外文翻译---永磁牵引电机的崛起(编辑修改稿)

外文翻译---永磁牵引电机的崛起(编辑修改稿)内容摘要：

ne 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 大连交通大学 2020 届本科生毕业设计（论文）外文翻译 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 electrical or mechanical limits. In the separatelyexcited DC motor, for example, the armature current is also reduced as a function of speed. Advantages and drawbacks The main reason why permanentmag machines are being more and more widely adopted for railway traction drives is that they offer very significant advantages pared with equivalent threephase induction motors. The level of efficiency is 1% to 2% higher across 80% of the operating range. The specific power is 30% to 35% greater, resulting in a machine that is about 25% smaller and lighter for the same power rating. Whereas in an asynchronous motor heating of the rotor is caused by the inherent slip power, this is virtually eliminated with a PM drive, avoiding the need for rotor cooling. Normally, PM machine stators are pletely sealed and cooled by means of a heat transfer fluid, thus leading to potentially more reliable drives. PMSMs also allow dynamic braking down to very low speeds and, in theory, it should be possible to produce a selfcontrolled retarder by electromechanically shortcircuiting the stator windings. Of course, these benefits are not available without promise. There are seven main drawbacks to the use of permanentmag traction motors, although appropriate mitigation measures have been developed. Limitations on the size and cost of the fourquadrant converter and machine do not allow operation across the whole speed range by the simple expedient of supplying the machine with a voltage that is sufficiently higher than the back 大连交通大学 2020 届本科生毕业设计（论文）外文翻译 EMF to permit the flow of current required to achieve the desired torque. This constraint is solved by means of field weakening, creating the constant torque and constant power regions. Since the field generated by the permanent mags cannot be adjusted, field weakening is achieved by injecting currents into the stator windings which set up fields to oppose those of the rotating permanent mags. These extra currents cause copper losses in the stator windings that negate, to some extent, the efficiency gains that are achieved by the use of the lowloss permanentmag rotor. In order to be able to control the currents that create the field weakening effect, it is necessary for the electronics to know the position of the rotor, to an accuracy of between 1176。 and 2176。 (the field angle). For a fourpole machine this requires a mechanical resolution of better than 176。 . If a sensor is used, its integrity and reliability must be extremely high to ensure adequate performance. Sensorless approaches can be used, such as that developed by Schr246。 dl1, but these can lead to a reduction in the accuracy of control. The magic flux is temperaturedependent in that the field。