Re: Primary Construction
Subject: Re: Primary Construction
From: MSR7-at-PO.CWRU.EDU (Mark S. Rzeszotarski, Ph.D.)
Date: Tue, 5 Dec 1995 15:39:27 -0500
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>>Quoting Jerry Biehler <jbiehler-at-TELEPORT.COM>:
>> What is the disance I should have between turns?
> 3/8 of an inch, or .375 inches
>This is a pretty good general purpose airspace for primary coils
>operating with 15 kvac rms inputs. You can space a little wider,
>but I would not gap the turns any closer.
>> And what diameter should I make the center hole?
>The secondary coil needs 1-1/2 to 2 inches of space between the
>base of the coil and the first turn on flat helical or inverted
>conical section ("saucer") primary. To allow this room all around
>you need to make the inside turn of the primary between three and
>four inches greater in diameter than the secondary coil. Go a
>little wider still rather than a little smaller. With the length
>of conductor you are going to use, a slightly roomier primary
>design will allow you to fire larger diameter secondary coils
>down the road. It is OK to giver yourself a little room to grow.
I agree with Richard's comments in general. I have been examining
the flat spiral as an inductor, investigating the Q, self inductance and
mutual inductance between a flat spiral and a solenoid. Several
observations deserve mention:
1. For maximum coil Q, the spiral should be wound so that the thickness of
the coil (outer radius minus inner radius) is about one fourth of the outer
diameter dimension. For example, a coil with an outer diameter of 30 inches
built with 3/8 inch copper with 3/4 inch center of wire to center of wire
turns spacing (3/8 inch air space) requires 10 turns of wire. The inside
diameter is 15 inches. I would probably wind it with a few more turns than
this, since Q losses due to the use of large copper tubing are not great,
and additional inductance in the primary is frequently useful for tuning
purposes. 10-15 turns is probably optimal.
2. The inner turns of the primary coil provide little inductance compared
to the outer turns of the coil. The turns not used act like excess
capacitance in the primary circuit, but contribute little to primary losses
in practice. (You might want to tap the primary using the outer turns,
although this alters the coupling significantly.)
3. I keep the inner turn about 1 1/2 to 2 inches away from the secondary
coil, primarily for safety reasons. I don't want my primary voltage arcing
to my RF ground accidently.
4. More often, the coupling coefficient K is more important, so the flat
spiral might be elevated into a saucer shape to increase the mutual
inductance between the primary and secondary. Alternatively, the secondary
can be moved up or down about the primary coil axis to adjust the degree of
coupling. Low coupling yields poor performance. Over-coupling is tough on
the primary components, and can cause spark breakout in the middle of the coil.
5. Wider turns spacing results in increased inductance for a fixed number
of turns, but the effect is minor. It is probably better to use the turns
spacing rule of thumb above provided by Richard, based on voltage breakdown
6. Generally, a "good" primary has an outer diameter about equal to the
secondary coil height for a conventional tesla coil (with about a 3:1
height:diameter ratio for the secondary).
7. Relatively high inductance values of 40-100 uH are desirable in the
primary inductor. Old texts advocating a few turns in the primary are
probably unwise. Otherwise, stray inductance from other wires in the primary
circuit (between the spark gap, coil and capacitor) cause appreciable
losses. This is called off axis inductance, and it contributes to a
lowering of the resonant frequency of the primary, but steals energy from
the secondary and reduces performance.
8. Here are some typical values for expected inductance from 3/8 inch
tubing with 3/4 inch center-center spacing:
inside diameter: 6" 6" 8" 8" 10" 10" 12"
# turns 12 15 12 15 12 15 12
inductance (microhenry) 51 87 63 104 75 122 87
(Note the above inductance assumes the primary is tapped out at the last
turn, which is not typically the case.)
Mark S. Rzeszotarski, Ph.D., Senior Physicist, Radiology Dept.
The Mt. Sinai Medical Center, Cleveland, Ohio 44106
Assistant Professor, Case Western Reserve University
Departments of Radiology and Biomedical Engineering
Voice:216-421-4689 FAX:216-421-5343 E-mail:msr7-at-po.cwru.edu