不同截面电缆并联问题
近日施工中，一个传动柜出线因为电缆不够用了，便用一条不同截面的电缆代替，但问了些同行都说不能这么用，并说其中的小截面电缆会首先出问题，想请教各位高见。
IGBT逆变器；
电机额定电流：780A， 辊道电机，不需连续运行。
设计的电缆：3根 185平方的变频专用电缆。
现在敷设：2根 185平方变频电缆 + 1根 120平方的变频电缆。
长度：25米
这样用三相电流会不平衡，但最坏情况下电流将怎样分配（在接触良好，材料相同，长度相同，绝缘等级也相同情况下）？
请各位不吝赐教，谢谢！
一般在施工中不采用这种方式，大多为相同截面的电缆并联使用；
从理论上说，二根电缆的电阻随截面成反比，当并联使用后，会按截面比分担电流；但由于我们传送的是交流电流，交流电流存在着“集肤效应”，电缆的额定载流量与截面并不严格地成正比，故可能存在二根不同截面电缆并联后，一根过热的情况；
从你说的总电流为780A算，送电距离也不算很长，两根185的电缆也能够传送，就不必再并上一根120的了。
从理论上说，二根电缆的电阻随截面成反比，当并联使用后，会按截面比分担电流；但由于我们传送的是交流电流，交流电流存在着“集肤效应”，电缆的额定载流量与截面并不严格地成正比，故可能存在二根不同截面电缆并联后，一根过热的情况；
从你说的总电流为780A算，送电距离也不算很长，两根185的电缆也能够传送，就不必再并上一根120的了。
08-12-26 09:08
没错，两根电缆基本上够用了，但裕量小了点。因此我当时才想保留3根。才引出一个讨论，现在大家都觉的有问题，但不知道问题会是怎么样，或是由于实际负载不会到100%等方面的原因，实际运行时不会有问题。
也因为是用于IGBT变频器输出，趋肤效应会更加显著，但我觉得阻抗大的电缆，电流的分配也会更小些，我不理解为什么有些人说小截面的电缆会“抢负荷”，因而先坏掉。不过，虽然电缆设计还有裕量，但我现在也担心起来，我想在运行时候观察一下，如果温度确实太高，就把120 导线去掉，因为没有这方面的理论解释，所以我也想知道会出现什么情况。
也因为是用于IGBT变频器输出，趋肤效应会更加显著，但我觉得阻抗大的电缆，电流的分配也会更小些，我不理解为什么有些人说小截面的电缆会“抢负荷”，因而先坏掉。不过，虽然电缆设计还有裕量，但我现在也担心起来，我想在运行时候观察一下，如果温度确实太高，就把120 导线去掉，因为没有这方面的理论解释，所以我也想知道会出现什么情况。
08-12-26 09:23
这是一个老外的观点：
What the "electrical power supply institute" said is correct.
Everyone knows that electrons follow the path of lest resistance.
The smaller conductor has more resistance/distance than a larger conductor. It should see less current than the larger conductors but more heat too.
This is only part of the story!
"The charge is located more at the point".
Electrons naturally travel toward the surface of a conductor and they will also congregate around the smaller radius of an irregular cross section. The conductor as a whole must be able to dissipate the heat! (I believe that using two conductors of different diameters creates and irregular cross section without being able to share the heat.)
----------------------------------------------
The two large conductors lead in heating up as they do more current is shifted to the smaller conductor increasing it's temp until the point of failure. The "balance" is only good as long as the current doesn't cause the smallest conductor to exceed it's rating (3 times that of the smaller conductor.)
The smaller radius just compress the charge, whether it is moving or static. This wouldn't play apart in our situation. (This only come in to play someone is trying to make or avoid making a spark. As in determining what the insulation requirement for cable at different voltages.)
What the "electrical power supply institute" said is correct.
Everyone knows that electrons follow the path of lest resistance.
The smaller conductor has more resistance/distance than a larger conductor. It should see less current than the larger conductors but more heat too.
This is only part of the story!
"The charge is located more at the point".
Electrons naturally travel toward the surface of a conductor and they will also congregate around the smaller radius of an irregular cross section. The conductor as a whole must be able to dissipate the heat! (I believe that using two conductors of different diameters creates and irregular cross section without being able to share the heat.)
----------------------------------------------
The two large conductors lead in heating up as they do more current is shifted to the smaller conductor increasing it's temp until the point of failure. The "balance" is only good as long as the current doesn't cause the smallest conductor to exceed it's rating (3 times that of the smaller conductor.)
The smaller radius just compress the charge, whether it is moving or static. This wouldn't play apart in our situation. (This only come in to play someone is trying to make or avoid making a spark. As in determining what the insulation requirement for cable at different voltages.)
08-12-26 09:25
楼主说的意思是其中两相是185平方的，另一相是120平方的电缆吗？
不知是你理解错了，还是实际上是这样敷设的
如果是你理解错了，2楼说的就对了
如果真是这样用的，那120的应该变为185的，780A对于120的电缆来说有点大了
不知是你理解错了，还是实际上是这样敷设的
如果是你理解错了，2楼说的就对了
如果真是这样用的，那120的应该变为185的，780A对于120的电缆来说有点大了
08-12-26 09:25
对了，看到上面的问题我也想问一下大家，就是应该是三根电缆并联的，常用解法就是三根电缆的每向并联，但是有人将一根电缆的三相作为一相接在一起，电缆是铠装的，那么是三根并一起的好还是单根的好呢，我们感觉是由于涡流效应，是三根并的好。
08-12-26 09:54
如果不考虑电缆连接点的接触不良风险，不考虑其它浪费、干扰因素；同厂同型号或者说电阻率相同的电缆并连没有什么 问题，如同一根多芯电缆的一根芯与其它芯，你说这一根芯会因为它的纤细而熔断吗？
引用 ljd007241 在 2008-12-26 9:54:00 发言【内容省略】
俺认为三根每相并联的好，一般三相均衡负载，这样能保证每根电缆电流矢量和为0，最大程度减少干扰与涡流效应
08-12-26 16:51
