1、英文原文：Motor Protection PrinciplesArijit Banerjee, Arvind Tiwari Jakov Vico, Craig WesterGE Global Research, GE MultilinBangalore, India Markham, Ontario, CanadaAbstract this paper discusses the fundamentals of motor protection principles. every motor is designed for a specific operating temperature d
2、epending upon its insulation. once this limit is exceeded its life decreases drastically. over heating condition may arise due to several factors like supply system disturbance, electrical unbalance conditions, faults, load variation and environment. it is essential to have a proper coordination bet
3、ween development of protection principles and the design of motors to understand the concept of motor heating and how thermal protection for motors can be used to prevent loss of motor life. in this paper an overview of existing protection methods for motors is reviewed with a special consideration
4、of thermal protection of motors.I. Introduction Three phase motors can be classified into two types: induction and synchronous. induction motors are the workhorses of most industrial systems. induction motors are used on fans, blowers, conveyors, crushers, compressors, cranes, pumps, shredders, extr
5、uders, refiners and chillers. despite heir ruggedness and simplicity in construction, the motor failure rate per year is significant 1. motor failure rate is conservatively estimated as 3-5% per year, but in some industries like mining, and pulp and paper, motor failure rate can be as high as 12%. m
6、otor failure cost contributors are, repair or replacement, removal and installation and loss of production. all failures can be divided in three groups. one third of failures are electrically related, one third are mechanically related failures and remaining one third of failures arerelated to envir
7、onment, maintenance and the other reasons. due to occurrence of such outages, the industry has to replace or repair the motor and incur huge downtime. this necessitates the requirement of proper the protection of the machine to ensure productivity unplanned outages. most of the motor failure contrib
8、utors and failed motor components are related to motor overheating. thermal stress can potentially cause the failure of all the major motor parts: stator, rotor, bearings, shaft and frame. there are two main risks for an overheated motor: stator windings insulation degradation (for stator limited mo
9、tors) and rotor conductors deforming or melting (for rotor limited motors, where motor thermal limit is defined by allowed motor stall time). Insulation lifetime decreases by half if the motor operating temperature exceeds its thermal limit by 10c (figure 2). apart from overheating, there are a numb
10、er of conditions that can result in damage to three-phase motors also. these damages may be due to operating conditions or internal or external faults.External faults and operating conditions include: under-voltage, asymmetrical loading, phase and ground faults on the motor feeder and overloading du
11、ring starting and running operation. Internal faults include: ground faults, faults between windings and inter-turn faults. The motor protection device (MPD) must be able to prevent any damage occurring to the motor. In the following sections, the requirements of several protection methods are brief
12、ly discussed.II. Overvoltage protectionWhen an induction motor runs in an overvoltage condition, slip (being inversely proportional to square of the voltage) decreases resulting in decrease in load current. Simultaneously, magnetizing current increases exponentially resulting in poorer power factor.
13、 Decrease in load current results in lower copper loss while increase in magnetizing current results in higher core loss. Although old motors had robust design, new motors are designed close to saturation point for better utilization of core materials and increasing the V/Hz ratio could cause satura
14、tion of air gap flux leading motor heating. This leads to higher overall loss, less efficiency and higher operating temperature of the motor2. In addition, the dielectric stress on the insulation of the motor is higher. This necessitates requirement of overvoltage protection for an induction motor.
15、The overvoltage element should be set to 110% of the motors nameplate unless otherwise stated in the motor data sheets.III. Under-voltage protection When an induction motor operating at full load is subjected to an under voltage condition, slip increases, power factor is increased and full load curr
16、ent Increases. Although stator core loss decreases, rotor copper loss and stator copper loss increases. This leads to poorer efficiency and higher temperature rise for the motor 2. The under voltage protection element can be thought of a backup protection for the thermal overload element. In some cases, if an under voltage condition exists it may be desirable to trip th
