1、中文 1880 字 电子信息科学系 外文翻译 （ 2012 届） 基于单片机的电动保护控制器系统的设计 学生姓名 学 号 专 业 班 级 指导教师 完成日期 一、 外文原文： A.C. Motor Protection AREVA INTRODUCTION There are a wide range of a.c. motors and motor characteristics in existence, because of the numerous duties for which they are used.All motors need protection, but fortun
2、ately, the more fundamental problems affecting the choice of protection are independent of the type of motor and the type of load to which it is connected. There are some important differences between the protection of induction motors and synchronous motors. Motor characteristics must be carefully
3、considered when applying protection； while this may be regarded as stating the obvious, it is emphasised because it applies more to motors than to other items of power system plant. For example, the starting and stalling currents/times must be known when applying overload protection, and furthermore
4、 the thermal withstand of the machine under balanced and unbalanced loading must be clearly defined. The conditions for which motor protection is required can be divided into two broad categories: imposed external conditions and internal faults. THERMAL(OVERLOAD) PROTECTION The majority of winding f
5、ailures are either indirectly or directly caused by overloading (either prolonged or cyclic), operation on unbalanced supply voltage, or single phasing, which all lead through excessive heating to the deterioration of the winding insulation until an electrical fault occurs. The generally accepted ru
6、le is that insulation life is halved for each 10 C rise in temperature above the rated value, modified by the length of time spent at the higher temperature. As an electrical machine has a relatively large heat storage capacity, it follows that infrequent overloads of short duration may not adversel
7、y affect the machine. However, sustained overloads of only a few percent may result in premature ageing and insulation failure. Furthermore, the thermal withstand capability of the motor is affected by heating in the winding prior to a fault. It is therefore important that the relay characteristic t
8、akes account of the extremes of zero and full-load pre-fault current known respectively as the Cold and Hot conditions. 2)/(1 1log IIt ReThe variety of motor designs, diverse applications, variety of possible abnormal operating conditions and resulting modes of failure result in a complex thermal re
9、lationship. A generic mathematical model that is accurate is therefore impossible to create. However, it is possible to develop an approximate model if it is assumed that the motor is a homogeneous body, creating and dissipating heat at a rate proportional to temperature rise. This is the principle
10、behind the thermal replica model of a motor used for overload protection. The temperature T at any instant is given by: )(max teITT where: maxT =final steady state temperature =hrating time constant Temperature rise is proportional to the current squared: )1(2 tR eKIT where: RI =current which,if flo
11、wing continuously,produces temperature maxT in the motor. Therefore, it can be shown that, for any overload current I , the permissible time t for this current to flow is: In general, the supply to which a motor is connected may contain both positive and negative sequence components, and both compon
12、ents of current give rise to heating in the motor. Therefore, the thermal replica should take into account both of these components, a typical equation for the equivalent current being: )( 2221 KIII eq Where: 1I =positive sequence current 2I =negative sequence current And: K =negative sequence rotor resistance/positive sequence rotor resistance At rated speed. A typical value of K is 3. Finally, the thermal replica model needs to take into account the fact that the motor will tend to cool down during periods of light load, and the initial state of the
