cable cross section for a dc motor and an ac motor

[Kataeb]

hello,
one small question:
Is it the same cable cross section for a dc motor and an ac motor power supply?
thank you

 

[juad]

hi kataeb
no it is not the same because in dc current we did not have the skin effect as in altearnating current,so the same cross section area of cable can carry dc current more than ac current if all other phesical effect is the same also it is the same if the cable is solid or it is strandede if the current is dc.

 

[DickDV]

The skin effect only becomes significant at much higher frequencies.

To figure the ampacity of any wire, consult the published tables. These are usually part of electrical safety codes in each country and may vary a little from country to country.

 

[juad]

dickdv
please if there is no skin effect can you answer me why is the current carrying capacity for solid cable is differ from the stranded cable in alternating current(if both in same cross section area) ??? i find that in BS STANDARED for current carrying capacity. thanks

 

[Mickey]

dickdv
please if there is no skin effect can you answer me why is the current carrying capacity for solid cable is differ from the stranded cable in alternating current(if both in same cross section area) ??? i find that in BS STANDARED for current carrying capacity. thanks

He didn't say no skin effect, just that an increase in frequency
makes it worse.

http://en.wikipedia.org/wiki/Skin_effect

 

[leitmotif]

dickdv
please if there is no skin effect can you answer me why is the current carrying capacity for solid cable is differ from the stranded cable in alternating current(if both in same cross section area) ??? i find that in BS STANDARED for current carrying capacity. thanks

Given that the outer diameter of a stranded conductor is the same as that of a solid conductor, the solid can have higher current carrying capicity because there are no gaps between the individual conductors of a stranded.

The stranded conductor cross sectional area is the sum of the individual strands' cross sectional area.

Dan Bentler

 

[Mickey]

Given that the outer diameter of a stranded conductor is the same as that of a solid conductor, the solid can have higher current carrying capicity because there are no gaps between the individual conductors of a stranded.

The stranded conductor cross sectional area is the sum of the individual strands' cross sectional area.

Dan Bentler

I disagree:

Stranded vs. Solid Wire

This one is a bit of a mind-boggler, but it's important. When electricity flows through a wire, it mostly flows on the surface of the wire, not through the middle. This effect is more pronounced on high frequency AC than it is on DC or low frequency AC. This means that a "wire" of a given size that made up of many smaller strands can carry more power than a solid wire - simply because the stranded wire has more surface area. This is one reason why battery cables in your car and welding cables are made up of many very fine strands of smaller wire - it allows them to safely carry more power with less of that power being dissipated as heat. However, this "skin" effect is not as pronounced in a typical 12V DC automotive application, and the wire and cable used there is stranded for flexibility reasons.

 

[DickDV]

Well, this subject came up in another BBS recently and I'm not going to get dragged into another discussion on it except to say that, when you lay a bunch of bare copper wires parallel to each other and then compress them tightly together with an insulation jacket so all of the "skins" are shorted to each other, there are effectively no individual wire "skins" to talk about---they are all shorted together into a single conductor.

As to why there is a difference in ampacity (is there?) between solid and stranded wire of the same gauge, I haven't a clue.

Anyone on here that works for a wire company?

 

[leitmotif]

Given that the outer diameter of a stranded conductor is the same as that of a solid conductor, the solid can have higher current carrying capicity because there are no gaps between the individual conductors of a stranded.

The stranded conductor cross sectional area is the sum of the individual strands' cross sectional area Dan Bentler

LET ME REPHRASE THAT
Given that the outer diameter of a stranded conductor is the same as that of a solid conductor, the solid can have higher current carrying capicity because there are no gaps
AS YOU WOULD SEE
between the individual conductors of a stranded.

The stranded conductor cross sectional area is the sum of the individual strands' cross sectional area.

Dan Bentler

 

[Kataeb]

According to the link http://en.wikipedia.org/wiki/Skin_effect (by the way Wikipedia is amazing) the skin effect is negligible for low frequencies (at 60 hz, copper skin depth is 8.47 mm). So I think it is the same cable cross section for dc motor and 50 hz ac.
Thanks

 

[BryanG]

I know that solid cables are rated to carry higher current than flexible, I can't remember if the same applies to stranded. The main difference between solid and non-solid cables it that you add air into the cable, air is a thermal insulator and so heat won't be able to move out of the cable as effectively. The same for anywhere that you can't loose heat so quickly, the cable will have to be de-rated. The effect with stranded will be much less than with flexible.

Bryan

 

[rdrast]

Ultra-high strand count cabling is used for batteries and welders (and locomotives, etc) simply because of two characteristics.

The first, is the fine strands more closely approximate the total cross sectional area of a solid lead, and secondly because they can actually flex repeatedly in a safe manner.

Solid wire does not handle bending well, causing failures.

Please just STOP worrying about the "Skin Effect" at low frequencies. The typical formula for the 37% point of:
mils = 2837/sqrt(Freq_in_Hz) does not tell the whole story. Even if it did, you would be looking at cables in the 750MCM range before you could even find the calculated 37% point of load carrying.

The skin effect does show up drastically as you move to higher and higher frequencies. It is very important to consider when working with RF especially, but the reasons even there aren't as simple as you might imagine.