Z. Azar

Influence of Electric Loading and Magnetic Saturation on Cogging Torque, Back-EMF and Torque Ripple of PM Machines

The torque ripple of a permanent magnet (PM) machine is mainly due to the cogging torque and distortions of the back-emf waveforms, winding inductances and current waveforms. Both the cogging torque and back-emf are normally calculated at open-circuit conditions. However, they are affected by the electric loading and magnetic saturation. This paper investigates the influence of the load conditions on the cogging torque and back-emf waveform by employing a frozen permeability finite element technique. Furthermore, the effectiveness of the rotor skew on the minimization of the cogging torque, thus torque ripple, is also highlighted. It is found that the cogging torque magnitude is significantly increased under load conditions due to more flux leakage through tooth tips. However, the more important issue is that the cogging torque periodicity also changes, thus the rotor skew technique becomes less effective. In addition, the back-emf waveform under load conditions contains more harmonics, which leads to more electromagnetic torque ripple. It is also proven that the cogging torque, back-emf harmonics and consequently the output torque ripple are effectively diminished if the machine is skewed by one actual cogging torque period, i.e., when the electrical loading influence is considered. The analysis results are supported by experimental measurements.

Influence of Additional Air Gaps Between Stator Segments on Cogging Torque of Permanent-Magnet Machines Having Modular Stators

In order to improve the electromagnetic performance and ease the manufacture process, particularly stator winding, it is often preferable to employ a modular stator, e.g., individual stator tooth/back-iron segments, or separate stator tooth segments and back-iron segments. However, due to manufacture limits, there always exhibit additional air gaps between the stator teeth and back-iron segments. In practice, such gaps are likely to be nonuniform due to manufacture tolerances. This paper investigates the influence of uniform and nonuniform additional gaps on the cogging torque of permanent-magnet machines having either unskewed or step-skewed rotors. It is found that the presence of these gaps increases the cogging torque magnitude due to increased flux leakage through the tooth tips, while the uneven gaps can cause a significant increase in both the peak and periodicity of cogging torque waveform, which causes the skew method ineffective, as confirmed by both finite-element analyses and experiments.

Investigation of Torque–Speed Characteristics and Cogging Torque of Fractional-Slot IPM Brushless AC Machines Having Alternate Slot Openings

This paper presents a comparative study of torque-speed characteristics and cogging torque of the fractional-slot interior permanent magnet brushless ac machines having different slot openings, viz. open slot, closed slot, and hybrid slot (sandwiched open and closed slots), and unskewed and step skewed rotors. The influence of such alternate slot openings on the magnetic cross-coupling and cogging torque is particularly emphasized. Furthermore, the repercussion of the manufacturing limitations and tolerances on the cogging torque of machines having these alternate slot openings is also investigated. The magnetic cross-coupling level and the sensitivity of cogging torque to manufacturing limitations and tolerances strongly depend on the slot opening materials. The finite-element analyses are experimentally validated.

Influence of end-effect and cross-coupling on torque-speed characteristics of switched flux permanent magnet machines

Switched flux permanent magnet (SFPM) machines have the synergy of PM machines, including high torque density and high efficiency, and switched reluctance (SR) machines, such as simple and robust rotor structure. Therefore, they are potentially attractive for many applications, ranging from traction to aerospace. Comparing with conventional SFPM machines, multi-tooth SFPM machines have additional advantages of using less magnet material but higher torque capability. This paper investigates the influence of the end-effect and cross-coupling on the torque/power-speed characteristics of a multi-tooth (6–4 stator pole/tooth, 19-rotor pole) SFPM machine. Various models at different cross-coupling levels are considered, viz. full cross-coupling on both PM flux linkage and dq-axis inductances, partial cross-coupling on the PM flux linkage only and without cross-coupling. It is found that the partial cross-coupling model, which is much easier and faster, exhibits almost the same accuracy as the full cross-coupling model. Furthermore, the end-effect causes a large reduction in torque-speed characteristics. However, such reduction is more significant in the flux weakening operation region, as confirmed by the 3D finite element (FE) and experimental results.

Investigation of electromagnetic performance of salient-pole synchronous reluctance machines having different concentrated winding connections

This paper presents a comparative study of the electromagnetic performance of 6-stator/4-rotor pole switched reluctance (SR) machines having AC sinusoidal bipolar excitation which is equivalent to salient-pole synchronous reluctance (SynR) machines having different non-overlapping concentrated winding connections, i.e. asymmetric, symmetrical and hybrid. The output torque of such machines is mainly generated by the self and mutual inductances, which are principally highlighted in this paper. In addition, the average value and quality of output torque and phase and line voltage waveforms are also predicted and compared. During the investigations the influence of the magnetic saturation on the machine performances is particularly examined. In order to confirm and verify the predicted results, a prototype is constructed and tested under the three different winding connections.

