GB2126059A

GB2126059A - Multilayer cylindrical induction coil

Info

Publication number: GB2126059A
Application number: GB08321076A
Authority: GB
Inventors: Dieter Schluckebier
Original assignee: Otto Junker GmbH
Current assignee: Otto Junker GmbH
Priority date: 1982-08-06
Filing date: 1983-08-04
Publication date: 1984-03-14
Also published as: FR2531596A1; FR2531596B1; GB8321076D0; DE3229374C2; DE3229374A1; GB2126059B

Abstract

A multilayer induction coil comprises a conductor 50 embodied by two electrically insulated partly overlapping and identical conductor sectional profiles 50a, 50b. A serving 52 holds the conductor profiles together. Each profile has a cooling duct 51a, 51b to ensure rapid and uniform cooling of the coil windings. <IMAGE>

Description

SPECIFICATION Multilayer cylindrical induction coil This invention relates to a multilayer cylindrical induction coil for heating or melting metal articles placed in its interior, with provision for cooling the at least one conductor of the coil.

An induction coil of this kind was disclosed, for instance, in Report No. 4, of Section lla of the Eighth Congress of the Union Internationale d'Electrothermie, entitled "The Theory of Multi Layered Windings for Induction Heating and their Application to a 1 MW, 50 Hz, Longitudinal Flux Billet Heater" by I. G. Harvey. This Report states how multilayered induction coils can be of higher efficiency than single-layered induction coils.

However, a problem arising with multilayered coils relates to conductor cooling. The known induction coil is cooled externally-i.e., the insulated coil windings are disposed axially and radially apart from one another and the complete coil or a number of component coils are disposed in a water-tight tank with an inflow and outflow of water, the water which flows through the between-conductors gaps being intended to provide the required cooling.

However, since cooling is required to be very uniform, the known induction coil Is unsatisfactory since considerations of flow technique make it virtually impossible to provide uniform cooling at every part of the coil. Local overheating of the coil may occur and possibly lead to the destruction of the coil insulation. Also, the cooling water may damage the insulation in the course of time, with the resulting risk of flashover in the windings, and, therefore, of the destruction of the coil. Yet another disadvantage is that the power factor is relatively low due to the considerable distance which the cooling system calls for between the article and the inside of the coil. The relatively complicated cooling facility makes repair work complicated. Dismantling the coil is a time-consuming operation.

It is the object of the invention to provide an induction coil of the kind specified which is of simple construction and high efficiency.

According to the invention, therefore, the cooling provision takes the form of a cooling duct which extends through the conductor lengthwise thereof. It has long been known for the conductor of an induction coil to have a hollow duct through which a preferably liquid coolant flows. However, the conventional thinking in the case of multilayered induction coils of the kind concerned in the present connection has basically been that the radial thickness of the conductors must not exceed 3 mm. For instance, it is stated on page 9 of the Report referred to previously that internally cooled conductors could be used instead of the external cooling using a water-tight tank but that it is impossible to use such a coil in most cases because of the need for the radial thickness of the conductors to be at most 3 mm.It is further stated in the Report that internal cooling is not worthwhile because internally cooled conductors of such a reduced cross-section are expensive and the reduced throughput of water would make cooling unsatisfactory. It is known that eddy current losses of a coil increase with increasing conductor thickness. According to the invention, the radial thickness of the conductors is more than 3 mm. Preferably, the radial thickness of the conductors is from 7.5 to 8.5 mm. The eddy current losses associated with conductors of this order of thickness are appreciable, but it has surprisingly been found that the compact coil construction made possible by the cooling according to the invention leads all round to a relatively high efficiency, so that eddy current losses caused by the increased conductor thickness are more than compensated for.

The induction coil according to the invention enables the insulated conductors to be wound directly one on another to give a coil of relatively reduced overall dimensions. The power factor is relatively high because the distance between the article and the coil interior is very small. The efficiency of an induction coil of the kind of interest here depends inter alia upon the copper factor, which denotes the extent to which the coil cross-sectional area is actually taken up by the copper conductor. The closer this factor is to unity, the better is the performance and the better is the electrical efficiency of the coil. The copper factor of the coil according to the invention is very near unity, at least radially of the coil.

Efficiency is much improved as compared with the known single-layered coils. One particular improvement on the multilayered induction coils hereinbefore referred to is that the coil according to the invention is more compact-i.e., it has smaller external dimensions. The risk of local overheating and of possible flashovers between conductors is obviated. Since no special water tank need be provided for the coolant, the coil can be repaired rapidly whenever necessary.

