Modulation control was used for speed control. The 3-level inverter promised lower torque ripples and copper losses as compared to the 2-level inverter. A maximum difference of approximately 16.82% and 66.1% lower copper losses and torque ripples respectively were reached at particular switching frequencies and motor speeds. On average the 3-level invereter promised a difference of 30% and 9.57% lower torque ripples and copper losses respectively. The results also showed that, the choice of the inverter in terms of lower torque ripples and lower copper losses do not only depend on the switching frequency but also on the motor speed.

Control Strategy for a Variable Winding Synchronous Reluctance Machine for Traction Applications

This paper presents a control algorithm for a synchronous reluctance machine with different windings. Due to the poor saliency ratio of the SynRM the use of field weakening alone limits the constant power speed range. In this work the machine uses two different windings, one with 18 turns and the other with 25 turns. The constant power speed range of the machine was extended by 76% when the windings were changed from 25 turns to 18 turns. The control algorithm implements field weakening before the windings are changed. When the machine winding configuration is changed constant power speed range can further be extended. Ideally the machine winding would be changed automatically by the use of power switches, however in this paper due to the unavailability of the power switches the machine windings were changed manually.