GB2111360A - Induction heating apparatus - Google Patents

Abstract

A metal slab 1 is induction heated by apparatus comprising an array of four fixed electromagnetic field members 2 each made up of radially extending polyphase windings 3 each powered so as to cause a wave of magnetic flux to rotate, sweeping an annulus of the slab 1, thus induction heating it. The windings 3 could instead be a circle of alternately opposed permanent magnets, each of the four circles being belt-driven from a common shaft, achieving the same effect. <IMAGE>

Description

SPECIFICATION Induction heating apparatus This invention relates to induction heating apparatus, particularly of flat conductive members used as metal slabs.

British Patent Specification 1513241 describes an induction heater using polyphase supply in a flat form for the heating of slabs, this heater being basically a linear induction motor with the slab as the stationary secondary, travelling wave magnetic fields being produced which travel along the slab. It would be preferable if the wave did not have to be generated from nothing at one end of the heater and was not lost at the other.

According to the present invention, an induction heating apparatus comprises an array of magnetic field members in fixed relationship to each other and a workpiece support arranged to hold fast, against forces induced by the field members, a workpiece to be heated, leaving substantially equal gaps from each field member, each field member being arranged to cause a magnetic flux wave to rotate, sweeping an annular path on the workpiece.

Each field member may consist of radially disposed circumferentially spaced stationary windings connectable to different phases of a polyphase supply, axially spaced from the workpiece.

An alternative, preferred, form of a field member is a flux-bearing rotating yoke disc carrying poles arranged in a circle in the plane of rotation, each pole carrying a steady flux. Each pole may thus be a permanent magnet or a d.c.

electromagnet. All the field members in the array may be rotated by a common motor linked mechanically to every field member (e.g. by belts or gearing), and this should be so arranged as to give the minimum practicable resultant torque on the workpiece.

The invention also extends to a method of induction heating a workpiece using the apparatus set forth above.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows in schematic underside plan view an induction heating apparatus according to the invention, and Figure 2 shows in schematic elevational crosssection a different embodiment of an induction heating apparatus according to the invention.

In Figure 1, a workpiece 1 in the form of a 6 mm thick ferrous slab 1.3 m square rests on a stainless steel table (not shown) backed by a layer of thermal insulation (also not shown).

Four fixed magnetic field members 2 are disposed in an array one at each corner of the slab 1, but all in the same plane under the table.

(Heating the corners of the slab 1 is particularly desired.) Each magnetic field member is an annular form (outside diameter 50 cm) and has radially extending circu mferentially spaced stationary polyphase windings 3. Under power, these windings cause a wave of magnetic flux to rotate, sweeping an annulus of the slab 1 being the projection on the slab of the field member. The slab thus is induction heated.

It will be appreciated that the format and number of the field members are chosen to suit the size and shape of the workpiece. For especially elongated or unusually shaped slabs, other configurations may be adopted, possibly involving relative motion of the array with respect to the workpiece.

However, there is the possibility of minor difficulties with the arrangement wherein the field members comprise polyphase windings: (a) The large airgap needed for thermal insulation reasons gives a bad power factor which might have to be corrected (expensively) by capacitors.

(b) The large magnetising current has be to carried by the polyphase windings, which are of somewhat delicate construction.

(c) The polyphase windings become congested (as a geometric inevitability) where they approach the central "hole". This presents manufacturing problems.

(d) Since the whole device is stationary and near a slab with an average temperature of 4000C (200 to 8000 C) cooling presents problems. On a big device of this type, water cooling would be natural inside the conductors but on relatively small devices (30 kW each) this would be difficult and uneconomic.

(e) Without special supplies, the frequency in the slab is limited to 50 Hz (mains frequency).

Turning to Figure 2, an induction heating apparatus has the same general format as that in Figure 1, and for simplicity only one field member of the four is shown.

A ferrous slab 1 measuring 1.3 m x 1.3 m x 6 mm rests on a stainless steel table 1 2 backed by thermal insulation 1 2a.

Underneath the table are disposed four field members 13, all drive by belts 14 from a common drive motor 1 5.

Each field member 13 comprises its belt-driven shaft 1 6, a circular yoke disc 20 (diameter 50 cm) and an even number of equally circumferentially spaced pole pieces 21. Whether the number of pole pieces is divisible by four or not depends on the specific heating patterns required at the centre of the member 13. Each pole piece 21 carries a steady flux, being permanently 'north' radially inwardly and 'south' radially outwardly (or vice versa), and could be a permanent magnet, such as of samarium cobalt. In the embodiment shown, however, each is an electromagnet energised by a d.c. supply through sliprings 22 on the shaft 1 6.

For a more efficient return for magnetic flux from the poles and passing right through the slab 1, a magnetic return circuit 23 of laminated steel is provided above the slab 1. Where the slab 1 was of such thickness that only negligible flux penetrated it, the magnetic return circuit 23 would be dispensed with.

Under power, the field members 13 spin round, whereby magnetic flux lines, intersecting the slab 1, sweep round an area of the slab. The slab is thus induction heated.

