Surface-mounted Magnet Research Articles

Reduction of cogging torque in interior-magnet brushless machines

An investigation into the cogging torque in a four-pole interior-magnet brushless machines having either a six-slot stator and a short-pitched nonoverlapping winding or a 12-slot stator and a full-pitched overlapping winding is described. It is shown by finite-element analyses and measurements that, by appropriately defining the pole-arc to pole-pitch ratio, the optimal pole-arc to pole-pitch ratio for minimum cogging torque, which has been derived for surface-mounted magnet machines, is equally applicable to interior-magnet machines.

Iron loss in permanent-magnet brushless AC machines under maximum torque per ampere and flux weakening control

The airgap flux density distribution, flux density loci in the stator core, and the associated iron loss in two topologies of brushless AC motor, having a surface-mounted magnet rotor and an interior-mounted magnet rotor, respectively, are investigated when operated under maximum torque per ampere control in the constant torque mode and maximum power control in the flux-weakening mode. It is shown that whilst the interior magnet topology is known to be eminently suitable for flux-weakening operation, due to its high demagnetization withstand capability, its iron loss can be significantly higher than for a surface-mounted magnet machine.

Improved analytical model for predicting the magnetic field distribution in brushless permanent-magnet machines

A general analytical technique predicts the magnetic field distribution in brushless permanent magnet machines equipped with surface-mounted magnets. It accounts for the effects of both the magnets and the stator windings. The technique is based on two-dimensional models in polar coordinates and solves the governing Laplacian/quasi-Poissonian field equations in the airgap/magnet regions without any assumption regarding the relative recoil permeability of the magnets. The analysis works for both internal and external rotor motor topologies, and either radial or parallel magnetized magnets, as well as for overlapping and nonoverlapping stator windings. The paper validates results of the analytical models by finite-element analyses, for both slotless and slotted motors.

Online optimal flux-weakening control of permanent-magnet brushless AC drives

An enhanced online optimal control strategy, which maximizes the flux-weakening performance of a brushless AC motor, is described, and applied to motors having different rotor topologies: interior (radial or circumferential), inset, and surface-mounted magnet. It enables the maximum inherent power capability of a brushless AC motor to be achieved independent of any variation in its parameters, and facilitates maximum efficiency over the entire speed range. It also results in good transient dynamic performance, since it is coupled with feedforward vector control based on optimal current profiles.

A permanent-magnet rotor containing an electrical winding to improve detection of rotor angular position

Sensorless detection of rotor angular position is desirable for decreasing the cost and increasing the reliability of permanent magnet AC motor drive systems. This paper describes tests performed on a rotor that has surface-mounted magnets and a conducting winding that links only the d axis of the rotor. The rotor winding increases the variation of the stator winding inductance that occurs as the rotor rotates and improves the accuracy of rotor angular position estimates.

Eddy-current losses in the segmented surface-mounted magnets of a PM machine

A gas-turbine driven, high-speed, high-efficiency generator system intended for use in series-hybrid electric vehicles is developed. It consists of a permanent-magnet generator with surface-mounted magnets and a six-pulse controlled rectifier. The stator currents of the rectifier-loaded generator contain time harmonics which cause eddy-current losses in the magnets. These losses can be so high that they result in demagnetisation of the magnets. To reduce these losses, the magnets may be segmented. The aim of the paper is to model the eddy-current losses in such segmented magnets. This is done by incorporating magnet loss resistances in the equivalent circuits of the permanent-magnet machine. The model is verified by means of locked-rotor tests. To show the usefulness of the model, the losses in the magnets of a rectifier-loaded permanent-magnet machine are calculated. The eddy current losses in the magnets can be decreased by increasing the number of magnet segments: these losses are proportional to the square of the magnet width.

