Oscillating circuit arrangements for high frequency industrial generators

A high frequency industrial generator, especially suitable for supplying power to a device for heating an electric poorly conducting substance such as a fleece of a cellulose material, comprises two hollow concentrically disposed conductors each of square section and supporting respective electrode plates of an associated capacitor. All the plates are vertically disposed and lie at right angles to the walls of the respective conductors. In the middle of the electric plates there is situated a triode electron tube electrically connected to the plates to form an oscillating circuit. The anode potential and cooling water supply for the electron tube is located within a field free space inside the internal conductor. One set of electrode plates can be adjusted relative to the other to adjust the frequency and the arrangement ensures only low electrical losses are incurred.

This invention relates to an oscillating circuit arrangement for high 
frequency industrial generators and has particular reference to a 
generator the output of which is suitable for subsequent use for the 
heating of relatively poorly electrically conducting substances preferably 
in the form of a fleece made of layers, tracks, balls or the like of a 
material containing ligno cellulose and/or cellulose and/or other 
substances; the generator having an electron tube associated with a 
resonant circuit and taking the form of a coaxial conduction system 
integrated with the interdigitated plates of a capacitor which form the 
entire capacity of the system. 
In high frequency electrical generators of this type exacting requirements 
are set in respect of power and frequency stability which up to now could 
only have been met through measures involving high constructional and 
manufacturing costs. 
An oscillating circuit for high frequency industrial generators is known 
from DE-AS 1,075,211 which has the form of a box closed on all sides and 
of rectangular cross-section and which has inside the box a free standing 
surface-like internal conducting member which forms a part of the 
inductance of the oscillating circuit and whose one edge is connected with 
a wall of the box and whose opposite edge carries a transverse plate, 
which together with the wall of the box opposite to the plate, forms the 
capacitor of the oscillating circuit. In this known oscillating circuit 
measures are taken to so couple and decouple the high frequency energy to 
both sides of the internal conducting member that the harmonic radiation 
is kept as small as possible. The principal disadvantage of the known 
oscillating circuit is above all that it has a relatively large 
inductance, because the surface around which the magnetic field circulates 
has a relatively large cross section, and relatively large conducting 
paths exist to the plates of the capacitor which lie opposite to the wall 
of the box, because the surfaces of the plates of the capacitor are at 
right angles to the lines of current in the surface-like internal 
conducting member. With such an arrangement associated relatively large 
losses must be catered for. 
A high frequency oscillating circuit in the form of a tank circuit is known 
from DE-PS 952,186 which is constructed in similar manner to the 
oscillating circuit of DE-AS 1,075,211 and in which measures are taken in 
order to reduce an undesired change of frequency due to unequal heating. 
This is achieved by flexibly connecting together, or to the capacitor, at 
at least one position, the surfaces which in general form the self 
induction of the circuit. The disadvantages described in connection with 
DE-AS 1,075,211 are however also present in this known construction of the 
oscillating circuit. 
An apparatus for dielectrically heating by a separately excited generator 
is known from US-PS 3,169,230 in which measures are taken to automatically 
tune the load circuit to the fixed oscillator frequency these measures 
comprise the cyclic changing of the resonant circuit in the power 
amplifier to achieve a tuning of the load circuit to the fixed frequency 
of the oscillator. The tank circuit used in this circuit arrangement 
comprises a hollow resonator with a cylindrical outer housing closed at 
both ends in which is provided a capacitor arrangement comprising three 
concentric cylinders which are alternatingly connected with the end plates 
of the tank circuit. This tank circuit arrangement, which is not simple to 
manufacture, has the disadvantage of a relatively large self inductance of 
the capacitor as a consequence of the arrangement of the surfaces of the 
capacitor parallel to the respective neighbouring external conducting 
surfaces which corresponds with an electrically unfavourable series 
connection of conducting parts. 
The object of the invention is to provide an oscillating circuit 
arrangement for relatively powerful high frequency industrial generators 
which in spite of relatively low manufacturing cost is characterized by a 
high mechanical and electrical stability and specially high operational 
reliability. 
This problem is solved by the invention in that the coaxial conducting 
system is of square cross sectional form and that the plates of the 
capacitor are respectively arranged at right angles to the associated 
surfaces of the internal and external conducting members of the coaxial 
conducting system. 