Compared with the Figure 1 embodiment, these features of the present invention are noteworthy: (i) The rotating field is supplied by the d.c.

excitation of synchronous-type poles which are mounted on a flux-bearing yoke disc. The field system is driven by a shaft running in mechanical bearings and is supplied by sliprings and bushes which pass d.c. from a stationary source, through the hollow shaft, to the field coils.

(ii) The poles carry only steady flux, and therefore can be cut from solid steel for cheapness and mechanical strength, though laminations could be used if desired. Since the poles may be tapered radially, a laminated fabrication could prove expensive.

(iii) Although the mechanical drive could be from a motor on the shaft 1 6 (direct drive), it is preferably to drive by belts 14 or gears. By this means, on large economical (say) 4 pole motor 1 5 can drive all four heaters and, at the same time, the belt can be used to produce a faster speed at the heated shaft. This, combined with an increase in the number of poles, means that the frequency at the workpiece can be increased considerably over mains frequency.

In our example, Main motor 4 pole: 1 500 rev/min (50 Hz) Heated shaft 2:1 belting: 3000 rev/min (50 rev/sec) 8 poles (four pole pairs, p = 4) on heater: f= p x n = 4 x 50 = 200Hz The heater shaft speed is limited from mechanical considerations and the number of poles by airgap configuration. The highest possible frequency at the workpiece is the most desirable, and therefore both the number of pole pairs (p) and the belting ratio (affects n) should be as high as possible.

(iv) It would be possible to replace the field windings by permanent magnets e.g. samarium cobalt. This presents some mechanical problems in ioading and unloading the slab, although if the slab 1 rises to greater than the Curie temperature and becomes non-magnetic (a quite probabie requirement) the unloading problem almost disappears. With permanent magnets, the possibility of 'switching off' the heater by disconnecting the field excitation current through the sliprings 22 is of course not available.

(v) For clarity, the support structure for the bearings and for the mechanical handling mechanism for loading and retaining the airgap have been omitted.

The foliowing consequences flow from these features: (a) Since the airgap in synchronous motors is less important than in induction motors, and the present heater can be regarded as derived from a motor with the armature member held stalled, a larger mechanical airgap can be tolerated in the Figure 2 embodiment, since the a.c. magnetising current problem has been removed, allowing room to accommodate better thermal insulation 1 2a.

(b) The power factor of the scheme is now the power factor of the drive motor since the bad power factor of the load and the gap has been dissociated from the supply system.

(c) The magnetic field excitation power (transmitted via the sliprings 22), being of the order of a kilowatt only, is small compared with the load (principally the drive motor 15, which consumes many kilowatts), so that the field power control can be a small electronic device rather than the large device needed to handle the total power of a conventional travelling wave induction heater.

(d) The frequency-changer property is advantageous both from increased heating and the ability (by exploiting the (at high frequency) skin effect) to heat thinner sheet.

(e) The efficiency of the overaii scheme will be the efficiency of the motor identified by the mechanical windage losses of the heaters and the small power in the field.

(f) The field winding is simple and robust, and thus much more tolerant of heat compared with an a.c. winding.

(g) The added distance of the field winding and the shielding provided by the pole shoes make cooling easier.

(h) The physical rotation cools the field winding automatically; added cooling means can be provided if needed.

(i) While, in both the Figures, the field members have the same diameter (50 cm), the Figure 2 embodiment can be more easily miniaturised, since the congestion problem at the narrower (radially inward) end of the tapering windings 3 of Figure 1 does not arise, the design restrictions imposed by ttie central hole being largely removed by the Figure 2 embodiment.

(j) By judicious choice of the pole number, the currents flowing in the central area can be controlled to give the most advantageous heating; an odd number of pole pairs has an additive effect across the centre, and an even number a repellent effect.

(k) After heating, and whilst the slab is being charged, the excitation can be removed, the cooling can continue and the whole device can return to room temperature before the cycie recommences.

Claims (8)

1. An induction heating apparatus, comprising: an array of magnetic field members in fixed relationship to each other and; a workpiece support arranged to hole fast, against forces induced by the field members, a workpiece to be heated, leaving substantially equal gaps from each field member; each field member being arranged to cause a magnetic flux wave to rotate, sweeping an annular path on the workpiece.

2. An induction heating apparatus according to Claim 1 , wherein each magnetic field member consists of radially disposed circumferentially spaced stationary windings connectable to different phases of a polyphase supply, the whole being axially spaced from the workpiece.

3. An induction heating apparatus according to Claim 1, wherein each magnetic field member is a flux-bearing rotatable yoke disc carrying poles aranged in a circle in the plane of rotation, each pole carrying a steady flux.

4. An induction heating apparatus according to Claim 3, wherein each pole is a permanent magnet or a d.c. electromagnet.

5. An induction heating apparatus according to Claim 3 or 4, wherein all the field members in the array are rotated by a common motor linked mechanically to every field member.

6. An induction heating apparatus according to Claim 1 and substantially as hereinbefore described with reference to and as shown in Figure 1, or in Figure 2, of the accompanying drawings.

7. A method of induction heating a workpiece, comprising placing the workpiece on the support of an induction heating apparatus according to any preceding claim, and actuating each field member to cause a magnetic flux wave to rotate.

8. A method according to Claim 7, further comprising relatively moving the array as a whole with respect to the workpiece.