Another and substantial advantage of the invention is that the multilayered induction coils according to the invention can be used for existing induction heating facilities without special outlay. This rapid and therefore low-cost conversion of existing facilities is impossible with the known external cooling hereinbefore described.

As previously mentioned, the invention provides relatively high efficiency. However, it would be desirable to further increase efficiency by a far-going reduction of eddy current losses.

According to the invention, therefore, the conductor is embodied as a multielement conductor combined from a number of conductor sectional profiles, more particularly as a twoelement conductor combined from two such profiles. Embodying the conductor as a multielement conductor with electrical insulation between the elements or sectional profiles contributes considerably to reducing eddy current losses when the coil is in operation.

According to a special feature of the invention, the two-element conductor is in cross-section substantially rectangular or oval and the sectional profiles, each of which has a cooling duct, overlap at least to some extent and are insulated from one another electrically in the overlap zone. The cross-section of the conductors embodied by a number of sectional profiles is preferably sub stantialiy rectangular, as in the case of the known conductors, so that a compact coil winding is possible. It is theoretically possible, as hereinbefore described, for the cross-section of the complete conductor to be of solid copper with a duct in it, but the development described is better when the duct in the conductor, just as in the case of the solid conductor, is of relatively large crosssection but the conductor is of reduced height.

When two conductors of the latter kind are combined, the relatively thin zones of the conductor overlap to provide overall a substantially rectangular cross-section. Although the conductor sectional profile, is, therefore, basically only 3-4 mm thick, the hollow duct is of relatively large cross-section, thus ensuring adequate cooling. Independent protection is sought for this particular construction of the conductor in which the sectional profiles overlap one another at least to some extent.

The overlapping feature proposed to reduce eddy current losses and improve cooling can be embodied in various ways. According to a feature of the invention, the two profiles engage with one another meshingly by way of their narrow sides distal from the respective cooling duct, more particularly by way of a groove in one profile and of a tongue in the other, In addition to the required partial overlapping of the sectional profiles, this feature also ensures that the two profiles are held together satisfactorily.

According to another feature of the invention, the two profiles are symmetrical and co-operate to form a substantially rectangular or oval crosssection. The two symmetrical sectional profiles when joined together form preferably a rectangle.

In one possible embodiment of the symmetrical profiles, the profiles are wedgeshaped, the cooling duct being disposed in the profile end which is remote from the wedge apex.

In an embodiment which is more advantageous from the production engineering point of view, the profiles are stepped on one side.

In a preferred embodiment of the invention distinguished by very low eddy current losses, the profiles have a portion which extends ring-fashion around approximately 75% of the cooling duct and which merges into a rib having a pointed end, so that two such profiles disposed at a 1 800 offset from one another co-operate to form a substantially closed rectangular surface. The cross-section of this embodiment of the invention can be compared with two figure nines placed one beside another with one the opposite way around to the other.

The two sectional profiles which form a conductor can in theory be electrically interconnected as required; preferably, however, the profiles embodying a conductor are connected in parallel.

For the conductor sectional profile according to the invention to be used optimally in an induction coil, the winding thereof must be such that the compact construction is in fact possible. This is ensured more particularly when the coil winding is devised as described in the Applicants' patent application, serial No. P filed on the same day. According to the present invention, therefore, the or each conductor of the coil forms one or more coil segments each embodied by two oppositely wound multilayer pancake windings disposed one beside another, the inner layers of the two pancake windings being interconnected, preferably by way of a bridge or link or the like which is at an inclination to the plane of the pancake windings.A winding of this kind is suitable for conductor sectional profiles which are fairly thick and wide, since the special nature of the winding is such as to make sharp kinks or bends unnecessary.

It has been found that eddy current losses can be reduced if the conductor is of rounded, preferably semi-circular, cross-section on at least one of its narrow sides. This featurej.e., that the conductor is semicircular preferably on its two narrow sides-helps to achieve an aproximately 10% improvement in electrical efficiency.

Embodiments of the invention will be described in greater detail hereinafter with reference to the drawings wherein: Figure 1 is a side view of a part of an induction coil arrangement; Figs. 2a to2c are each a view in cross-section of a coil conductor having a cooling duct; Fig. 3 is a view in cross-section of a conductor sectional profile having a cooling duct; Fig. 4 is a view in cross-section of two conductor sectional profiles combined to form a coil conductor, and Figs. 5 to 7 are views in cross-section of various embodiments of coil conductors.

