M. L. Mohd Jamil

Influence of Slot and Pole Number Combinations on Unbalanced Magnetic Force in PM Machines With Diametrically Asymmetric Windings

Unbalanced magnetic forces (UMFs) exist in permanent magnet machines with diametrically asymmetric windings, e.g., machines having pole and slot numbers differed by one and two, even when there is no rotor eccentricity. In machines having pole and slot numbers differed by two, the on-load UMF results only when the single-layer winding is employed and the number of coils is odd. This paper investigates the influence of slot and pole number combinations on the UMF by using an analytical method based on the finite-element predicted airgap flux densities. A UMF may be divided into two components due to the radial and circumferential traveling stresses. It is found that they are additive for machines having pole number 2p = 3k - 1, e.g., 8-pole/9-slot and 8-pole/6-slot, but partially canceling for machines having 2p = 3k + 1, e.g., 16-pole/15-slot and 16-pole/18-slot. Furthermore, due to the additive and canceling effects, for the same slot number, the machine having a smaller pole number exhibits a larger UMF when the slot/pole numbers differ by one, whereas the machine having a larger pole number exhibits a larger UMF when the slot/pole numbers differ by two and the slot number is higher than 6. For the same pole number, machines having slot/pole numbers differed by two exhibit a smaller UMF than machines having slot/pole numbers differed by one. It is also found that, if 2p = 3k + 1, the UMF gradually diminishes to 0 with the increase in pole number. On the other hand, when 2p = 3k - 1 , it reduces to a certain value with the increase in pole number. The experimental results are provided to verify the analysis.

Distortion of Back-EMF and Torque of PM Brushless Machines Due to Eccentricity

The paper investigates the influence of slot/pole number combination and winding configuration on the electromagnetic performance, such as flux density, back-EMF and electromagnetic torque, of permanent magnet machines having static or rotating rotor eccentricities. It shows that the eccentricity does not affect the back-EMF and electromagnetic torque of rotational symmetrical machines whose configuration repeats every certain amount of angle. However, it significantly distorts the back-EMF and torque waveforms of rotational asymmetric machines. Further, the static eccentricity does not change the harmonic contents of back-EMF, but results in unbalanced three phase back-EMF. Hence, the temporal order of the main torque ripple due to the static eccentricity is equal to the pole number. On the other hand, the rotating eccentricity results in more back-EMF harmonics of orders equal to the PM magnetisation order plus/minus one. Therefore, the temporal order of the main torque ripple due to the rotating eccentricity is equal to the triplen number closest to the pole number. The prediction is validated by the experiment.

Influence of pole and slot number combinations on cogging torque in permanent magnet machines with static and rotating eccentricities

With aid of finite element and analytical methods, this paper investigates the influence of static/rotating eccentricities on the cogging torque of permanent magnet machines having various pole/slot number combinations (2p/N_s), including fractional-slot topologies, such as 2p=N_s±1, 2p=N_s±2 and 2p/N_s=8/12, as well as an integer-slot machine 2p/N_s=4/12. It is found that the influence of eccentricity reduces significantly with the number of structure cyclic symmetry, which is equal to the greatest common divisor of slot and pole numbers. Hence, the eccentricity shows the largest influence in machines having 2p=N_s±1, relatively large influence in machines having 2p=N_s±2, but very small influence in the other machines. As for machines having 2p=N_s±1 and 2p=N_s±2, the static and rotating eccentricities result in (2p)th and (N_s)th order cogging torque components, respectively. Both static and rotating eccentricities produce similar cogging torque magnitude. The experimental results validate the FE and analytical investigation.

High efficiency of switching strategy utilizing cascaded H-bridge multilevel inverter for high-performance DTC of induction machine

This paper presents about the control switching strategies of switching frequency by using the multilevel inverter. In previous topology inverter, 2-level inverter is commonly implemented by using direct torque control technique. However, this inverter has some major problem issues which occur due to the limitation of capability the semiconductor device. Basically, there exist two problems; Firstly, high torque ripple and secondly variable switching frequency. Both problems will affect the performance in controlling the AC drive. Besides that, the 2-level inverter is not capable to operate in high power application. Over the past decades, the multilevel topology was constructed to solve the problem in high application. In general, there are three types of multilevel inverters proposed, Firstly, Cascaded H-bridge Multilevel Inverter, secondly, Flying diode multilevel inverter and lastly, Neutral-Clamped multilevel inverter. Thus, in this paper, the Cascaded H-Bridge Multilevel Inverter (CHMI) is chosen to improve the switching frequency as well as a enhance response for motor drive. An algorithm is introduced to improve the torque ripple by using information of flux status.

