This invention relates to induction heaters for metal strip.

In process lines for thermal treatment of continuously moving metal strip by transverse flux induction heating it is usual for successive coils of strip to vary in nominal width, one from another, over a range of 2:1 or more. Moreover it is common for the width of strip to vary gradually by small amounts within the length of any given coil.

The requirement to obtain uniform heating in strips of different widths can be addressed by providing interchangeable inductors, each designed for a specific width. In a production line of typical width spread this may entail the provision of as many as eight or more inductors, which is clearly uneconomic in utilisation of equipment.

The objective of the present invention is to obtain uniform heating in any width of strip using a single inductor with variable-width capability. Such an inductor should preferably be capable of adjusting its width whilst under power; also it is preferred that width adjustment should be stepless and should not entail movement of major parts of the main inductor core or windings, since this may lead to premature failure under the severe conditions of vibration and heat which are encountered in this application.

It is known that in an inductor for the heating of metal strip by transverse flux induction a desired distribution of heating effect across the width of the strip can be obtained by the addition to the inductor poles of auxiliary pole pieces of magnetically permeable material (flux modifiers) which serve to enhance the magnetic flux in the vicinity of the edges of the strip, and also by the provision of electrically conductive shields

(shading plates) which are positioned generally outboard of the edges of the strip so as to screen the outlying portions of the inductor.

Accordingly a first feature of the present invention provides a transverse flux induction heating system for metal strip, comprising at least one inductor having a fixed pole assembly, and a pair of adjustably positioned auxiliary pole pieces of magnetically permeable material each of which is positioned towards one side of the path of the strip to modify the distribution of the induction field.

A second aspect of the invention provides a transverse flux induction heating system for metal strip, comprising at least one inductor having a fixed pole assembly and a pair of adjustably positioned shading plates each of which is positioned towards one side of the path of the strip.

According to a further aspect of the invention there is provided an adjustable transverse flux induction heating system for metal strip, comprising a fixed pole assembly, and at least one movable assembly including an auxiliary pole piece and/or a shading plate, at each side of the strip path.

Preferably, the carrier assembly includes both an auxiliary pole piece, and a shading plate, and also includes means for adjusting their relative positions on the carrier.

Accordingly, a preferred embodiment of the invention comprises a transverse flux induction heating system for metal strip including a fixed pole assembly and at least one transversely movable carrier at each side of the path of the strip, which is slidably mounted for movement across the strip path, and carries a shading plate; first actuator means for controlling the position of the carrier; an auxiliary pole piece mounted in the carrier, and second actuator means for moving the pole piece in the carrier.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figures la and lb are schematic cross-sectional views of an induction heating system showing adjustments for different width strip;

Figure 2 is a view as seen looking at the face of the strip of an induction heating system according to the invention; and

Figure 3 is a perspective view of a part of the apparatus of Figure 2.

Referring to the drawings each shading plate (2) and flux modifier pole piece (4) is provided with an individual positioner (6, 8). The design takes into account the conditions experienced in an inductor of substantial power rating, where the shading plates and flux modifiers are subject to complex electro-magnetic forces of considerable magnitude and are also subject to constant vibration and to heat. This necessitates a form of construction which is both rugged and vibration resistant and at the same time suitably compact so that it can be accommodated in the restricted space available between the two halves of the inductor.

A system of runways of substantial construction shown in the illustration as slideway channels (10) fabricated from insulating materials having suitable strength and heat resistant properties, is attached to the inner face of each half of the inductor. The runways are arranged so that a shading plate (2) mounted in a sliding carrier (12) is positioned above each end of each pole face (14) . An actuating device (6) . the outer end (16) of which is anchored to the main frame (18) of the inductor, can traverse the shading plate from its outermost position to its innermost position. In the preferred arrangement shown, this range of movement being at least half the difference between the maximum and minimum widths of the strip widths

under consideration. The actuator can be a manually operated adjuster or, where remote control is required, it may be an electric positioner, or similar hydraulic or pneumatic device, of suitably long stroke.

Housed within the shading plate carrier (12) is the associated flux modifier (4) ; this itself is mounted on a sliding or rolling member which can move within the carrier to adjust the flux modifier position relative to the shading plate. The positioning of the flux modifier is controlled by an actuator (8) attached to the carrier and this again may be a manually operated adjuster or a remotely controlled positioner similar to that used for shading plate adjustment; however, since the flux modifier movements are relatively small, its actuator need only be of short stroke.

It will be seen that the long stroke positioner (6) can move the shading plate to whatever position is required, carrying with it the flux modifier (4) and its short stroke positioner (8) ; this movement constitutes the primary width adjustment of the inductor relative to a specific strip width as illustrated in Figures la and lb. If necessary the flux modifier can then be adjusted over a short distance as required to trim the inductor performance to suit the particular type of strip material.

In an industrial installation it is preferred that all the actuators should be of the remote control type. This makes it possible rapidly to set up the pattern of shading plate and flux modifier positions which result in the desired distribution of heating for a strip of given width and characteristics, either by selection from a previously gathered database or by direct control from devices such as scanning pyrometers which monitor the temperature profile across the strip.