A new optimum structural design is presented here in order to shift the natural frequencies of the stator structure in brushless DC motor to the highest values at different mode shapes and increase the stator stiffness. Shifting the stator natural frequencies to higher values reduces or eliminates the severe effects of the structural resonances as the coincidence of the natural frequencies and the components of the motor electromagnetic excitation forces occur at higher frequencies at which the forces have lower amplitudes. In the proposed structural design, some parameters relating to stator slot are optimised to shift the stator's natural frequencies to the highest values without changing the slot areas and the stator outer diameter as the problem constraints. The optimisation is done by the response surface methodology combined with the modal analysis by the finite element method (FEM). The validity of the optimisation method is confirmed by FEM simulations and experimental tests at different speed regions. Moreover, the performances of the prototype and optimum motors are analysed by FEM and experimental tests to show that the stator parameters optimisation process does not cause any negative influences on the quality of the motor performance.

High temperatures may damage power transformers. These problems leading to the high temperature are more revealed under non-rating operating conditions such as unbalanced supply voltage. The aim of the present study is thermal analysis of such supply and obtaining its temperature distribution. Existing thermal analysis methods through thermal equivalent circuit (TEC) have some drawbacks; in these models, thermal parameters of different regions of transformer such as core, tank, metallic parts and winding are not defined. On the other hand, those models are not applicable for thermal analysis of transformer with unbalanced supply voltage. Here, a novel TEC model is proposed which is able to define temperatures of different components of oil-immersed power transformers individually under unbalanced supply voltage. The merit of the introduced model is that losses of different parts of transformer are considered as heat generating sources which are used as the inputs of thermal model. At this end, a three-dimensional finite-element method is suggested which is able to estimate the losses of different parts of power transformer. Finally, the results of applying the TEC to the transformer are compared with the temperature distribution of finite-element modelling and high accuracy of the TEC model in estimation of the temperatures of different regions are emphasised.

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Sensorless brushless DC (BLDC) motor control is one of today's industry's requirements. The quality of this control is strongly dependent on the exact identification of the rotor angle for switching. In this study, a new sensorless BLDC motor control is introduced that could determine the exact time of the commutation. This method is based on the line-to-line flux linkage. The unscented Kalman filter method is used to identify the parameters in the proposed method. In comparison with back electromotive force based methods, this approach has advantages such as reduction of torque ripple due to commutation operations and resistance to motor parameters' changes. The correctness of the method and the performed comparison is proved by the computer simulation and practical results.

In this study, a new analytical model is provided to calculate the effect of the pole offset on the output parameters of brushless DC (BLDC) motor. The proposed model is based on the two-dimensional analysis model and the direct solution of Laplace and Cauchy-Poisson equations for the magnetic field in the air gap and the magnets area. In this model, radial and tangential components of the magnetisation vector are calculated. Effect of pole offset on the radial and tangential components of magnetisation vector is derived. It is shown that in the case of a motor with pole offset although the length of the magnet decreases at the edges of the pole, the effective air-gap length remains constant. This study proposed two coefficients to model the reduced thickness of the magnets at the edges of the poles. Therefore, the flux density is calculated in the air gap and motor magnets. Based on the proposed model, effect of pole offset on the cogging torque ripple and back-electromotive force is achieved. These results are compared with the results of finite element and experimental tests and accuracy of the model is presented.