Magnetic field analysis and inductances of brushless DC machines with surface-mounted magnets and non-overlapping stator windings

A 2D analytical model is developed for calculating the self- and mutual-winding inductances of brushless DC machines having surface-mounted permanent magnets and non-overlapping stator windings. The model is validated against both finite element analyses and measurements, and used to predict the influence of machine design parameters. >

Achieving a constant power speed range for PM drives

Flux weakening has long been a standard approach in obtaining a constant power speed range for traction drives with commutator motors. There is much interest in using permanent magnet (PM) commutator motors or, preferably, electronically commutated PM motors for such applications. Some PM motor designs, such as those using inset or buried magnets, have been proposed to achieve flux weakening. This paper questions the desirability of the flux weakening approach for a number of applications where energy efficiency is of prime importance, such as in electric vehicles. For an acceptable increase in the rating of some of the semiconductor devices, standard PM commutator motors and switched PM motors with surface-mounted magnets can be used and a significant reduction in power loss can be achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Instantaneous magnetic field distribution in brushless permanent magnet DC motors. III. Effect of stator slotting

For pt.II see ibid., vol.29, no.1, p.136-142 (1993). A method for modeling the effect of stator slotting on the magnetic field distribution in the airgap/magnet region of a radial-field brushless permanent-magnet DC motor equipped with a surface-mounted magnet rotor is presented to allow the analytical prediction of the open-circuit, armature-reaction, and consultant magnetic field distribution under any specified load condition. It is shown that the conventional 1D relative permeance model, which is used extensively in the analysis of induction and synchronous machines, is inappropriate for permanent magnet motors, for which the proposed analytical method accounts for stator slot openings by the application of the conformal transformation method and a 2-D relative permanence function. The results of predictions from the analysis are compared with corresponding finite element analyses. >

Instantaneous magnetic field distribution in permanent magnet brushless DC motors. IV. Magnetic field on load

For pt.III see ibid., vol.29, no.1, p.143-151 (1993). An analytical technique is developed for predicting the instantaneous magnetic field distribution in the airgap-permanent magnet region of radial-field topologies of brushless DC motors equipped with a surface-mounted magnet rotor and operating under any specified load condition. It accounts implicitly for the stator winding current waveform and the effect of stator slot openings. The 2-D field analysis, in polar coordinates, combines the armature reaction field component, derived from an analytical determination of the relative temporal and spatial position of the two field components. The predicted open-circuit, armature-reaction, and load field distributions all show excellent agreement with results from corresponding finite element analyses.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Induced voltage harmonic reduction of PM cylindrical machines

Typically, permanent magnet (PM) machines with surface-mounted magnets have trapezoidal induced voltage waveforms with high harmonic content that can adversely affect torque pulsations, noise, and efficiency. A simple methodology for reducing these harmonics by selecting the appropriate magnet shape and dimensions has been introduced previously by the authors (see IEEE Proc. PES 1989 Summer Mtg.). For illustrative purposes, however, the method used a simplified analytical model of the magnetic circuit, ignoring important factors such as curvature and magnet leakage. The authors presently apply the proposed methodology to cylindrical machines using a more precise model. The results, showing the potential for reducing induced-voltage harmonics by using a double-magnet per pole configuration, have been verified by comparison with results obtained from finite element analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electromechanical forces and torque in brushless permanent magnetic machines

An analytical method was developed to calculate the electromechanical forces and torque directly from design parameters in a brushless permanent magnetic machine having surface-mounted magnets and a surface-wound stator. Analytical equations, which are very easily programmable, were derived for both radial and parallel magnetization of the rotor magnets. The accuracy of the model was verified by using finite element analysis. The utility of the model in parametric studies is demonstrated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Torque production in permanent-magnet synchronous motors

An analytical method was developed to determine cogging as well as commutating torque in permanent-magnet machines of both buried and surface-mounted magnet types. A detailed analysis of the torque production in a permanent magnet synchronous motor drive with rectangular current excitation is presented. The effect of skewing is considered. The proposed method takes advantage of the knowledge of the flux density distribution in the air gap as a function of design parameters. The equations can be programmed easily and are very useful for parametric studies and optimization.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>