【正文】 ity and reliability must be extremely high to ensure adequate performance. Sensorless approaches can be used, such as that developed by Schr246。 (electrical) angle by the action of the mutator. In a seriesconnected machine, the same current flows through the stator and rotor windings (Fig 3), while a separatelyexcited machine allows the armature and stator fields to be controlled independently (Fig 4). 大連交通大學 2020 屆本科生畢業(yè)設計（論文）外文翻譯 In a classic synchronous three phase machine, the rotor field is produced by a current supplied via slip rings, and the orientation of the field is determined by the physical position of the rotor winding (Fig 5). The stator field is created by currents flowing in the stator windings and rotates at the speed determined by the inverter frequency. The angle between stator and rotor fields increases as more torque is produced, but the rotor speed is the same as that of the stator field. A braking action develops if the angle bees negative. In an asynchronous threephase machine, the magic field rotating in the stator induces currents in the rotor cage (Fig 6) that, in turn, generate a magic field which interacts with the stator field to produce either motoring or braking torque. In motoring, the rotor speed is lower than the rotating stator field speed set by the inverter, and in braking it is faster. No torque is produced if the two speeds are the same. This difference can be expressed as slip frequency or percentage slip. In a PMSM, the rotor field is created by mags that are either distributed on the surface of the rotor or buried in openings in the rotor laminations (Fig 7). The latter arrangement offers greater mechanical strength and much lower eddycurrent losses in the rotor. The material with the strongest magic properties is Neodymium Iron Boron (Nd2Fe14B). The stator field is generated by means of a relatively standard threephase multipole winding on a laminated core. In all electric machines, the rotating magic field leads to the generation of voltages that oppose the supply voltage(s), the socalled back EMF. At zero speed this is zero, but it grows linearly with speed. Thus the supply voltage must be increased to maintain a constant torque in region 1. The torque supplied or absorbed by an electric machine is given by the product of the magic flux and current. It is the role of the electronic power converter to condition the DC or singlephase AC supply voltage such that a suitable current or currents flow in the motor. Many different types of converters are available, but most modern traction systems use insulated gate bipolar transistors (IGBTs) and some form of pulsewidth modulation. In the region of constant tractive effort, the voltage (and frequency in the case of induction machines) applied to the terminals needs to increase linearly with motor speed so as to maintain the product of flux and current, that is the torque, at a constant level. Beyond the base speed, the applied voltage cannot be 大連交通大學 2020 屆本科生畢業(yè)設計（論文）外文翻譯 increased further due to the limitations of the power electronics and the insulation capability of the machine. However, mechanically, the machine can go faster. So region 2 is entered by field weakening, thereby reducing the level of back EMF or, in the case of a PMSM, counteracting its influence. In DC machines this is achieved by reducing the current flowing through the field windings (see the resistance RFW in Fig 3) and in a conventional synchronous machine it is achieved by reducing the current supplied to the rotor. In an induction machine, field weakening happens automatically as the supply frequency is increased while the supply voltage is kept constant. In a PMSM, field weakening is more difficult to implement because the rotor field is created by permanent mags. In region 3, the flux and current are reduced at a greater rate than in the constant power region to avoid exceeding the machine39。s perspective that modern traction systems control the torque accurately and smoothly throughout the speed range. Excellent torque control results in optimum use of available adhesion between wheel and rail, along with smooth acceleration and the ability to cruise at a constant speed and to brake the train electrically (dynamic braking). Tractive effort, power and speed The torque produced in a traction motor is translated into a linear force at the whee