Performance Analysis of Synchronous Reluctance Machines Having Nonoverlapping Concentrated Winding and Sinusoidal Bipolar With DC Bias Excitation

This paper describes the operation principle and analyzes the electromagnetic performance of synchronous reluctance (SynR) machines having the structure of switched reluctance machines but excited by an ac sinusoidal current with dc bias, i.e., salient-pole SynR with nonoverlapping concentrated windings and a dc bias excitation. Under such excitation, these machines become similar to permanent magnet (PM) counterparts. The dc bias, which is equivalent to the PM excitation, produces back electromotive force (EMF) in the ac coils as well as cogging torque. Thus, injecting the ac coils by sinusoidal currents results in output torque on the machine shaft. The produced back EMF and cogging torque, and torque ripple magnitudes and periods, as well as the optimal ac current angle and the ac/dc current density ratio, strongly depend on the winding connection configuration. The maximum average torque can be achieved when the ac/dc current density ratio is 2 and the symmetrical winding connection, which results in a lower torque ripple, is employed. A prototype machine is built and tested to confirm the analyses and conclusions.

Comparative study of electromagnetic performance of switched reluctance machines under different excitation techniques

This paper analyses and compares the electromagnetic performance of switched reluctance (SR) machines having three different excitations, i.e. conventional unipolar, AC sinusoidal bipolar with/without DC bias. Under the AC sinusoidal bipolar excitation, the SR machine becomes synchronous reluctant (SynR) machines having concentrated winding. On the other hand, in principle the SR machine under sinusoidal and excitation with DC bias is similar to the switched flux permanent magnet (SFPM) machine. During the finite element investigation, the torque capability and torque-speed characteristics are particularly investigated, whilst the losses and efficiency are also highlighted. In conclusion, the SR machines having AC sinusoidal bipolar excitations with DC bias exhibits relatively smooth output torque, widest torque-speed characteristics, lowest iron loss as well as relatively high efficiency. Furthermore, comparing with the conventional unipolar excitation, both AC sinusoidal bipolar excitations can result in much lower torque ripple percentage and same or even higher average output torque, as well as lower iron loss and higher efficiency. A prototype is constructed and experimental results are presented to verify the predictions and analyses.

Torque speed characteristics of switched flux permanent magnet machines

Purpose – The purpose of this paper is to investigate the influence of end‐effect and cross‐coupling on the torque‐speed characteristics of switched flux permanent magnet (SFPM) machines.Design/methodology/approach – The torque‐speed characteristics are predicted using two different methods. These are direct and indirect finite element methods, at different cross‐coupling levels, namely, full cross‐coupling on both PM flux linkage and dq‐axis inductances, partial cross‐coupling on the PM flux linkage only and without cross‐coupling.Findings – The influence of the cross‐coupling on dq‐axis inductances of the studied machine is relatively small. However, it is more significant on the PM flux linkage. Therefore, the partial cross‐coupling model, which is much easier and faster, exhibits almost the same accuracy as the full cross‐coupling model. Furthermore, the end‐effect causes a large reduction in torque‐speed characteristics. However, such a reduction is more significant in the flux weakening operation re...

Influence of pole number and stator outer diameter on volume, weight, and cost of superconducting generators with iron-cored rotor topology for wind turbines

This paper investigates the influence of pole number and stator outer diameter on the performance of superconducting (SC) generators. The SC generator has an iron-cored rotor topology. First, the generator structure is introduced and the optimization procedure is described. Then, the influence of design parameters on performance, in terms of generator volume, weight, SC wire utilization, active material cost, etc., is presented. Some relationships for the optimal combinations for different performance attributes are established. In addition, the influence of SC material price on the determination of optimal stator outer diameter and pole number is discussed. Finally, the influence of SC coil area per pole on performance is also investigated.

Electromagnetic performance analysis of synchronous reluctance machines having non-overlapping concentrated winding and AC sinusoidal bipolar with DC bias excitation

This paper describes the operation principle and electromagnetic performance of synchronous reluctance (SynR) machines having the structure of the switched reluctance (SR) machines but excited by AC sinusoidal current with DC bias, i.e. salient-pole SynR with non-overlapping concentrated windings and DC bias excitation. Under such excitation, the machine becomes very similar to the permanent magnet (PM) counterparts. The DC bias, which is equivalent to the PM excitation, produces back-emf in the AC coils as well as cogging torque. If the AC coils are injected by 3-phase sinusoidal currents, output torque will be generated on the machine shaft. The produced back-emf, cogging torque magnitude and period, and optimal AC current angle as well as optimal AC/DC current density ratio strongly depend on the way of winding connection. The maximum average torque can be achieved when the AC/DC current density ratio is 2 and symmetrical winding connection, which results in lower torque ripple, is employed. A prototype machine is built and tested to confirm the analyses and conclusions.