EP3145275A1 - Induction heating coil - Google Patents

Abstract

The invention relates to an induction heating coil. This induction heating coil according to the invention has a first inner winding W 1 and at least one further outer winding W 2. In this case, the inner winding W 1 has a radius r 1 and the outer winding W 2 has a radius r 2. r 1 is smaller than r 2. Furthermore, the inner winding W 1 is located within the outer winding W 2. The windings W 1 and W 2 are connected at a position 100 at one end of the coil, so that at this point 100, a current flow i through the coil changes its sign in the z-direction.

Description

The present invention relates to an improved induction heating coil which is particularly suitable for heating low temperature plasmas.

State of the art

Electromagnetic coils are in electrical engineering windings and winding goods, which are adapted to generate or detect a magnetic field. They are often part of an electrical component or device, such as a transformer, relay, electric motor or speaker. An important application of coils is the field of inductive heating of electrically conductive materials. Coils usually comprise at least one winding of a conductor. This consists for example of wire, enameled copper wire, silver-plated copper wire or high-frequency stranded wire. Coils are often often wound on a bobbin (coil carrier). The bobbin can be either solid or hollow. Hollow bobbins may comprise a core of another material (e.g., a soft magnetic material). The winding arrangement and shape, the wire diameter, the winding and the core material determine the value of the inductance and other (quality) properties of the coil.

The operating principle of coils is based on the fact that when they are traversed by the electrical alternating current, a magnetic field B builds up in their interior. This causes an electric field E, which is an acceleration causes the charge carrier inside the coil. This field essentially comprises the two components E _φ and E _z when the coil is a cylindrical coil. The first component E _φ causes the charge carriers are accelerated on a circular path perpendicular to the main coil axis. The main coil axis is also referred to as the z-axis. The plane of this perpendicular to the main coil axis is also referred to as in the azimuthal plane. The second component E _z causes an acceleration of the charge carriers along the main coil axis.

The electrical properties of coils are greatly determined by the way the electrical conductor from which they are wound is wound up. Important here is the resulting geometric structure of one or more turns or layers. This structure is also called winding.

In long coils ( Fig.1 ), whose length l is usually greater by at least a factor of 10 than their diameter, the field lines of the magnetic field of the coil are substantially parallel. It follows that the electric field resulting from the magnetic field essentially comprises only one component E _φ . The component E _z and thus the acceleration along the main axis of the coil can thereby be neglected. It follows that the charge carriers are accelerated on a fixed circular path and do not move in a helical movement towards the coil end.

In many applications, this idealization does not apply. In these cases, the component E _{z of} the electric field can not be neglected. This is especially true for short coils whose length is not significantly greater than their diameter. These often only involve a few turns. In these cases Charge carriers inside the coil also experience an acceleration along the z-axis. ( Fig. 3 )

Coils that serve as induction heating coils usually comprise only a few windings and are to be regarded as short. This means that charge carriers inside the coil usually experience a non-negligible acceleration in the z direction. If the material to be heated inside the coil consists of condensed matter, i. it is present as a solid or liquid, the acceleration along the z-axis is irrelevant, since the charge carriers of these materials can not leave the coil.

This is not the case when plasmas, in particular low-temperature plasmas, are produced and heated inductively. Here, the charge carriers are freely movable. This is the case, for example, with radio-frequency ion engines (RIT).

Here, the charge carriers can leave the induction heating coil and thus be lost to the heating process. This reduces the efficiency of the heating process. So far, the influence of the escape along the coil main axis and thus not contributing to the heating charge carriers is achieved by increasing the injected electromagnetic energy. However, this solution is energy-consuming and lowers the efficiency of the induction heating coil.

task

Proceeding from this, the object of the invention is to provide an induction heating coil for inductive heating, which has a reduced acceleration of the charge carriers in it along the z-axis.

Solution of the task

This object is achieved by a device having the features of claim 1. Advantageous embodiments and further developments of the present invention will become apparent from the dependent claims.

The induction heating coil according to the invention has a first inner winding W ₁ and at least one further outer winding W ₂ . In this case, the inner winding W _{1 has} a radius r ₁ and the outer winding W _{2 has} a radius r ₂ ( Fig. 3 ). r ₁ is smaller than r ₂ . Furthermore, inner winding W ₁ lies within the outer winding W ₂ . The windings W ₁ and W ₂ are connected at a point 100 at one end of the coil, so that at this point 100 a current flow i through the coil changes its axial direction in the z-direction.

This causes the components E _z and E _{-z of} the resulting electric field to cancel along the z-axis. Due to the superposition of the currents at each point, in addition, a larger field strength E _{φ is} effected. This means, on the one hand, that less charge carriers leave the coil along the z axis during inductive heating and, on the other hand, that higher heating power is achieved with the same current intensity.

The radii r ₁ and r _{2 of} the windings W ₁ and W ₂ are preferably 10 mm to 100 mm.

The length of the induction heating coil is preferably between 10 mm and 100 mm.

The number of turns N of the windings W ₁ and W _{2 of} the induction heating coil is preferably 3 to 20, more preferably 5 to 10. Coils with smaller numbers of turns have a larger z-component of the resulting electric field. With larger numbers of turns, the efficiency of the induction heating coil decreases due to the greater electrical resistance.

In particularly advantageous embodiments of the induction heating coil, this has a rectangular, round or ellipsoidal shape. This allows an adaptation of the coil to the bobbin. Furthermore, such a regular shape of the coil leads to a more homogeneous field inside the coil.

A use of the induction heating coil according to the invention is in inductively or inductively capacitively excited ion or electron sources. Here it serves the coupling of electromagnetic energy.

