Permanent magnet synchronous motor design for compressor application

Regulations and standards related to motor efficiency used in white good applications are continuously changing. Energy efficiency is one of the most current topics in present day and has even become more critical day by day. It can be seen that the global companies are spending a significant portion of the capital budget for energy efficiency purpose taking into consideration the competitive world market. The permanent magnet used in motor technology in terms of performance and efficiency comes into prominence in today's world. Considering the compressor applications, one phase induction motors are being used in a widespread manner. These types of motors directly fed from the alternative current network without any motor drive are working at a constant speed. To be operating at a constant speed is known as an enormous disadvantage with reference to the compressor system performance. The motor interrupts the operation after a certain time. When the refrigerant cooling capacity decreases the rated value, motor runs again. The starting current is high value when the motor operating is in an undesirable situation with regard to energy consumption. By taking all these parameters into consideration, permanent magnet motors can be easily controlled by the power electronics motor drive circuits and are becoming more popular for refrigerator applications from day to day. Permanent magnet synchronous motors become attractive for refrigerant compressor applications have much precedence such as high torque value per unit volume, to be controlled easily at low speeds, to be able to rise to the high speed, low decibel value, high efficiency, compact structure etc. Particularly, the advances of the magnetic materials and the magnet technology, bringing down the cost of manufacturing and improvement in power electronics are the reason for preference for white good applications. To begin with, many academic studies in the literature detailed scrutinized about the permanent magnet motors. Electrical motors consist of rotor and stator parts. When analyzed the academic researches, it is seen that there are two main points about the design consideration related to stator and rotor. Each reason concerned the design improvements are to set to zero the cogging torque value, to reduce the torque ripple and the minimization of losses. Choosing a proper match of slots and poles number, skewing armature lamination, selecting the width of slot opening, teeth notching and pairing and the proper winding layout are the design parameters for stator part. Selecting a proper pole arc to pole pitch ratio, magnet skewing, magnet segmentation, changing the magnet profile could be applied for the rotor design improving. The motor design was completed in the light of these design criteria. Requirements regarding a motor model were determined after the preparatory work. The speed-torque curve of the motor, maximum current value, power value etc. were specified in the beginning of the thesis project. Hereupon, the reference model was chosen and its two dimensional finite element analysis (FEA) was solved by software program. This motor has six slots and four poles and magnets were mounted onto the rotor surface. It is typically brushless direct current (BLDC) motor and has a trapezoidal back-electoromotor force (back-EMF), driving by the trapezoidal pulse current. Cogging torque, back-EMF peak-to-peak values, torque output, motor losses were given as graphics and tables after the analysis studying. After two dimensional solution, reference model was also prepared for three dimensional analysis. The length of magnet and rotor stacks are five milimeters longer than the stator stacks. The aim of three dimensional analysis is to see the effect of length of magnets. The peak value of back-EMF and the average electromagnetic torque is higher than the two dimensional anaylsis result. The alternative motor design process was started after the completion of the reference model analysis. Firstly, the proper slots and poles configuration were chosen for the permanent magnet motor. After that, the pole type was selected as a surface mounted. The first modeling process was made by using the software program, named as Speed®, which uses the magnetic equivalent circuit model based on the analytical solution. Then, the model was solved and the results were handled. At the first stage, back-EMF of the motor was attracted notice. It is seen that the back-EMF had trapezoidal wave shape. Since, the purpose of this thesis is to design the motor having a sinusoidal back-EMF waveform. For that reason, next step of the design and changing phases were done by using FEA program ANSYS Maxwell®. It is possible to design the motor having a sinusoidal back-EMF wave shape thanks to the changing of the magnet profile. This procedure is known chamfering operation considering the manufacturing process. Fractional slot double layer winding configuration was selected for the best choice to minimize the stator slots harmonics. After the creating the final model, two dimensional FEA simulations were run and results were discussed. The cogging torque, back-EMF, torque output and motor losses were evaluated. It is seen that the back-EMF waveform is pure sinusoidal curve after simulation results. After two dimensional analysis, three dimensional model was created and prepared for simulation. The magnet length and rotor stacks are four milimeters longer than the stator stacks. The difference between two and three dimensional analysis results was shown. The back-EMF value was obtained higher after three dimensional simulation. Both two dimensional and three dimensional motor model were driven by the same current value. The electromagnetic torque in the airgap was gained high value in the three dimensional analysis. These results were related to the length of magnet. Afterwards, the reference model and the original motor design were compared and 3D analysis results were discussed in detail. First, the cogging torque was handled. It is seen that the original motor model has better cogging torque value. The reference model has very high torque ripple value and its value is %57. The original motor model has only %10 ripple value. In the light of this information, the original model has better electromagnetic torque value. It is seen that the original model total harmonic distortion value is under %1. The motor efficieny for the original motor is two-three perceantage higher the reference motor. Thereafter, the motor was prototyped. The lamination and magnet drawings were prepared as engineering drawing and the order was made. Magnets were shocked directionally. Motor was collected and made available for testing after the material had been obtained. The motor was tested in the laboratory and experimental results were carried out. The thesis was written after the 3D analysis results and the experimental results compared and validated. Overall, it is shown how to design good motor design process. It is possible to reduce the motor dimensions and the material consumption in order to get more compact motor structure and high efficiency when applying the proper motor design parameters.