Fig. 1 shows some of a multilayered cylindrical induction coil, for use for heating a metal article 1 which extends through the coil interior. The coil part shown in Fig. 1 comprises a coil segment2 embodied by two oppositely wound pancake windings 3, 4. There are a total of five winding layers in the embodiment shown. The inner two layers of the pancakes 3, 4 are directly interconnected by way of a bridging element or link or the like 5, the same extending at an inclination to the plane of the two pancakes 3, 4.

The led-out ends of the outer layers are offset from one another-i.e., offset inwards and outwards from the plane of the illustration. The electrical connections are made by way of lugs 6, 7. Spigots 8, 9 for water are disposed at the ends of the coil conductors.

The complete coil is embodied by a number of segments2 disposed side by side and close together.

A description will now be given, with reference to Figs. 2 to 7, of various embodiments of coil conductors (hereinafter called "conductors") which are used for the pancake winding described with reference to Fig. 1.

Fig. 2a shows a very simple and low-cost embodiment of a conductor. The conductor 20 is of solid copper formed by a known and conventional process with a cooling duct 21 To insulate the windings from one another the conductor 20 can have around it in known manner an electrically insulating serving 22. The conductor20 is 8.5 mm high and 40 mm wide.

When the coil is in operation water flows through the duct21 to provide adequate and uniform cooling of the various coil windings. In this embodiment, that narrow side of the conductor 20 which is on the right in Fig.2a is flat whereas that narrow side of the conductor20 which is on the left is of semicircular cross-section. This rounding of the conductor cross-section reduces eddy current losses.

Figs. 2b and2c show other embodiments of a conductor according to the invention; they differ from the conductor 20 of Figs. 2a in that the conductor cross-section of both the narrow sides is semicircular. The conductor20' of Fig.2b has a substantially oval cooling duct21' which is disposed symmetrically in relation to the conductor cross-section. The cooling duct2 1" of the conductor20" of Fig. 2c is in the left-hand half of the conductor cross-section.In the embodiments which still remain to be described, at least one narrow side of the conductor is preferably semicircular, in order to ensure very reduced eddy current losses, Tests on a six-layer coil wound from a conductor of 8.5 mmx23 mm cross-section have shown that the electrical efficiency of a conductor of completely rectangular cross-section is 0.52, as compared with 0.57 for a conductor whose narrow sides are rounded.

A multilayered cylindrical coil constructed as shown in Fig. 1 can have a relatively high efficiency if embodied by means of the conductor shown in Figs.2a to2c. However, due to the relatively large radial thickness of 8.5 mm, eddy current losses are still appreciable; they can be reduced very considerably by means of the conductor sectional profiles shown in Figs. 3 to 7.

Fig. 3 shows a conductor sectional profile 30 having a cooling duct 31. Two such profiles 30 are combined, at a 1 800 offset from one another, to form a conductor 40 in Fig. 4. The two profiles are held together by an electrically insulated serving 42 and are separated from one another by an electrically insulating layer 43.

Fig. 5 shows a variant which is preferred for reduced eddy current losses. In contrast to the embodiment of Fig. 4, which is of substantially rectangular cross-section, the cross-section of the embodiment of Fig. 5 is substantially oval. A conductor 50 is embodied by two sectional profiles 50a, 50b which are disposed at 1 800 offset from one another and which in the state shown form a substantially closed cross-section.

The two profiles 50a, 50b are isolated from one another electrically by an insulating layer 53. As can be gathered from Fig. 5, the two profiles 50a, 50b are of identical construction to one another and are arranged symmetrically of one another.

The profile 50a, for instance, has a portion 54 which extends ring-fashion about 75% of the way around the cooling duct 51a and which merges into a rib 55 nearly half as thick as the complete conductor 50. The rib end distal from the cooling duct 51 a is pointed, for adaptation to the ringshaped portion of the profile 50b.

A serving 52 holds the two profiles 50a, 50b together. As Fig. 5 shows, the thickness of either of the overlapping profiles 50a, 50b is nowhere greater than half the thickness of the complete conductor 50. This feature ensures high coil efficiency because of low eddy current losses in the conductor.

In the embodiment of Fig. 6 a serving 62 holds together two identical profiles 60a, 60b having cooling ducts 61 a, 61 b to form a conductor 60.

An insulating layer 63 separates the two profiles 60a and 60b from one another.

Referring to Fig. 7, two conductors sectional profiles 70a, 70b engage meshingly to form a conductor 70. Profile 70a is formed with a groove 76 in which a tongue 77 of the profile 70b engages with the interposition of an insulating layer 73. This tongue and groove construction holds the two profiles together satisfactorily, retention being further improved by a serving 72.

