AVR Freaks

Mains frequencies and high currents require low resistance wiring. Thus bigger cross section and even current distribution is desirable, so for flex cables stranded wire is used. Welding for instance. Fixed connections usually are made from thick flat copper or alu buses.

I will do an inspired guess. I am sure that there are qualified people out there.

Once you start having > 500kW motors you turn to higher supply voltages. You definitely start having high voltage distribution around your site.

1000A is relatively hefty current at 440V. The copper size is not that unmanageable. But I bet you use stranded cables.

Where you start getting serious currents s with low voltages.

Anyway, surely you look up in the cable tables for the power rating of your motors. Then do a swift sum on the length of cabling. You will have massive inrush currents but the run currents will not heat the cables too much. You design for the inrush current.

David.

I do not know what this means. You normally rate motors in kW (or horsepower for US citizens). A horsepower is about 0.75kW. So if your 'cv' is horsepower, this means motors from 38kW to 225kW.

If you are installing these motors, there are tables for the recommended wire cross-section-area. The motors and switchgear come with the overload and starter gear.

I very much doubt that skin effect will make any difference at 60Hz. Even at 2.5kHz it will be trivial.

It sounds very much to me like the gold knobs and 'solid silver wire' for ultimate hi-fi quality.

I am sure you can do a lengthy mathematical analysis. Otherwise, you look in the tables, do some rough calculations on the back of the cigarette packet. Add a good safety margin and drive down to your electrical wholesaler.

David.

Brunoi,

in that section You popinted to the telephone cable is specified in terms of resistance per 1000 feet of length as a function of frequency.

Please note:

Frequency resistance per 1000 feet

1Hz------------52.50
1KHz-----------52.51
10KHz----------52.64

for a change in frequency in the order of FOUR ORDERS OF MAGNITUDE you have a change in resistance of 1%

In summary insignificant change in resistance with frequency at the lower end of frequencies ( audio frequencies)

It helps to illustrate CHANGE in resistance with frequency.

As for eddy current losses they are more the domain of laminated magnetic path rather than copper conductors.

Why do you have different cables?

Surely you have 3 identical cables for each phase. You can measure the individual phase currents with a clamp meter. Each phase should be about the same. If not then you have a serious problem.

I presume that you mean that you have 50m run of 3-phase 400mm2 from your supply to the VFD and a 10m run of 2x120mm2 from the VFD to the motor.

The VFD run will be at 2.5kHz and the supply run will be at 50Hz.

If the 400m2 cable has the 2.5kHz, then I am very surprised by the difference in temperature. I would expect 50Hz to have trivial skin-effect. Even 2.5kHz should be no problem.

Replace the 400 mm2 by 120 mm2 cables. The scrap value should go towards paying for the smaller cables.

David.

Sounds like the run from VFD to motor uses solid wire for a portion of the run, and stranded for another portion.

The fact that two significantly different sizes of cable are used indicates that there could be confounding factors to the correlation between cable temperature and power loss within that cable. For instance, the first segment of the run might be larger because it runs through a higher ambient temperature area (and therefore was derated). If that's the case, then it could also explain why that part of the run is getting hotter.

Plus it's hard to trust a touch-test comparison--perception of temperature is not very consistent or accurate.

Bruno,

when You do the calculation for rectangular conductor make sure You do take into consideration corner effects.

Conformaly map the rectangle into upper half plane ( using Greens theorem) solve for current distribution and map back into real space.

It would be interesting to see just how significant the effect is at 60Hz