These sources contain a gas (e.g., xenon) and high frequency electrons within an isolated vessel, the discharge vessel. To the discharge vessel, an induction heating coil for feeding a necessary for plasma excitation high-frequency energy is wound. The current flow in the coil results in a magnetic field, which accelerates electrons inside the coil. These electrons now encounter the gas atoms or gas molecules in the discharge vessel and ionize them.

By damping the electric field component acting parallel to the z-axis, the inductively-coupled plasma is heated more efficiently because relatively more field energy is induced in the azimuthal plane and is not lost by the acceleration along the z-axis. This causes the through The electric fields accelerated electrons are forced on spiral tracks whose axial slope is getting smaller. The latter causes a greater proportion of the accelerated electrons to remain within the coil and assist in the heating of the plasma.

In a further embodiment, a radio frequency ion motor (RIT) comprises the induction heating coil according to the invention. In a radio-frequency ion engine, a high-frequency plasma to be stimulated within an insulated vessel, the so-called discharge vessel. To the discharge vessel is often referred to as a coupling coil induction heating coil for feeding a necessary energy for plasma excitation. The plasma is thus inside the coil. When an ion engine incorporates the induction coil of the present invention, it is more efficient because almost all of the electrons in the plasma discharge help to maintain it because they are prevented from leaving the heated areas.

The Induktionsheizspule invention causes the axially induced fields in the z-direction due to the symmetry almost cancel, since the same current flows in both the positive and in the negative z-direction. That is, the component E _{z of} the resulting electric field is attenuated. Since the current flows through both windings of the induction heating coil in parallel, the resulting field components E _φ overlap at the same time and are thus amplified. In addition, due to the increasing coil inductance L, a lower current I current is needed to provide the magnetic field. This favors the thermal behavior of the coil, since smaller currents lead to less ohmic power dissipation. This is despite almost twice the length the current-carrying conductor of the coil of the case, since the power loss to P = I ² R, linear to the resistance R and thus to the length of the current-carrying conductor grows, but at the same square with the smaller required current I decreases.

The radius of the outer winding W ₂ is greater than the radius of the inner winding W ₁ , thus the field induced by W _{2 is} somewhat smaller in magnitude than that induced by W ₁ . Nevertheless, the component E _{z of} the resulting electric field is negligibly small.

Embodiment:

An exemplary embodiment is an ion engine comprising the coil according to the invention as induction heating coil.

Comparative measurements were taken on a first unmodified RIM-4 ion engine with an induction coil with conventional winding and another modified RIM-4 mod ion engine. each with an inventive induction heating coil at an ion current I _b = 10 mA at a flow rate of the gas used of 0.2 sccm made. The comparison values of the electrical parameters and the performance data are here Fig. 5 refer to. The difference between unmodified and modified engine with respect to the required coil power P _s is 1.5 W at a total power P _s of 15.23 W. Thus, the engine, which comprises the induction heating coil according to the invention, about 10% efficient.

Less power is needed to heat the plasma as fewer electrons are lost. Also of great importance is that with about half the current amplitude I _c can be worked, which also leads to a better electromagnetic Compatibility (EMC) leads. Thus, the other engine components are less affected by the induction heating coil.

Brief description of the illustrations

• Fig. 1 shows a long coil. The field lines of the internal magnetic field are parallel over a wide range of the length I here. This causes the charge carriers in the interior to accelerate along the z-axis only in the peripheral regions of the coil.
• Fig. 2 shows a typical short coil with few turns. In this type of coil, the charge carriers experience acceleration along the z-axis. In the case of an inductively heated plasma within the coil, these charge carriers would leave the coil.
• Fig. 3 shows the induction heating coil according to the invention. In this coil, the forces acting on the charge carriers cancel each other along the z axis due to the opposite winding. As a result, freely movable charge carriers remain in the coil. Due to slightly different radii of the two coils, the fields do not completely cancel. To a good approximation, however, it can be assumed that the charge carriers experience almost no acceleration in the z direction and the electrons disappear on the axis.
• Fig. 4 shows in part of Figure 4a, the axially induced in the z-direction electric field strength component E _{z of} a conventional single-wound induction heating coil in an ion engine and in sub-image 4b the axial field strength component induced axially in the z direction of a coil according to the invention.
• Fig. 5 shows the operating parameters of the ion engine RIM-4 and the modified ion engine RIM-4 mod. η _m here denotes the mass efficiency, η _el the electrical efficiency, R _s the electrical resistance of the induction coil, H _s the magnetic field strength of the coil, ζ the coupling factor and t the transmission efficiency.

Claims (7)

Induktionsheizspule characterized in that it comprises a first inner winding W ₁ and at least one further outer winding W ₂ , wherein the inner winding W _{1 has} a radius r ₁ and the outer winding W _{2 has} a radius r ₂ , wherein r _{1 is} smaller than r ₂ , wherein the inner winding W _{1 is} within the outer winding W ₂ and the windings W ₁ and W _{2 are connected} at one end of the coil at a location (100) so that at this point (100) there is a current flow i through the coil changes its flow direction with respect to the z-axis of the induction heating coil. Induction heating coil according to claim 1, characterized in that the radii r ₁ and r _{2 are} between 10 mm and 100 mm. Induction heating coil according to claim 1 or 2, characterized in that the length of the induction heating coil is between 10 mm and 100 mm. Induction heating coil according to one of claims 1 to 3, characterized in that it has a rectangular, round or ellipsoidal shape. Induction heating coil according to one of claims 1 to 4, characterized in that the number of turns of the windings W ₁ and W _{2 is} between 3 to 20. Radio-frequency ion motor (RIT), characterized in that it comprises an induction heating coil according to one of the preceding claims. Use of an induction heating coil according to any one of claims 1 to 5 for the coupling of electromagnetic energy in an inductively or inductively capacitively excited ion or electron source.

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