Influence of slot and pole number combinations on unbalanced magnetic force in permanent magnet machines

Unbalanced magnetic forces (UMF) exist in permanent magnet machines having pole and slot numbers differed by one and two, even when there is no rotor eccentricity. In machines having pole and slot numbers differed by two, the on-load UMF results only when the single layer winding is employed and the number of coils is odd. This paper investigates the influence of slot and pole number combinations on the UMF. UMF may be divided into two components due to the radial and tangential traveling force waves. It is found that they are additive for machines having a pole pair number p=3k+1, e.g. 8-pole/9-slot and 8-pole/6-slot, but cancelling for machines having p=3k−1, e.g. 16-pole/15-slot and 16-pole/18-slot. Further, due to the additive and cancelling effects, for the same slot number, the machine having a smaller pole number exhibits larger UMF when the slot/pole numbers differ by one, while the machine having a larger pole number exhibits larger UMF when the slot/pole numbers differ by two and the slot number is higher than 6. For the same pole number, machines having slot/pole numbers differed by two exhibit smaller UMF than machines having slot/pole numbers differed by one. It is also found that if p=3k−1, the UMF gradually diminishes to 0 with the increase of pole number. On the other hand, when p=3k+1, it reduces to a certain value with the increase of pole number. The experimental results verify the predictions.

A dual mode DTC of open-end winding induction motor drive with reduced torque ripple

In this paper, a new control strategy for Direct Torque Control (DTC) of dual two-level VSI fed 3-phase induction motor drive in an open end winding configuration is presented. The drive can be operated in dual mode; open-end winding or conventional single-sided supply configuration; based on the condition of the motor speed. A block of Optimum Status Detection & Mode Selector (OSD & MS) is introduced to the original structure of the DTC in order to analyze the motor speed condition and also to monitor the torque operating condition. Based on the information from the block, a new lookup table is devised for selecting of the most optimum voltage vectors. Since the open-end winding providing greater numbers of voltage vector, its have the capability to minimize torque ripple. A simulation has been conducted using MATLAB/Simulink in order to achieve the results.

Unbalanced magnetic force in permanent magnet machines having asymmetric windings and static/ rotating eccentricities

This paper presents finite element investigation and experimental validation on the characteristics of unbalanced magnetic force (UMF) with static/rotating rotor eccentricities in permanent magnet machines having asymmetric windings. It shows that the magnitude of UMF due to eccentricity increases with the pole number, p. The open-circuit UMF varies linearly with the eccentricity ratio. The static/rotating eccentricities result in similar average UMF. However, the harmonic contents of either x/y components or magnitude of the UMF due to static eccentricity as well as the x/y components of the UMF due to rotating eccentricity are the constant, and the (4p)th, the (8p)th, ...., whilst the harmonic contents of the magnitude of the UMF due to rotating eccentricity are the constant, and the 3rd, the 6th, ..., of which the constant and the triplen one close to the pole number are the main components. The experiments on two 9-slot machines validate the prediction.

Electromagnetic performance of interior permanent magnet machines with eccentricity

This paper comparatively investigates the electromagnetic performance, such as airgap flux density, flux linkage, radial stress, back-EMF and output torque of two rotational symmetrical and asymmetric interior permanent magnet BLDC and BLAC machines having static/rotating eccentricities by FE model and a simple analytical model. It shows that the static/rotating eccentricities hardly affect the back-EMF and output torque of symmetrical machine, although they significantly change its airgap flux density and radial stress. On the contrary, both eccentricities can significantly distort the back-EMF and output torque of asymmetric machines. Asymmetric machines can have larger torque ripple with eccentricity than symmetrical machines, although asymmetric machines produce smoother torque without eccentricity. The static and rotating eccentricities result in many different orders of the back-EMF and torque components.

Electromagnetic performance of high-torque and low-speed PM brushless machines

This paper presents investigation of electromagnetic performances such as open-circuit flux-density, back-EMF and output torque on two fractional-slot PM brushless machines. Their ratios of slot-number to pole number are similar respectively, i.e. one with slot-number to pole number is differ by one and the other differs by two. Theirs main parameters such as split-ratio, axial length as well as rated torque and rated speed are fixed for the purpose of electromagnetic comparison. The investigation is performed by using 2D finite-element analysis in which saturation condition is considered. It is shown that optimum magnet thickness and machine split-ratio are desirable for a given electromagnetic torque. On the other hand, winding factor and winding topology have direct influence on the profile of machine back-EMF, which may affect the performance of electromagnetic torque. From the investigation, the machine designs are adequately reliable for robotic application as they are calculated for high torque and low speed performance.

Torque ripple minimization technique in fractional-slot PM brushless machines

This paper presents a technique to reduce torque ripple in a fractional-slot PM brushless machine. A 12-slot/10-pole machine equipped with various stators design is the subject of investigation. A better torque performance at rated condition is desirable when additional permanent magnets are inserted on the stator part. The design optimization process on motor topology is performed in order to obtain the best torque performance. It is shown that significant reduction of torque ripple is achieved without losing the desired average torque. The influences of the proposed stator design on the back-EMF, output torque and cogging torque are analysed and compared with the initial machine design.