Note: Descriptions are shown in the official language in which they were submitted.
~77379
-1
Danfoss A/S, Nordborg (Denmark)
High frequency ignition transformer for
oil or gas burners
The invention relates to a high frequency ignition transformer for
oil or gas burners, comprising at least one primary coil on the
low voltage side and one secondary coil on the high voltage side.
A known ignition transformer of this kind is fed by pulses of a DC
transistor-oscillator with a frequency of about 20 kHz and is
additionally provided with a feedback winding of the transistor-
oscillator. To operate the transformer with such a high frequency
has the advantage that it can have a small construction in com-
parison with an ignition transformer operated at mains frequency.
However, the hi6h frequency can lead to the power of the ignition
spark being inadequate for igniting heavier gases or atomized or
vapourized heavier combustion oil because the ignition current is
limited by the unavoidable stray inductances of which the impedance
is correspondingly high~because of the high frequencies.
It is the problem of the invention to provide an Ignition trans-
former of the aforementioned kind which permits a higher ignition
current and thus more certain ignition to be obtained while retain-
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ing its small construction.
Specifically, the invention relates to an ignitionsystem for generating a spark for lighting a combustible fluid.
The system includes a high frequency voltage supply, an ignition
transformer having in its input connected to tho voltage ~upply
and its output connected to two electrode~ defining a spark gap.
The ignition transformer includes a magnetic core ha~ing an
elongate rod-like core member with opposed ends not in flux
communication with any further magnetic core members, a low
voltage primary coil means arranged coaxially on the core member
intermediate its ends and being connected to the high frequency
voltage supply, and high voltage secondary coil means arranged
coaxially on the core member to either side of the primary coil
means and being substantially co-extensive with the core member
and the aggregate axial length of the secondary coil means ~o
that, in use, on pa~sage of current across the spark gap on
generation of a spark, the flux genera~ed by the secondary coil
means and pa6sing wholly through the secondary coil mean3 is le~s
than the flux generated by the secondary coil meanæ and passing
wholly through the primary coil mean~, whereby the effective
inductance of the secondary coil means is reduced and the
ignition current is relatively large.
According to the invention, this problem is solved in
that the primary coil and the ~econdary coil are in ~he form of
coaxial multi-layer coils and the axial length of the
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secondary coil is a multiple of the axial length of the primary
coil.
With this construction, the entire magnetic flux
produced by the secondary coil flo~s through the primary coil~
In this way, the secondary ~tray i.nductance be~omes practically
negative upon occurence of the ignition spark, so th~t the entire
impedance become~ smaller and thus a larger ignition current i5
obtained.
Preferably, the primary coil is of annular disc form
and the secondary coil is subdivided into annular disc-shaped
sections symmetrically disposed to both sides of the primary
coil. With this subdividing of the ~econdary coil, one obtains
greater safety against a high voltage flash-over b~tween the
individual coil sections. The primary coil and the annular
disc-shaped sections of ~he secondary coil may be disposed in a
respective annular chamber of an insulating coil carrier
comprising a plurality of axially juxtaposed annular chamber
This permits one to obtain in a simple manner the annular di~c
form for the primary coil ~nd secondary coil sections while
simultaneously increasing ~he resistance to flash-overs between
the sections of the secondary coil.
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Preferably, the halves of the secondary coil disposed to both
sides of the primary coil lie close to the primary coil. In this
way, a still stronger ignition spark can be achieved.
In another embodiment, the secondary coil may be axially wound
throughout and the primary coil may surround the axial middle of
the secondary coil.
The ignition transformer may comprise a rod-shaped ferrite core
which permits a particularly small construction to be obtained for
the transformer.
; .
The invention and its developments will now be described in more
detail with reference to the drawing, whereln:
Fig. 1 shows the basic principle of a transformer with a primary
coil comprising only one winding and a secondary coil comprising
only one winding.
'
Fig. 2 is a diagrammatic representation of an example of ignition
transformer according to the invention.
Fig. 3 shows a more detailed example of a transformer according to
the invention.
Fig. 4 is a no-load equivalent circuit of a transformer according
to the invention, a~d
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Fig. 5 is an equivalent circuit of a transformer according to the
invention when producing an ignition spark.
For a better understanding of the invention, reference will first
be made to the basic principle of a transformer as shown in Fig.
1, wherein the primary coil 1' has only one winding and the second-
ary coil 2' likewise has only one winding. If a current flows
only in the primary coil 1', it produces a magnetic flux 011 which
passes through the entire primary coil 1' but only part of the
secondary coil 2'. If th0 part of the total flux 011 flowing
through the secondary coil is designated 012 and the part of the
total flux 011 not flowing through the secondary coil 2' is
regarded as stray flux 01s' then:
011 01s + 012 (1)
If a current flows only in the secondary coil 2' then the total
flux 022 produced by the secondary coil can likewise be subdivided
into a part flux 0z1 flowing through the primary coil 1' and a
stray flux 02s~ so that:
022 = 02 ~ ~Z1 (2)
In the example of an ignition transformer according to the inven-
tion as illustrated diagrammatically in Fig. 2, the primary coil 1
comprising a plurality of windings and the secondary coil 2 are in
the form of cylindrical multi-layer coils, the primary coil
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being axially much shorter than the secondary coil 2 and surround-
ing the secondary coil 2 substantially at its axial middle.
