【正文】 Most of the motors in the market are radial field type (cylindrical or salient pole construction). However, the axial field motors have some advantages over the conventional radial field construction in terms of power density, torque to inertia ratio, peak torque, less mag weight, low inductance, short winding turns, pact design etc. Axial field motors are designed in package, disk and sandwich type construction and have no iron in the rotor, resulting in low inertia. Axially directed magic field from rotor mag interacts with radially directed currents in these axial field motors. The mags are encapsulated in resin or plastic. Because of their construction, they are considered most suitable for robotics, puter equipments, machine tools etc. Radial field motors are also designed with varying desired flux linkage waveforms such as sinusoidal or trapezoidal, different shapes and positions of mags in the rotor such as buried or surface mounted etc. They are widely used since stator design is similar to conventional ac synchronous or induction motors. Figure 1 shows the typical crosssections of these two types of popular PMBLDC motors. Shape and location of PM in rotors Permanent mags are placed in the rotor in PMBLDC motors. In axial field type of motors, the mags are encapsulated in resin or plastic in disc form as shown in figure la. These mags are placed in such a manner that induced back emf are either sinusoidal or trapezoidal waveforms. In radial field motors, the 武漢工程大學(xué)畢業(yè)設(shè)計 (外文文獻(xiàn)翻譯） 15 mags are placed in different form such as surface mounted for low speed motors and interior radially oriented or interior tangentially oriented in high speed PMBL motors. Figure 2 shows such rotor geometries. They are also designed to achieve sinusoidal or trapezoidal back emfs depending upon applications. Sinusoidal and rectangular fed motors PMBLDC motors are designed to have either sinusoidal or trapezoidal (excited) induced back emfs. Sinusoidal excited motors are fed with sinusoidal polyphase currents similar to conventional synchronous motors for ripplefree torque with unity power factor for constant torque operation below base speed with frequency control and having leading currents to affect field weakening for constant power operation. Maximum speed of operation is restricted with demagization caused by armature reaction and mechanical construction. Magic saliency on rotor with reluctance torque helps to achieve wide speed range of constant power operation. Trapezoidal excited motors need polyphase balanced rectangular currents with 120 electrical degree width and adjustable magnitude and direction. Constant flux interaction with constant amplitude polyphase currents develops ripple free torque similar to conventional dc motor with electronic mutation. Because of these rectangular currents they are also called switched PM motors, brushless dc motors and electronic mutated PMBLDC motors. Figure 3 shows the ideal current waveforms for these two types of motors. Position sensors requirement is accordingly changed to realize these ideal current 武漢工程大學(xué)畢業(yè)設(shè)計 (外文文獻(xiàn)翻譯） 16 waveforms in the motor windings in self synchronous control mode. 3. Closed loop controllers Irrespective of sinusoidal or trapezoidal excitation, PMBLDC motors are used for position control, speed control and torque control in motion control applications. Figure 4 shows a typical position closed loop control with inner speed and current loops. For speed control system outer position loop is not required and speed reference is the mand signal. Torque control is incorporated in high performance motion control through closed loop regulation of phase currents in synchronization with shaft position feedback. In the majority of PMBLDC motors, torque is linearly related to currents and torque mand maps into current mands with only a simple proportionality constant. In some typical cases, a nonlinear mapping is required between torque and current mands. The constant power operation of PMBLDC motor drive is extended through field weakening control techniques which also requires another mand signal for torque to current mapping. Current regulation in the phase windings of PMBLDC motors either in sinusoidal or rectangular manner is carried out using current controlled voltage source inverter(VSI). PWM, hysteresis and predictive current controllers are used to issue the switching signals to the devices of the inverter to realize winding currents close to mand currents. Speed control is generally achieved by using a speed feedback and speed mand through speed controller which outputs a mand signal for torque controller. Position control is implemented through position 武漢工程大學(xué)畢業(yè)設(shè)計 (外文文獻(xiàn)翻譯） 17 feedback and position mand using position controller. The output of the position controller is the speed mand for inner speed loop. Both position and speed closed loop controllers are realized using wide varying closed loop controllers such as PI (proportionalintegral), PID (proportionalintegralderivative), SMC (sliding mode controller), adaptive controllers, fuzzy based control and neural based controllers. These classical (PI or PID) controllers or advanced closed loop controllers such as SMC, fuzzy and neural workbased ones are implemented using DSP, microcontroller and specific application integrated chip (ASIC) for speed and/or position control. Many manufacturers have developed ASICs for typical applications of PMBLDC motors. 4. Recent developments in inverters and converters PMBLDC motors are invariably fed from variable frequency inverters to provide electronic mutation. At small ratings, MOSFETbased VSIs are used to achieve ideal current control with reasonable high switching frequency. In med