Inductance vs Ac Resistance?

[jimi]

Can someone please explain the difference between inductance and AC resistance? I'm aware of what inductance and reactance is but not AC resistance. Thanks.
In simple language please

[Joe Gwinn]

Can someone please explain the difference between inductance and AC resistance? I'm aware of what inductance and reactance is but not AC resistance. Thanks.
In simple language please

The best analogy for inductance and capacitance is mechanical: inductance is like mass and capacitance is like springiness. In both cases, energy is stored and released at a different time. This shuttling of energy is the root cause of reactance.

AC resistance is completely independent of inductance, as both inductors and capacitors can have an AC resistance, and this AC resistance is independant of the reactance. How to tell?

With all resistances (AC or DC), the current wave is exactly in time phase with the driving voltage wave. By contrast, with a pure reactance (L or C; no R), the current wave leads or lags the driving voltage wave by 90 degrees.

In inductors, one can measure the DC resistance. This is the minimum possible resistance for that inductor. If one measures resistance using a AC test signal (versus DC), one will get a resistance value that will be larger than the DC value by an amount that's proportional to the square root of the frequency of that test signal. The excess resistance (of AC resistance over DC resistance) represents that part of the test signal that is lost in eddy currents. In an air-core coil at audio frequencies, AC and DC resistances are the same because there is no nearby metal in which to induce eddy currents. Bring a piece of steel nearby, and the AC resistance will rise. If one has a sufficiently powerful test signal, that piece of metal will become warm.

In film capacitors, the DC resistance is usually too high to measure. However, when one tests at AC, one will see a series resistance that varies with frequency (but it's more complex than the square root). At low frequencies, the series resistance is basically the DC resitance of the very thin metal layers that form the capacitor plates. At high frequencies, the dielectric between the plates becomes lossy, and some of the AC signal is lost as heat. (Transmitter capacitors have maximum power ratings for this reason.)

[Just Bob]

AC resistance really isn't good nomenclature. I'd assume you meant the magnitude of the impedance. Impedance is a vector composed of the sum of resistance plus reactance. Resistance is the "real" part i.e. the current is in phase with the applied AC voltage. Reactance is the "imaginary" part i.e. the current is not in phase with the applied voltage.

For an ideal inductor, the magnitude of the impedance ("AC resistance") is 2 x pi x Frequency x inductance. So, the AC resistance gets bigger as either the frequency increases or the inductance increases.

[Dave Kerr]

For an ideal inductor, the magnitude of the impedance ("AC resistance") is 2 x pi x Frequency x inductance.
AC resistance is completely independent of inductance, as both inductors and capacitors can have an AC resistance, and this AC resistance is independant of the reactance. How to tell?

So, who's correct, JB or JG? Or is my feeble non-EE mind just not getting it?

[Joe Gwinn]

AC resistance really isn't good nomenclature. I'd assume you meant the magnitude of the impedance. Impedance is a vector composed of the sum of resistance plus reactance. Resistance is the "real" part i.e. the current is in phase with the applied AC voltage. Reactance is the "imaginary" part i.e. the current is not in phase with the applied voltage.

For an ideal inductor, the magnitude of the impedance ("AC resistance") is 2 x pi x Frequency x inductance. So, the AC resistance gets bigger as either the frequency increases or the inductance increases.

No. AC Resistance is the in-phase component, not the vector sum of resistance and reactance. The only difference between DC resistance and AC resistance is the frequency of the test signal.

For an operational definition, AC resistance is what one measures as "resistance" when measuring inductance using a Maxwell-Wein Impedance Bridge (or an Extech 380193 LCR Meter). For the bridge, see http://home.comcast.net/~joegwinn/.

The Extech is the only ~$200 handheld LRC meter that is known to work correctly with pickups, which have very high resistance for their inductance.

[Just Bob]

Technically, AC resistance is skin effect resistance and other assorted loss mechanisms. But, I didn't think that's what the original post was talking about. Any resistance is the in-phase component. Z = R+jX . Both R (the in-phase component) and X can be frequency dependent. AC resistance is still a confusing term and is misused extensively. Should be "equivalent resistance" or "effective resistance".

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

[Joe Gwinn]

Technically, AC resistance is skin effect resistance and other assorted loss mechanisms. But, I didn't think that's what the original post was talking about. Any resistance is the in-phase component. Z = R+jX . Both R (the in-phase component) and X can be frequency dependent. AC resistance is still a confusing term and is misused extensively. Should be "equivalent resistance" or "effective resistance".