Whereas in the transformer according to Fig. 1 the stray fluxes
01s and 02 are both always positive, in the example of Fig. 2 it
happens that, in the case of a secondary short circuit as occuring
in practice when producing an ignition spark, the flux 022 passing
through the entire secondary coil is only part of the total flux
021 produced by the secondary coil 2 but this total flux 021
passes completely through the primary coil 1 and part of the flux
021 of the secondary coil 2, namely the stray flux 02 ~ passes
through only part of the secondary coil 2. After transposing the
Equation (2), one obtaines:
'
02s = 022 ~ 021 (3)
Since 021 is larger thar 022' the stray flux 02s is negative.
In the equivalent circuit of Fig. 4 for no-load on the secondary
side and the equivalent circuit of Fig. 5 for secondary short
circuit (production of an ignition spark), an indication is given
of a stray inductance L1 corresponding to the stray flux 01s and
the stray inductance L2 corresponding to the stray flux 02s in
conjunction with a main or transverse inductance L.
On production of an ignition spark corresponding to a short circuit,
the following equation applies according to Fig. 5 to the short
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circuit current Ik referred to the primary side (having regard to
the winding ratio):
K - L + L2 ~4)
According to Equation (4), the short circuit current Ik referred
to the primary side is larger than the current I on the primary
side because the stray inductance LZs on the secondary side is
negative. Although the number of turns of the secondary coil 2 is
thus substantially higher than that of the primary coil 1, because
the ignition transformer is a high voltage transformer, the igni-
tion current on the secondary side can be extremely large on
production of an ignition spark and it becomes all the larger the
more negative is the stray inductance L2S.
The ignition transformer illustrated in Fig. 3 correspond~ in
principle to the ignition transformer of Fig. 2 because here, too,
the secondary coil 2 is axially much longer than the primary coil
1 and both coils 1 and 2 are wound in layers. The equivalent
circuLtq shown in Figs. 4 and 5 therefore also apply to the Fig. 3
ex_mple, the stray inductance L2 11kewLse being negative and the
ignition current Ik on the secondary side referred to the primary
side is very much larger than the current I on the primary side.
The coilsl and 2 are, however, wound on a coil carrier 3 made from
insulating material~ The coil carrier 3 is substantially rod-
shaped and has a plurallty of coaxial annular chambers 4 and 5
juxtaposed in the axial direction of the coil carrier 3, the
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annular chambers 5 being axially narrower than the single annular
chamber 4 disposed at the axial centre of the coil carrier 3 and
being symmetrically disposed to both sides of the more central
annular chamber 4. Seven annular chambers 5 are shown to both
sides of the central annular chamber 4. However, the number of
annular chambers 5 may be smaller or larger than illustrated so
that the total axial length of the annular disc-shaped sections 6
of the secondary coil 2 disposed in the annular chambers 5 and
thus the axial length of the secondary coil amounts to a multiple
(about twice to five times) of the axial length of the primary
coil 1 disposed in the central annular chamber 4. The halves of
the secondary coil 2 disposed to both sides of the primary coil 1
are separately wound and the axial inner ends of the two coil
halves are led outwardly and are then interconnected so that one
obtains a secondary coil ~ with a central tapping ~not shown).
The axial outer ends of the primary coil 1 and the secondary coil
2 are likewise constructed as terminals but are not illustrated
for the sake of simplicity.
The central annular chamber 4 contains a further coil 7 which is
wound above the primary coil 1, serves as a feedback coil and, in
the associated eiectronic ignition device, is connected to the
base of the transistor of a DC fed transistor-oscillator which
operates the ignition transformer with high frequency pulses of
about 20 kHz.
All coils 1, 2 and 7 are wound in layers coaxial to the longi-
tudinal axis of the coil carrier 3, and a rod-shaped ferrite core
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9 is disposed in an axial throughgoing hole 8 of the coiL carrier
3.
The wall thickness of the walls 10 bounding the central annular
chamber 4 is so selected in the axial direction of the coil carrier
3 that the axial spacing a of the primary coil 1 from the adjacent
coil halves of the secondary coil 2 is so small that the secondary
stray inductance L2S has as high a negative value as possible and
therefore produces a correspondingly stronger ignition current.
Subdividing of the secondary coil 2 into a plurality of annular
disc-shaped sections 6 has the advantage that the ignition trans-
former can deliver on the secondary side a very high ignition
voltage of about 15 kV without fear of flash-over, as is required
for the burner of a boiler. The transformation ratio of the
transformer can then be about 50 to 60.
The entire ignition transformer may be embedded in plastics resin
so that only the seven terminals (not shown) project from it.
