不同截面电缆并联问题

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！

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。

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.)

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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.)

是不同截面电缆并联运行，即每相都是2根 185+ 1根 120，是3+3的变频专用电缆。屏蔽层两端接地。