Re: Calculation of Q?
Re: Calculation of Q?
Thu, 13 Mar 1997 07:00:09 +0000
"John H. Couture" <couturejh-at-worldnet.att-dot-net>
Tesla List <tesla-at-pupman-dot-com>
>> Steve Falco
> TESLAC calculates coil Q by first determining the RF (not DC!!)
>of the wire used in the winding, and then dividing that value into the
>reactance of the winding at the coils self resonant frequency. This is
>classical method of Q calculation for series resonance. Then the Q value
>optained is "derated" in a attemp to compensate for the coil form,
>the primary and such other things that reduce the effective Q of the
> TESLAC calculation of Q is NOT absolute, and no other programs
>is either, they are best used in comparisons between one coil design and
>another. The main thing I've found in high Q coils is LOTS of inductance
>coil diameter, it rather seems that any wire size that will allow 800 to
>turns close wound at a H/D ration of 2.5 to 3.5 to 1 will result in a
>secondary coil in a standard tesla coil.
> Mark Graalman
Steve & Mark -
The Q factor is a function of Q = Xl/R and several other electronic
equations. The Xl is easy to find but the R is not. The R for Tesla
consists of three parts, DC resistance, AC resistance, and the
due to corona and electric fields around the Tesla coil.
The Rdc and Rac resistances are well covered in radio books. However,
third resistance is not mentioned. It has been found that this R varies
about 100 ohms for small TCs to about 1800 ohms for large TCs. These
have been determined by calculation of coils with known values of Q
and Xl reactances. Note that this R is equal to the impedance Z times
cosine of the phase angle. R is also equal to the X reactance divided by
tangent of the phase angle. The proper R value must work in all of these
A graph of these Rs is shown in my books. This R must be coordinated
other electronic parameters such as power factor, dissipation, log
decrement, VSWR, etc. When the above Rs are used the arbitrary
modifications, etc are not required.
The secondary Q factor of a TC is important because it should be
with the operating spark gap timing. A high Q factor could mean that
secondary dampened wave would interfere with the spark gap firing
shorter spark length.
It is interesting to note that Tesla said in the CSNotes that the
coil Q factor should equal the secondary coil Q factor for his large
The JHCTES printout of his coil is shown in my TC Notebook and it also
Qp = Qs. This is a rare occurance for the program. I found this amazing
correlation years after the program was made. I do not know if this is
coincidental or Tesla looking over my shoulder when I was working on the
program. Please take it easy on the jokes.