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Re: AC coil resistance equation



You are so right.


-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Friday, August 28, 1998 7:25 PM
Subject: Re: AC coil resistance equation


>Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
>
>James and all,
>
>Calculating Rac ahead of time is not trivial, since there are many
>effects at work that can make the AC resistance of a coil markedly
>different than the DC resistance. Quite honestly, it's much easier to
>simply measure it with a signal generator and oscilloscope AFTER you've
>made the coil. For those who want to grind through the calculations and
>charts, probably the best single reference is Frederick Terman's "Radio
>Engineer's Handbook", McGraw-Hill Book Company, 1943, 1019pp. Refer to
>the section dealing with copper losses for air-core coils, pages 74-80.
>There appear to be no closed-form equations that cover all of these
>losses. While this treasure is, unfortunately, out of print, it may
>still be found in libraries and hamfests, and is available for sale on
>the Web. Check out:  http://www.mxbf-dot-com/ OR
>http://www.bibliofind-dot-com/cgi-bin/texis.exe/search.vor
>
>Physically measuring Rac is best done by connecting a non-inductive
>resistive decade box in series with the base of your resonator (toroid
>in place), and driving it with a low impedance sinusoidal RF signal
>generator. A batch of low valued carbon resistors can also be used, or a
>low resistance pot (with some loss of accuracy if it's wirewound). By
>setting the signal generator to match the resonant frequency of the
>resonator and toroid pair, you can then carefully adjust the series
>resistance so that the voltage drop across the series resistance is
>exactly HALF of the applied voltage.
>
>At resonance, the impedance of the resonator drops to the AC resistance
>(Rdc plus any additional AC resistance due to skin and proximity
>effects). By adjusting the series resistance so that HALF of the applied
>voltage is across the series resistance, you've now set Rseries = Rac.
>BTW, do not try to measure the voltage at the top of the toroid, since
>the loading will really mess up your other measurements. Now carefully
>remove the series resistance and measure its value (or read it if using
>a decade box). Depending upon the coil and operating frequency, Rac will
>typically be in the range of 3-5X Rdc.
>
>One other note: To get a better estimate of the non-breakout Q of your
>secondary system, you'd need to add the groundpath resistance of your
>resonator's base ground to the above value of Rac in order to get an
>estimate of the overall "effective" Rac of the resonator/toroid/ground
>system. Using this value in the denominator for the equation for Q will
>get you much closer. A poor grounding system will significantly reduce
>Q, AND your system's performance.
>
>Safe coilin' to you!
>
>-- Bert --
>
>Tesla List wrote:
>>
>> Original Poster: "bmack" <bmack-at-frontiernet-dot-net>
>>
>> From: "bmack" <bmack-at-frontiernet-dot-net>
>>
>> ----------
>> > From: Tesla List <tesla-at-pupman-dot-com>
>> > To: tesla-at-pupman-dot-com
>> > Subject: Re: AC coil resistance equation
>> > Date: Thursday, August 27, 1998 9:46 PM
>> >
>> >
>> > From: "James" <elgersmad-at-email.msn-dot-com>
>> > "JAMES" wrote:
>> >" I thought the AC resistance equation for coils was 2 pi freq(in Hertz)
>> H(in
>> > Henrys)."
>> >
>> > James:
>>
>> You have the equation for inductive reactance, Not the resistance!
>> They are very different. I don't know how to explain it without going
>> into a long disertation that may only confuse you.
>> Resistance and reactance combined is called impedance.
>> Impedance is the pathagrean sum of the two.
>>
>> Z=sqrt(R^2+X^2)
>>
>> Z= impedance
>> R=resistance
>> X=reactance
>>
>> Unloaded Q, Qu=X/Rac  -This determines the Quality factor of the coil.
>> Energy is lost via heat due to the Rac ( real, resistive part).
>>
>> An impedance also will cause a phase shift between E and I
>>
>> But then there's conjugates and resonance ....etc.
>>
>> Like I said, it can get involved.  I suggest you curl up with a
>> basic electronics book to fill in the blanks. Then we can help
>> take the rough edges off the experience
>>
>> Jim McVey
>
>