远距离低压电动机动力电缆选择

Selection of Power Cable for Long-distance Low-voltage Motors

针对石油化工装置电气设计中经常出现的远距离低压电动机电缆选择错误,结合《工业与民用配电设计手册》(第四版)中的相应公式,选取了一个有代表性的工程案例分析电缆选择要点和规律,校验电动机线路的电压降,指出计算公式的一处疏漏及修正电缆工作温度的方法,校验电动机启动时端子电压降和母线电压降,分析了多根电缆并接时线路电抗值的取值。通过定性分析电动机启动时端子电压降对启动转矩和启动时间的影响,得出端子电压降与启动时间的对应关系。汇总计算结果表明:随着电缆长度的增加,电动机启动时对母线电压的影响逐渐减弱,而端子电压降越来越大;放大电缆截面对减小电动机正常电压降效果显著,但对降低端子电压降影响较小。相同距离的电动机功率大的端子电压降更严重,因此在校验端子压降时优先校验大功率电动机;远距离低压电动机电缆选择的主要依据是正常状态电压降,并根据情况校验较远、较大电动机的端子电压降。

In view of the selection errors of long-distance low-voltage motors power cable often occurring in the electrical design of petrochemical equipment,and combined with the relevant formula in Industrial and Civil Power Distribution ( 4th ed.),a representative engineering case is chosen to analyze its main points and laws. The voltage drop of motor line is checked,and a mistake in the calculation formula is pointed out,and the working temperature of cables is corrected. The terminal voltage drop and bus voltage drop of motor start-up are checked,and the value of cable reactance when multiple cables are connected in parallel is analyzed. Through qualitative analysis of the effect of terminal voltage drop on start-up torque and start-up time,the corresponding relationship between terminal volt- age drop and start-up time is obtained. The summary of calculation result shows that with the increase of cable length,the influence of motor starting on bus voltage gradually decreases,while terminal voltage drop increases;the effect of enlarging cable cross-section on reducing motor normal voltage drop is significant, but it has little effect on reducing terminal voltage drop. The terminal voltage drop with larger motor power at the same distance is worse, so when checking terminal voltage drop, priority should be given to high-power motors;the main basis for cable selection of long-distance low-voltage motors is the normal voltage drop,and the terminal voltage drop of long-distance and larger motor should be checked as the circumstances may require.