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

Ahh, well, I think "AC resistance" is pretty clear, and with terms like "equivalent resistance" or "effective resistance" people will always ask something to the effect of "equivalent to what?". So, I keep it simple, in parallel to DC resistance, which the pickup world is very familiar with (albeit called simply "resistance").

[jimi]

So the AC resistance is all losses that don't cause a phase shift like the resistance of the copper and eddy current losses etc ?
I guess to measure this the meter would have to ignore all phase shifted signals?

[Joe Gwinn]

So the AC resistance is all losses that don't cause a phase shift like the resistance of the copper and eddy current losses etc ?
I guess to measure this the meter would have to ignore all phase shifted signals?

Yes to both.

I suspect that the problem with most LCR meters is that they assume that the AC resistance is small compared to the reactance, and compute the inductance by assuming that the measured impedance (being the magnitude of the vector sum of resistance and reactance) is all reactance. In other words, they ignore phase, which simplifies the design of the meter. This works well enough with relatively pure components like transformer and inductor windings, but fails miserably with guitar pickups.

As for AC Resistance, it turns out that the term is widely used in audio. Go to http://www.audioholics.com/techtips/...ples/index.php and seach for "AC resistance".

[jimi]

How much difference is there usually between AC resistance and DC resistance?

[Joe Gwinn]

How much difference is there usually between AC resistance and DC resistance?

With an air-core coil, or a coil wound upon a ceramic magnet, with no metal (aside from the coil wire) nearby, they are essentially equal. The more metal nearby, the more magnetic and/or conductive the nearby metal, the higher the AC resistance; the DC resistance is unaffected.

[R ski]

Ac Dc resistance

In a tube amplifier, the output transformer, deals with both DC resistance
and AC resistance. DC resistance of the tranny governs, the current
flow, that is determined by the output tube biasing. Ohms law, can be
used to determine, the current flow through the tranny, in terms of bias current.

AC resistance, is better known as impeadance. Since your output tranny
is subject to fundemental frequency of your guitar note. The impeadance
is effected by frequency, resonance, and speaker characturistics. Because
of these impeading factors, the impeadance would be done on a graph
with the frequecy vs impeadance. If you were to look at speaker review
in audiophile magazines, they illustrate the impeadance from 20 -20000 hz.
The graph has dips and peaks simular to oil price trading.

So if you were to measure a typical output tranny with a ohm meter, the
speaker side appears as less than one ohm. The tube side would probably
appear as 60 ohms, even though the manufacture specs 6000 ohms
impeadance.

Some ampagers gave a good discription of determining actual impeadance
in this forum some time ago

[David Schwab]

With an air-core coil, or a coil wound upon a ceramic magnet, with no metal (aside from the coil wire) nearby, they are essentially equal. The more metal nearby, the more magnetic and/or conductive the nearby metal, the higher the AC resistance; the DC resistance is unaffected.

OK, I have a question that I haven't been able to find an answer to. I had mentioned over at the MIMF forum that an air core and a ceramic magnet core coil are pretty much the same as far as inductance. I have head this numerous times. Someone there pointed out that ceramic materials, like the kind used for ferrite beads, have high permeability. So he claimed that even though they are not ferrous, they increase the inductance of the coil. OK that makes sense for ferrite beads, but what about ferrite magnets?

Recently he made a humbucker that use ferrite beads for pole pieces, and on top of each of those he had a neo magnet. He only wound the pickup to about 5k, but had a pretty low resonant peak like you would from a strat pickup. he claims this is because of the ceramic poles. (he had some computer generated plots of resonant peaks he did).

Obviously metal parts also produce counter-emf/eddy currents, but I wonder if you can effect the inductance (as in magnetic coupling) without increasing the AC resistance?

[Just Bob]

You need to look at the BH curve of a magnetic material. Beyond a certain flux level, the material is saturated and further increases of the magnetic field will cause much smaller increases in the flux. The small-signal permeability is much lower. A magnet is generally pretty close to saturation. (excepting alnico or another material that has been "tempered"). Magnetized material in saturation will have less effective permeability than if it is demagnetized. If not saturated, depends on the details of the BH curve. Using ferrite beads with rare-earth magnets may or may not saturate the beads. Depends on the type of ferrite and the external magnetic circuit.

[David Schwab]

OK, I thought magnets would be different, so that makes sense.