By the choice of a square cross-sectional form of the coaxial conducting 
system a resonant circuit of especially high reactive power is obtained 
with an optimal utilization of the entire volume which is available for 
the generator, this is of decisive significance for the required operation 
of the generator at a stable frequency. The chosen arrangement of the 
plates of the capacitor results in a capacitor for the oscillating circuit 
of higher capacity and with extremely low self inductance and low losses. 
This low self inductance of the capacitor comes about because the 
individual part capacitors can be considered as parallel connected 
conducting pieces arranged closely one by the side of the other. The 
inductive portion of the resonant circuit is formed by the inherent 
self-inductance of the capacitor plates and it is, therefore, not 
identifiable as a separate component. 
In contrast with the conventional use of horizontally disposed plates in 
the capacitor the construction of the oscillating circuit of the present 
invention has the additional advantages that, any foreign particles 
finding their way into the apparatus, which in the prior art capacitors 
could be expected to give rise to electrical break down, will fall down 
between the plates into a region of lower electrical field strength and 
can be relatively easily removed. Furthermore the cooling necessary in 
consideration of the high powers is made easier because the plates for the 
cooling medium used which is typically air, stand vertically and offer 
only a small resistance to flow. 
Preferably the electron tube is arranged centrally with respect to the 
coaxial conducting system and lies at least partially in a rectangular 
space surrounded by the plates of the capacitor. 
The central arrangement of the electron tube in the oscillating circuit 
means the connection of the electron tube electrodes with the oscillating 
circuit can be made exceptionally short so that the danger of the 
formation of parasitic resonances is significantly diminished and the 
operational reliability of the generator is thus increased. 
Preferably the simultaneous supply of DC current energy and cooling water 
to the anode is made possible by leading them through the field free 
internal space of the interior conductor and in this manner decoupled from 
parts conducting high frequencies. The necessity of supplying the circuit 
via chokes and the associated increased danger of the formation of 
parasitic oscillations is thus obviated. 
In accordance with an advantageous form of the invention one of the 
electrodes of the capacitor formed of plates vertically upstanding is 
fixed to and electrically connected with the internal conductor whilst the 
other electrode of the capacitor which is electrically connected with the 
external conductor is axially displaceable and can be subsequently fixed 
in position. This constructional form makes a possible a simple and 
accurate adjustment of the resonant frequency of the oscillating circuit 
by changing the position of the movable electrode of the capacitor. 
A further advantageous form of the invention comprises the connection of 
the centrally arranged electron tube via a plurality of symmetrical 
conductors of large areas with the elements of the oscillating circuit 
which control the frequency. 
This form of electrical connection is favourable taking into account the 
extremely low resistances of the reactive elements of the oscillating 
circuit which control the frequency and which are present because of the 
requirement for higher reactive power. 
Preferably the internal conductor and also the external conductor are 
respectively constructed of individual sheet parts which are mechanically 
fixedly connected together by welding soldering or riveting. 
This particular construction of the internal and external conductors brings 
the advantage of a comparatively low constructional cost because the 
respective sheet parts can be accurately manufactured with simple tools to 
the desired dimensions and a plurality of similar constructional parts can 
be used for the construction of the oscillating circuit. 
It is also of considerable significance that a high degree of mechanical 
stability is obtained by the vertical arrangement of the sheet parts of 
electrodes of the capacitor which permits the use of sheet metal parts of 
relatively small thickness without disadvantageously influencing the 
electrical stability of the circuit so that a saving of weight and 
material is achieved. 
In accordance with a specially advantageous form of the invention the 
individual sheet metal parts are provided with lips at right angles at 
their edges and connected at the lips by end face seams. 
The provision of lips for the sheet metal parts brings about a desired 
increase in the mechanical stability and simplifies the mutual connection 
through welding, soldering or riveting whereby lower electrical contact 
resistances are achievable by this kind of construction in contrast with 
the customary clamped connection. 