The invention was tested on a test ring using the conductor shown in Fig. 5. The.induction coil consisted of a total of 1 5 component coils of the kind described with reference to Fig. 1. With six layers of windings, therefore, there were twelve turns per component coil. Coil diameter was 0 mm, coil length was 720 mm and the coil conductor has a radial thickness of 8.5 mm and an axial width of 46 mm. The coil thus devised was used to maintain at a temperature of 1200C a water-cooled hollow copper block having a diameter of 165 mm and a length of 650 mm.

The electrical power was 122 kW. The throughput of cooling water per cooling duct of each conductor sectional profile was 02 m3/h. The inlet water temperature was 9.30C and the outlet water temperature was 1 9.20C. The current was 1165 A, the voltage per turn 3.35 V and the electrical resistance was 1.70x10-3.

The dissipation of the total coil was found to be 34.77 kW and the power factor was 0.175.

These data give an electrical efficiency of 0.71 5.

By way of comparison, the electrical efficiency of single-layered internally cooled induction coils is about 0.425.

The invention is not limited to the embodiments hereinbefore described. Other different conductor sectional profiles which help to reduce eddy current losses can be used; for instance, as a variant of the embodiment shown in Fig. 7, the two sectional profiles can be formed with a number of grooves and tongues, a feature which further reduces eddy current losses. The multielement conductors need not necessarily be two-element conductors and three or more sectional profiles can be combined to form a single conductor. A disadvantage in such cases, however, is that the various sectional profiles are not of identical construction, and so various sectional profiles must be available to produce a conductor, a factor which increases the costs of the complete facility.

The induction coil according to the invention is of use not only for heating metal articles but also for melting ovens.

Claims

1. A multilayer cylindrical induction coil for heating or melting metal articles placed in the interior thereof, with provision for cooling at least one conductor of the coil, which cooling provision takes the form of a cooling duct which extends through the conductor lengthwise thereof.

2. A coil according to Claim 1, wherein the radial thickness of the conductors is in the range of from 7.5 to 8.5 mm.

3. A coil according to Claim 1 or2, wherein the conductor is a multielement conductor combined from a number of conductors sectional profiles.

4. A coil according to Claim 3, wherein the conductor is a two-element conductor combined from two conductor sectional profiles.

5. A coil according to Claim 4, wherein the two-element conductor is, in cross-section, substantially rectangular or oval and the sectional profiles, each of which has a cooling duct, overlap at least to some extent and are insulated from one another electrically in the overlap zone.

6. A coil according to Claim 4 or 5, wherein the two profiles engage with one another meshingly by way of the narrow sides thereof distal from the respective cooling duct, more particularly by way of a groove in one profile and of a tongue in the other.

7. A coil according to Claim 6, wherein the two profiles engage by way of a groove in one profile and of a tongue in the other.

8. A coil according to Claim 4 or 5, wherein the two profiles are symmetrical and co-operate to form a substantially rectangular or oval crosssection.

9. A coil according to any one of the preceding claims, wherein the profiles are wedge-shaped, the cooling duct being disposed in the profile end which is remote from the wedge apex.

10. A coil according to any one of Claims 3 to 8, wherein the profiles are stepped on one side.

11. A coil according to any one of Claims 3 to 8, wherein the profiles have a portion which extends around approximately 75 percent of the cooling duct and which merges into a rib having a pointed end, so that two such profiles offset by 1 800 from one another co-operate to form a substantially closed rectangular or oval cross-section.

12. A coil according to any one of the preceding claims, wherein the profiles embodying a conductor are connected in parallel.

1 3. A coil according to any one of the preceding claims, wherein the or each conductor of the coil forms one or more coil segments each comprising two oppositely wound multilayer pancake windings disposed one beside another, the inner layers of the two pancake windings being interconnected, by a bridge or link or the like which is at an inclination to the plane of the pancake windings.

1 4. A coil according to Claim 1 3, wherein the inner layers of the two pancake windings are interconnected by a bridge or link or the like which is at an inclination to the plane of the pancake windings.

15. A coil according to any one of the preceding claims, wherein the conductor is of rounded cross-section on at least one of the narrow sides thereof.

16. A coil according to Claim 15, wherein the conductor is of semicircular cross-section.

1 7. A multilayer cylindrical induction coil substantially as hereinbefore described with reference to, and as shown in, any Figure of the accompanying drawing.

1 8. Any novei feature or combination of features described herein.