【正文】 5. However, some of these sensors in PMBLDC motor drive may be reduced from the view point of size, cost, maintenance and reliability. Typically mechanical rotor position and speed sensors have the drawbacks of increasing the number of connections between motor and controller, increased interference, limitation in accuracy of sensors due to environmental factors such as temperature, humidity, vibrations etc., increased friction and inertia and additional space in motor housing. Because of these problems recently there has been wide interest and developments in the techniques for elimination of mechanical rotor position/speed sensing by estimating the rotor position and speed using sensed currents and voltages. Moreover, the number of voltage and current sensors may be reduced through using intelligent processors for the control of inverter feeding PMBLDC motors. Various techniques for these sensors elimination and reduction are briefly discussed in the following section. 武漢工程大學畢業(yè)設計 (外文文獻翻譯） 23 Mechanical sensors elimination One of the most recent developments in PMBLDC motor drives has been the rapid evolution of new techniques for eliminating the rotor angular position sensor. Elimination of the shaftmounted position sensor is a very desirable feature in a number of applications since this sensor is one of the most expensive and fragile ponents in this drive. Some of the position sensorless schemes are classified in brief. Back EMF based position estimation The most mon methods of rotor flux position sensing is based on deriving the back emf signals. There are many methods for rotor position estimation based on the back emf and are briefly discussed below. Direct back EMF detection: This method is quite popular for rectangular fed PMBLDC motors. In these PM motors, particular phase winding is excited for 2/3 of each electrical period and ideally there is always one phase which is not excited. The direct sensing of back emf of unexcited phases in sequence is used to generate discrete rotor position signal for current synchronization with rotor flux. It has been applied to many industrial applications including disk drives, pact stereo players and room air conditioners. Estimation of back EMF: This method is applied to both sinusoidal fed and 武漢工程大學畢業(yè)設計 (外文文獻翻譯） 24 rectangular fed PMBLDC motors. This method is based on the reconstruction of the back emf by using voltage equation of the motor (e = v iRLdi/dt). Reconstruction of the back emf involves either simulating this equation with operational amplifier or solving this equation in online digital processor normally used for control. In this method, the terminal voltage and line current are measured directly and the above equation is used to achieve back emf and rotor position. 武漢工程大學畢業(yè)設計 (外文文獻翻譯） 25 武漢工程大學畢業(yè)設計 (外文文獻翻譯） 26 。 this paper concentrates on the recent advances in PMBLDC motors in terms of motor construction, closed loop controllers, semiconductor power modules, sensors and their reduction, torque ripple minimization, impact of microelectronics, cost reduction and potential applications. 2. Latest developments in the PMBLDC motors Permanent mag (PM) excitation has been used in place of dc excitation in different electric machines such as dc machines, synchronous machines and new PMBL machines such as PM stepper motors, hybrid stepper motors and PMBLDC motors. High cost of PM materials has been a major bottleneck for use and development of these electric machines. Gradual growth of better PM materials, improved manufacturing technology, varying nature of construction of these motors to suit specific applications have brought them at a level where they are considered one of the best motors available nowadays. PM machines have a wide spectrum but this paper is restricted to PMBLDC motors. Presently PM materials used in PMBLDC motors are classified in the following three broad categories, namely Alnico (A1NiCoFe), Ceramics also include ferrites and rareearth materials such as samariumcobalt (SmCo), neodymiumironboron (NdFeB). Alnico and ferrites have long been used in the development of PM motors as they are cheap and easily available. Rareearth PM materials, namely SmCo, are used nowadays because of the high energy density 武漢工程大學畢業(yè)設計 (外文文獻翻譯） 13 caused by its high residual flux density, coercive force and low temperature coefficient. NdFeB is considered one of the best PM materials presently since it offers much higher residual flux density and coercive force. However, its only drawback is the temperature limit. Continuous efforts are being made to overe this and it is hoped that this will enable PMBLDC motors to attain higher efficiencies and lower sizes along with other advantages. PMBLDC motors may be classified into different categories such as number of phases, radial or axial field, cageless or with cage bars, surface mounted PMs or buried mags, sinusoidal or rectangular fed motors etc. Some of them are briefly discussed in this section. Number of phases PMBLDC motors are developed in single phase in low power ( 50W) for tube axial fans to cool electronics equipments. They are manufactured in two phase construction for home appliances such as solar PV fed refrigeration system, servo control etc. Most of the medium and high power rating motors are designed in threephase construction similar to conventional ac motors. In some electric vehicles and megawatt rating motors for submarine propulsion etc., designers have pelling reasons to increase the number of phases to five, six or more in order to reduce the per phase power handling requirements. Radial and axial field motors 武漢工程大學畢業(yè)設計 (外文文獻翻譯） 14