Magnetic effects can wreck havoc with some switchmode inductor designs too,
particularly any design that handles AC and has a gap in both the middle and
outer poles.

----- Original Message -----
From: "Jack Smith" <Jack.Smith@COX.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 28 April 2003 18:14
Subject: Re: [OT]: Safety issues: AC vs DC


> Skin effect can be important at 60 Hz.
>
> Westinghouse's "Electrical Transmission and Distribution Reference Book,"
> 3rd ed (1944) notes:
>
> "For small conductors this ratio [skin effect] is entirely negligible, but
> for larger conductors it becomes quite appreciable and must be considered
> when figuring the 60-cycle resistance of large conductors."
>
> The following is extracted from the table associated with the foregoing
> text:
>
> Circular Mills                  Diameter (inches)               Skin Ratio
> --------------                  -----------------               ----------
> 300 000                         0.630                           1.006
> 800 000                         1.031                           1.046
> 1 000 000                               1.152
1.068
> 2 000 000                               1.631
1.239
> 3 000 000                               1.998
1.439
>
>
> The skin ratio is the ratio of 60 Hz resistance to theoretical DC
> resistance. 1,000,000 CM cable and larger is not uncommon in power
> transmission.
>
> In addition to skin effect, there is something called the "proximity
> effect," where close spaced cables, cables in proximity to steel plates,
> cables inside conduits, etc. have magnetic field distortions that force
the
> current into non-uniform distribution.  Proximity effect increases the
> effective resistance, in some cases by a large factor.
>
> In addition to skin effect, proximity effect is also found in radio
> frequency inductors where it causes the effective inductance and Q deviate
> from values calculated under uniform current sheet assumptions.
>
>
> Jack
>
>
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