Sürekli mıknatıslı senkron makine

Abstract

Bu tezin genelinde, kaliteleri her geçen gün artan ve ucuzlayan mıknatıs malzemelerinin, sürekli mıknatıs olarak, elektrik makinelerinde kullanılması ve uygulama örneği olarak Sürekli Mıknatıslı Senkron Makine incelenmiş, böyle bir tasarımın gerçeklenmesinin gerekçeleri ve makineye kazandırdığı özellikler, indüksiyon makineleri ile karşılaştırılarak verilmniştir. 5. Bölümde, sürekli mıknatıslı senkron makinede hava aralığındaki alan dağılımı ve makine parametreleri, makine magnetik devresi ile belirlenmiştir. 6.Bölümde, sürekli hal davranışı, gösterici diyagramı yardımı ile kayıplarını içeren eşdeğerdevre üzerinde incelenmiş ve gerçeklenmesi öngörülen bir sürekli mıknatıslı senkron motora ait sürekli hal çalışma büyüklükleri hesaplanmıştır.

In this thesis, the AC electrical machines that is used of permanent magnet are studied and are the per manent magnet synchronous machines are compareted with inductions machines in possibility of design. The spe cial feature of permanent magnet synchronous machines have been given. Permanent magnet synchronous machines (PMSM) are attracting growing international attention for a wide va riety of industrial applications. The attractive power- density and efficleny characteristics exhibited by these machines. The large majority of commercially available PMSM are constructed with the permanent magnets mounted on the periphery of the steel rotor core, exposing their surface magnetically and sometimes physically, to the air gap. These machines, referred to there as surface PMSM, are also known as brushless dc machines, inside-out machines, electronically commutated machines, as well as by wide variety of manufacturer-specific trade names. This range of terminology obscuers the fact that, in most cases, they are varions of the same class of machines. The PMSM rotor has mechanical reliability equal to that of an induction machines and eliminates excita tion losses found in induction or synchronous machines. The PMSM offers the potential for the highest electrical efficiency than induction machines in similar applica tions. The use of the permanent magnet in the rotor of PMSM makes it unnecessary to supply magnetising current through the stator for constant air gap flux; stator cur rent need only be torque-producing. Hence for the same output, the PMSM wiil operate at higher power factor (because of the absence of magnetising current) and will be more efficient than the induction machines. The con ventional wound-rotor synchronous machine on the other -VI- hand must have dc excitation on machine, which ia often supplied by brushes and sliprings. This implies rotor losses and regular brush maintenance, which implies downtime. Note that the key reason for the development of the PMSM was to remove the foregoing disadvantages of wound-rotor synchronous machines by replacing its field coil, dc power supply and slipring with a permanent magnet. The PMSM, therefore, has a sinusoidal induced EMF and requires sinusoidal current to produce constant torque just like wound-rotor synchronous machines. Current research in the design of the PMSM indicates that it has a higher torque -inertia ratio and power density when compared to the induction machines or wound-rotor synchronous machines, which makes it preferable for certain high-performance applicatons like robotics and aerospace actuators. Aproximately three-quarters of all electrical mo tors are induction motors. In order to reduce energy consumption, manufacturers have cosentrated on improving classical induction motors desings, and in addition, now offer high efficiency motors at premium prices. The efficiency improved by reducing the air gap power density, by replacing aluminum conductors with copper conductors, by increasing conductor cross section, and by using higher quality magnetic iron. These tecniques can be combined to reduce total losses by 20... 40 prcents. These tecniques, however, go only up to a point, beyond which it is not economical, and in some cases it is impossible, to increase effciency by adding materials. In addition, whit increasing loss reductions, it becomes more dif- fucult to maintain a specified minimum starting torque, maximum starting current, and minimum power factor. The challence of improving the induction motor efficinecy can be best understood by a brief rview of the various loss compenents. The following electrical losses are the priciple in all induction motors: -VII- - stator load current losses (I2R) - stator magnetizing current losses (I2R) - rotor slip losses (I2R for rotor) - iron losses -stary load losses The stator I2H losses can be improved by either current reduction or conductor cross section increase. The first requires more flux and thus more iron, which at a point will increase the I2R losses again because of the in creasing lenght of conductor. Increasing the conductor area tends to increase the iron loss because of either high loss density or increased iron volume. The rotor I2R losses are the cost of torque development in induction motor. Reducing the rotor resistance has the same impact on the iron losses as the stator resistance change. Also it reduced the starting torque. Those losses which depend on the fundamental flux are counted as iron losses. Besides a change to better material, there is little to be done which does not nega tively impact the other losses. The stary load losses consist basically of loses in the iron and windings which are current dependent. Since they are still not well enough understood to be precisely modelled. A way out of that loss dilemma is the change to a different machine concept where some loss components are eliminated. Introducing permanat magnet excitation to a synchronous motor can eliminate the excitation losses. The rotor losses I2R losses and the related stray load loss compenents disappear also. Therefore, the PMSM concept has been selected. From a manufacturing point of view, the stator of this machine is identical to the stator of a multiphase induction motot or of an electri cally excited multipahse syncronous motor. The new com ponent is the rotor which, in contrast to conventional -VIII- syncronous machine rotor, relies on permanent magnets as the source of excitation rather electric current car rying windings. The optimum rotor configuration, rotor electromagnetic and mecanical desing, and the stator electromagnetic desing must be matched to achieve the desired performance. The two basic configurations are shown in Figure 2.4. Because syncronous machines produce torque only at synchronous speed, there is no inherent starting torque. Both rotor configurations allow for the incorporation of amortisseur windings (squirrel-cage windings) to facilitate line stsrting of the PMSM as an indiction motor. The basic dif frence in the rotor is in the orien tation of the magnets. Figure 2.4a shows the magnetic axis of the magnets oriented radially. In this case, the no-load flux density in the air gap is slightly less than the flux density in the magnets due to rotor leakage flux. Machines using this kind of rotor configuration tend to have low air gap flux dendities. The configuration in Figure 2.4b showes magnets whit their magnetic axis oriented in the circumferential direction. This aligment can achieve a maximum air gap flux density (due to magnet) larger than that for the radial magnet orientation due to "flux squeezing". The magnetomotive force to drive the magnetic flux through the magnetic circuit in this case is provided by one mag net only. This configuration has been successfully ap plied in motors and generators for applications demanding low weigh and high power density. The PMSM is a synchronous machine and thus ex hibits the same generic perfomance characteristic as the wound rotor synchronous machine. Difference are due to the constant excitation and the fact that magnetically the magnet space behaves like air. The result is a direct -IX- (d) to quadrature (q) reactance ratio of less than one and a torque torque-angle curve with the pullout torque at angles larger than 90°. As is well known the pullout torque of an induction motor is proportional to the terminal voltage squared, which makes the induction motor performance very sensitive to voltage change. The PMSM including the reluctance torque and magnet torque which is linearly de pendent on the voltage, the PMSM is less sensitive to voltage changes. Damper windings are used to run the machine up the to speed on induction motor action with the machine pulling into synchronism by a combination of the reluc tance and synchronous motor torques provided by magnet. During the startup, the magnet exerts a braking torque that opposes the induction motor type torque provided by the damoer windings. Vector control is normally used in ac machines to convert them, performance wise, into equivalent seperat- ely excited dc machines which have highly desirable con trol characteristics. For high performance servo drives, hysteresis or PWM current controllers are used to ensure that the actual currents flowing in the motor are close as possible to the sinusoidal references.