Permanent-magnet brushless machines with unequal tooth widths and similar slot and pole numbers

This paper presents a comparative study of three-phase permanent-magnet brushless machines in which the slot and pole numbers are similar, with reference to conventional brushless dc machines in which the ratio of the slot number to pole number is usually 3 : 2. Three different motor designs are considered. Two have equal tooth widths, with one having a coil wound on every tooth and the other only having a coil wound on alternate teeth, while the third machine also has coils wound on alternate teeth but these are wider than the unwound teeth while the width of their tooth tips is almost equal to the rotor pole pitch in order to maximize the flux linkage and torque. Analytical and finite-element methods are employed to predict the flux-linkage and back-electromotive-force waveforms, and the self- and mutual-inductances, and these are shown to be in good agreement with measured results. It is also shown that the third machine is eminently appropriate for brushless dc operation

Modelling barriers for coal dust suppression

Airborne dust originating from the transport and storage of raw coal has detrimental effects on the environment. Warkworth Mining is seeking to reduce the dust emissions caused by the dumping of raw coal at their facility in Singleton, NSW. The suggested strategy was the construction of windbreaks, for which commercial designs exist. The MISG was asked to advise on the placement and design of such windbreaks. The problem was approached by studying results in the literature on windbreak design, selecting a few possible configurations, and then testing these by solving numerically for the wind velocity around the dumping site. It was concluded that a long fence on the upwind side of the dumping hoppers would provide moderate protection, but would interfere with current operating procedures. As a better option, a long downwind fence immediately behind the hoppers would provide a similar reduction in the dust emission, and allow more effective use of water sprays. Although fine details of the fence design could not be modelled numerically, we concluded that it was desirable for the fence to have an angled overhang in the vicinity of the hoppers, and a porous section near the base to reduce turbulent flows

Analysis of cogging torque in brushless machines having nonuniformly distributed stator slots and stepped rotor magnets

A simple analytical technique is proposed for synthesizing the cogging torque waveform of a permanent magnet brushless machine from the cogging torque waveform that is associated with a single stator slot. The machine may have either uniformly or nonuniformly distributed stator slots and/or a skewed rotor, in which the skew is realized by circumferentially displacing the magnets of each pole. The technique is validated by finite element analysis and measurements

Improved speed estimation in sensorless PM brushless AC drives

The application of flux-observer-based sensorless control to permanent-magnet brushless AC motor drives is described. Current methods of speed estimation are assessed, both theoretically and experimentally, and an improved method, which combines the best features of methods in which speed is derived from the differential of rotor position and from the ratio of the electromotive force to excitation flux linkage, is proposed. Its performance is verified experimentally

Calculation of d- and q-axis inductances of PM brushless ac machines accounting for Skew

A hybrid two-dimensional (2-D) finite-element/analytical technique is described for predicting the d-axis and q-axis inductances of permanent magnet (PM) brushless ac machines, with due account for the influence of skew. Predicted inductances are compared with measured values for two machines having identical stators, which are skewed by one slot-pitch, but which have different rotor topologies, one having surface-mounted magnets and the other having interior magnets

Non-global Logarithms at 3 Loops, 4 Loops, 5 Loops and Beyond

We calculate the coefficients of the leading non-global logarithms for the hemisphere mass distribution analytically at 3, 4, and 5 loops at large Nc . We confirm that the integrand derived with the strong-energy-ordering approximation and fixed-order iteration of the Banfi-Marchesini-Syme (BMS) equation agree. Our calculation exploits a hidden PSL(2,R) symmetry associated with the jet directions, apparent in the BMS equation after a stereographic projection to the Poincare disk. The required integrals have an iterated form, leading to functions of uniform transcendentality. This allows us to extract the coefficients, and some functional dependence on the jet directions, by computing the symbols and coproducts of appropriate expressions involving classical and Goncharov polylogarithms. Convergence of the series to a numerical solution of the BMS equation is also discussed.Comment: 42 pages, 6 figures; v2: small typos correcte

Rotor resonances of high-speed permanent-magnet brushless machines

For high-speed machines, in particular, it is very important to accurately predict natural frequencies of the rotor at the design stage so as to minimize the likelihood of failure. Finite-element analysis and experimental measurements are used to establish the natural frequencies and modes of the rotor of a high-speed permanent-magnet brushless motor, and to assess the influence of leading design parameters, such as the active length, the shaft diameter and extension, the bearings, and the material properties

Direct torque control of brushless DC drives with reduced torque ripple

The application of direct torque control (DTC) to brushless ac drives has been investigated extensively. This paper describes its application to brushless dc drives, and highlights the essential differences in its implementation, as regards torque estimation and the representation of the inverter voltage space vectors. Simulated and experimental results are presented, and it is shown that, compared with conventional current control, DTC results in reduced torque ripple and a faster dynamic response

Improved current-regulated delta modulator for reducing switching frequency and low-frequency current error in permanent magnet brushless AC drives

The conventional current-regulated delta modulator (CRDM) results in a high current ripple and a high switching frequency at low rotational speeds, and in low-frequency current harmonics, including a fundamental current error, at high rotational speeds. An improved current controller based on CRDM is proposed which introduces a zero-vector zone and a current error correction technique. It reduces the current ripple and switching frequency at low speeds, without the need to detect the back-emf, as well as the low-frequency error at high speeds. The performance of the modulator is verified by both simulation and measurements on a permanent magnet brushless ac drive