Why does the resistance of an inductor vary with frequency?

[ragavcit]

Hi,
How does an INDUCTOR AND A STRAIGHT WIRE differ?how come the resistance offered by an inductor changes with frequency??

 

[berkeman]

This should help as an introduction:

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

And it's the inductor's "impedance" that changes with frequency, not its "resistance". Welcome to the PF, BTW.

 

[Enthalpy]

Any length of circuit has an inductance. Now, when designing an inductor, you may want special characteristics, for instance a higher inductance, especially at frequencies <1GHz.

A first step is to wind wire in the form of a coil. The consequence is that turns interact with another, in an additive way because currents close to another are in the same direction. Then, the same current flowing in N turns creates a vector potential A which is multiplied by N. The time variation of A gives the potential gradient along the wire, which is also multiplied by N. And as the potential sums over N turns, the inductance is multiplied by N².

Another step is to use a magnetic core, which helps the induction flow. As the flux increases, so does the inductance.

Designing an inductor is really nothing obvious. Constraints can be radically different: frequency, maximum current, AC or mainly DC, high voltage, precise value... So materials, forms, performances differ a lot.

Resistance (the real or lossy part of the impedance, the imaginary being the reactance) also varies with frequency. Causes are plentiful and difficult to find and evaluate. The magnetic core, if present, is an important one. Skin effect can be another. Radiation can exist. Less obvious, you can have eddy currents within the wires themselves.

Very few books are good for inductors. As the company still existed, RTC edited a very good guide for inductor design - probably impossible to find now. You may try at other core manufacturers.

Radio-ham books often contain chapters for inductor design. They make more sense than academic books.

 

[Akaldine]

just as a matter of interest. in many RF circuits , big inductors are not useful. so they just "draw" a length of PC board conductor track on the pc board and it serves as an indcutor. smith chart techniques become useful.