The lips are so chosen in accordance with a further significant form of the 
invention that each of the four sheet metal parts has only one lip for the 
vertical connection seam and is connected at this lip with the non lipped 
edge of the following sheet metal part. In this manner the required 
cross-sectional geometry can be achieved through simple assembly devices 
in both cross-sectional axes and above all this system can be welded 
without distortion. The use of seams at the end faces means additional 
filler material can be dispensed with during welding. 
Lastly it should also be mentioned that on account of the construction of 
the internal and external conductors from individual sheet metal parts and 
despite the unavoidable tolerances of these individual metal parts the 
necessary cross-sectional dimensions of the internal and external 
conductors can be extremely accurately maintained through the use of 
simple assembly devices, and especially through the use of guide jigs.

Turning now to FIGS. 1 and 2 an oscillating circuit device for a high 
frequency industrial generator comprises a coaxial conduction system of 
square cross section having an external conducting member 1 and an 
internal conducting member 2 with a decoupling space 3 defined between the 
internal conducting member and the external conducting member. 
The entire capacity of the device is formed by vertically disposed plates 4 
and 5 of a capacitor which also run at right angles to the surfaces of the 
internally and externally conducting members 2 and 1, and in which the 
plates 5 are electrically connected to the internal conducting member 2 
and fixedly attached thereto whilst the plates 4 of the capacitor are 
electrically connected with the external conducting member 1 and are 
axially displacable relative thereto and can be locked in position 
relative to the fixed plates 5 of the capacitor so that the resonant 
frequency of the oscillator circuit can be adjusted without problem by 
changing the vertical position of the plates 4 of the capacitor. 
Since the capacitor plates 4, 5 are arranged perpendicular to the external 
conducting member 1 they define rectilinear grids of upper and lower 
plates. FIG. 2 illustrates the rectilinear grid formed by the lower 
capacitor plates 5. The upper capacitor plates 4 interleaf with the lower 
plates 5 and they form a corresponding rectilinear grid of upper plates. 
The construction, which is closed by a screening cover 6, contains a 
centrally arranged triode electron tube 9, the electrodes of which are 
preferably connected by symmetrical multiple connections of large area 8 
with the reactive elements which determine the frequency of the oscillator 
circuit. 
In FIG. 1 the connection for the heater and for the grid potential is 
schematically shown by the reference numeral 10 and the location for the 
supply of cooling water and anode potential is shown by the reference 
numeral 11. A snap closure valve 12 for the water is schematically 
illustrated. 
A space 13 lies radially inwardly of the internal conducting member 2 and 
beneath the plates 4 and 5 of the capacitor and is maintained completely 
field free by the base member 7 of the fixed electrode 5, and can be used 
for housing the DC supply for the anode and the insulated supply tube or 
hose for the cooling water. By this means the supply is completely 
decoupled from high frequency components. 
FIG. 2 shows the prefered construction of the coaxial conduction system 
using sheet metal parts provided with lips and the position and 
arrangement of the vertically disposed fixed electrode plates 5 of the 
capacitor. 
The space 14 inside the plates of the capacitor is of square cross section 
and serves to receive triode electron tube 9 as is to be seen from FIG. 2. 
The prefered form of the connection of the sheet metal parts, whose edges 
are provided with lips and which are used in the construction of the 
coaxial conduction system, is indicated at 15 where a weld in the form of 
a seam along an end face is illustrated. 
The use of the previously described construction does not cause any 
difficulties in operating at powers of 60 kW or more so that operation at 
a stable frequency and the avoidance of the danger of the formation of 
parasitic resonances can be guaranteed and a high operational reliability 
is present. 
While the means for adjusting the vertical position of the electrode 4 of 
the capacitor and for securing it in position have not been shown it will 
be appreciated by those skilled in the art that this needs to be done only 
once to tune the resonant frequency of the circuit to the desired value. 
The means for securing the electrode 4 in position can simply comprise a 
series of spot welds or clamp screws joining plates 4 to the external 
conductor 1 at the double lined position of the drawing as indicated at 
16. 
Adjustment of the vertical position of the electrode 5 can be simply 
effected by lifting means attached through the cover 6 and operable from 
outside. In its simplest form said lifting means could simply comprise two 
headed screw threaded members passing through the cover into respective 
screw threaded fittings on the electrode 5. The heads of the screw 
threaded members bearing on the top of the cover and being freely 
rotatable with respect thereto.