Not replying to any single post here:
Considering only resistance (okay, a motor is largely inductive - it makes the maths slightly trickier, but there is a significant resistive element to the motor and the basic premise remains)
Simply, given the same conductor type, 0.75mm2 cable has a higher resistance per unit length than 1.5mm2. Similarly, 1.5mm2 has a higher resistance per unit length than 2.5mm2. That fact is irrefutable.
The power dissipated in the cable run is proportional to the resistance.
Also, the voltage drop across a cable run is proportional to the resistance. The higher the resistance, the greater the voltage drop across the cable run.
The voltage drop across the cable run is proportional to the square of the current drawn. So, under load, with greater current draw, the voltage drop across the cable run increases at the square-law of the current, and implicitly, the load.
The voltage at the spindle is the terminal voltage of the VFD, minus the voltage drop across the cable run.
The mechanical power provided by the spindle is proportional to the terminal voltage of the VFD.
So, the lower the CSA of the cable run, the lower the voltage at the spindle. This drop increases as the load increases. This impacts directly the available power of the spindle. Cable CSA is related to performance.
Of course, the standard cable ratings easily found on the interweb don't consider the performance of the spindle, only the safe operating mode of the cable under domestic and industrial applications - given the heating effect of the dissipated energy on the cable, and - importantly - the longevity and integrity of the insulation. You might find that a particular cable works, but that the insulation fails after prolonged use.
The spindle manufacturer will provide a design specification for the spindle. This /should/ (I have to provide that caveat for our illustrious Chinese manufacturers who are not unknown to put anything that sounds good in the manual without reference to reality) allow the spindle to operate as designed. It is not unreasonable to expect then, that if you compromise that design, you'll compromise the performance of the spindle.
Finally (I need to get back to the bottle of wine), don't design a system assuming any strands on a multistranded cable will fracture - at that point the mechanical design of the system has already failed and catastrophic failure of the cable will occur in short time.
An analogy. If an architect specified a 152x89 RSJ. Would you chose to replace this with a 127x76 RSJ that you had lying around? It might well work, the building would probably stay upright after you've finished it. Hey!, the architect is clearly a fool.
Last edited by Doddy; 27-05-2016 at 11:14 PM.
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I might be a bit fick but did you just say "Fatter cable will make spindle go faster"?
Last edited by lucan07; 28-05-2016 at 12:00 AM.
Right. Well... this has all taken quite a turn. Who'd have thought all hell would break loose several months later. I'm the OP and just to justify my choice of cable, I must admit I had no idea what these cables looked like in real life, so getting the lightest possible seemed like the right thing to do. It's working very well, and since there still are forum voices that say it's okay, I'll stick with it. I do understand that more is better in these things, but it's all installed and, I'd like to think, behind me.
It's been interesting, though, and thanks to all. This is a great and helpful forum!
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