Electric heating – Inductive heating – Specific heating application
Patent
1994-05-25
1996-04-23
Leung, Philip H.
Electric heating
Inductive heating
Specific heating application
219662, 219646, H05B 606
Patent
active
055106005
DESCRIPTION:
BRIEF SUMMARY
This invention relates to induction heating apparatus, particularly for heating elongate metal workpieces of uniform width. GB-A-1546367 discloses such an apparatus in which magnetic pole pieces extend transversely of the length of the workpiece the associated windings being energised from an alternating current supply. A difficulty which arises with induction heating apparatus is obtaining a uniform temperature profile across the width of the workpiece being heated. In the prior art, attempts to obtain such a uniform temperature profile have involved seeking to control the flux density produced per unit width across the workpiece. In GB-A-1546367, flux per unit width is controlled by appropriate shaping, construction or arrangement of the pole pieces, or by the use of appendages attached to these pole pieces.
An alternative form of induction heating apparatus that has been proposed is described in GB-A-712066. Here the magnetic pole pieces extend longitudinally along the length of the workpiece, so that currents are induced in the major surfaces of the workpiece flowing longitudinally rather than transversely across the width. This arrangement can assist in avoiding the heat distortions caused by transversely flowing current loops being completed by longitudinal currents at the workpiece edges. However, longitudinally extending pole pieces imply alternative magnetic poles across the width of the workpiece, in turn implying a periodic distribution of longitudinal eddy currents across the width. Clearly, such a periodic spatial distribution of eddy current can result in a corresponding variation in heating effect over the workpiece width.
GB-A-712066 suggests that this may not be a problem provided the spacing between adjacent magnetic poles across the workpiece width is sufficiently small, e.g. 1 to 2 cms. However, the magnetic efficiency of such an arrangement would be very low due to these small magnetic pole spacings. This prior art specification does also propose an arrangement in which "it is not even necessary to use particularly small pole pitches" in which the width of the workpiece is equal substantially exactly to an even number of magnetic pole pitches. Then it is stated that a completely uniform heating across the width of the strip can be obtained by energising the windings of the inductor to provide both sine and cosine magnetic field distributions across the width of the workpiece. This is achieved either by energising the windings at two different frequencies, or at the same frequency but in phase quadrature.
It is believed that no practical apparatus has every resulted from the concepts disclosed in GB-A-712066 for the following reasons. The simple winding arrangements disclosed in the examples do not adequately control the generation of eddy currents in the workpiece to have the required sine and cosine profiles across the workpiece width. No account is taken in this prior art specification of the effect of the edges of the workpiece which provide discontinuities which effect the magnetic field distribution. Most fundamentally, it is seldom in practice required to produce precisely uniform heat input across the width of a workpiece. Especially if the workpiece has significant thickness, there can be increased heat loss from the edges so that it may be desired to slightly increase heat input at the edges. Also, induction heating apparatus is commonly employed for heating continuous strip and there is difficulty in maintaining uniform heat input across the width of a strip as it enters the heating apparatus and as it leaves. This non-uniform heating at the ends of the apparatus can be compensated by an appropriate degree of non-uniformity of heat input as the workpiece travels along the length of the apparatus.
As will become more apparent later herein, the arrangement disclosed in GB-A-712066 cannot be used to produce a desired non-uniformity of heat input across the width of the workpiece, even ignoring edge effects.
According to one aspect of the present invention, there is provided indu
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patent: 4122321 (1978-10-01), Cachat
patent: 4321444 (1982-03-01), Davies
patent: 4484048 (1984-11-01), Travers et al.
patent: 5055647 (1991-10-01), Heyes et al.
patent: 5126522 (1992-06-01), Katayama et al.
EA Technology Limited
Leung Philip H.
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