On Thu, 08 Dec 2005 02:22:25 GMT,
JS
wrote:
On Wed 07 Dec 2005 18:24:48, Victor Roberts
wrote:
But you must never use the cable whilst tightly coiled up
(unless you have deliberately overspecified its capacity). A
coiled cable has a much higher AC impedance due to induction and
can overheat alarmingly.
I have seen a
coiled 13A extension cable melt with a 3kW load! Typically a 9m
extension rated 13A unwound is derated to only 8A when fully
wound.
A coiled cable does not have enough inductance to matter at
50 Hz. In addition, inductance does not generate heat, only
resistive loss can do that. High frequency coils will have
higher skin effect losses than straight wires operated at
the same high frequency, and this can generate extra heat
since the skin effect will decrease the effective area of a
conductor and therefore increase its resistance. However,
there is little to no skin effect at 50 Hz.
Coiled cords get hotter than equivalent straight cords
simply because the heat generated by the resistive losses
cannot escape as easily.
I have often wondered about how much of an effect my very neat
coils of mains flexes for my PC was having! And I really do seem
to have a lot of devices which plug into the mains.
What about the magentic or ERF effects of a tightly coiled main
flex? Say, 12 neat-ish turns with a diameter of 5 or 6 inches
.... How much of a nuisance might such a thing be to electronic
equipment like my PC?
As has been said, the net magnetic flux far from the coil is
zero since you have two identical currents flowing in
opposite directions. The local effects are also just about
zero since the frequency is only 50 Hz. If this were 50 MHz,
then there would be turn-to-turn coupling and additional
heating.
--
Vic Roberts
http://www.RobertsResearchInc.com
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