CA1092662A - Two-phase, sliding-field electromagnetic inductors - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Electromagnetic device with sliding field comprising two two-phase inductors placed on the opposite sides of a con-ductive member acting as an armature. The two windings of each inductor are connected in series. The assembly defines a Wheat-stone bridge in which each of the diagonals is supplied by one phase of a two-phase generator. This device can be used for all the applications of the electromagnetic sliding fields where it is possible to associate two identical inductors indentically disposed with respect to an armature.

Description

The invention relates to polyphase electromagnetic in-ductors producing electromagnetic fields which undergo -transla-tional movement, known as sliding fields, of the kind used as linear motor inductors or as inductors for heating plane-surfaced conductive articles.
It is known that when use is made of polyphase induc-tors of this kind with a small number of poles and of short length with respect to the other dimensions, either in linear motor type applications in which the translational force produced by the sliding field is exploited, or in heating applications in which the energy dissipated in the induced body, is used the geometric positional asymmetry of the phases of the inductor with respect to the ends of the system as well as with respect to the induced circuit, involves a considerable lack of electric-al balance which is shown by differently phases currents and voltages. This is manifested by a considerable difference in load between the phases of the generator, the phase which is most loaded being that which supplies the winding which is located at the inductor input (the end through which the sliding field "enters" into the system), while the output phase hardly works at all.
It should be noted that for a three-phase inductor, the ; lack of balance is due to two different reasons: a spatial asym-metry arising from the different position of the phase circuits with regard to the ends of the system, a temporal asymmetry due to the phase difference of the currents running through these circuits, the combination of these two asymmetries being neces-sary for the creation of a sliding field. The result of this is to differentiate the part played by the ends, one being located at the input and the other at the output of the sliding field, and to create in that way à lack of electromagnetic balance be-tween the circuits near to one or the other end. ~ith a two-`:''"

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, phase inductor, a perfect spatial symme-try can be obtained, each phase beiny able to occupy a position symmetrical of the other with regard to the ends. On the other hand, the electromagnetic asymmetry is maintained since the phase currents are shifted by _, this causing one of the ends to act as input and the other to act as output.
The object of the present invention is to remedy these disadvantages and to provide an electromagnetic sliding field system of which the electric supply is balanced both in voltage and in current.
To this end. the subject of the invention is an induct-ive electromagnetic device comprising two flat-surfaced, sliding-field inductors disposed face to face which demarcate a substan-tially parallelepipedal space in which can fit an electrically-conductive member acting as an armature, characterised in that each inductor is provided on its working face with an even number of associated rectilinear electrical conductors in order to form two standard interlaced two-phase windin~s, themselves connected in series to the middle portion of each inductor, the input con-ductor of each of the two inductors being connected -to the output conductor of the opposite inductor, so as to form a closed aggre-gate Wheatstone bridge winding, ~he supply of electrical energy being effected by a two-phase generator one phase of which is connected between the junction points of the two-phase windings of one and the same inductor and the other phase of which is connected between the junction points of the two inductors.
In one preferred embodiment the electrical conductors of each inductor are connected in order to form interlaced two- -phase windings series with divided poles.
It will be manifest that the basic concept of the in-vention may be considered from two different points of view, closely complementary with one anotherO From a first point of .~ .

_ ~ _ ~326~ -view, it is a question of connecting with an "lnput" winding of one inductor an "output" winding of the other inductor and vice versa, so that the two assemblies thus made up behave in the same manner in so far as the electric load absorbed is concerned.
From a second point of view, it is a question of connecting the four windings in a balanced Wheatstone bridge branch-circuit the diagonals of which are supplied, respectively, by the two phases of a two-phase generator, in such manner that the two phases are electrically disengaged from one another and do not discharge into one another. These two means co-operate closely in order to obtain the total result, namely the production of a sliding field of high power in a short polyphase electromagnetic system with a low number of poles, with a balanced polyphase supply, i.e. with-out large reverse current. Of course, the structure according --to the invention necessarily requires that the electrical supply be two-phase. As the industrial electrical power is usually available in three-phase, use is made of a standard means in order to change from three-phase into two-phase, the most suit-able appearing to be the transformer called the "Scott".
With a two-phase inductor wound in accordance with the invention there can thus be obtained at the same time the spatial symmetry and the electromagnetic symmetry.
The system perfected according to the invention may be used for various applications of the sliding magnetic fields, at all times that it is possible to connect two identical inductors identically arranged with regard to an induced or secondary cir-cuit, where it is a question of applications in which what is mainly sought after is the acquiring of powers or a movement (linear motors, pumping of liquid conductors, stirring, etc.) or ` 30 in which what is mainly sought after is to obtain heating (heat-ing furnaces, thermal treatments, etc.).
There will now be described, by way of example in no ;

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. - . . . , " . . , ~ . , way limitative and with the sole purpose of illustrating and making the invention well understood, one example of use of the invention in an induction furnace for the heating of metallurgic-al products, reference beiny made to the accompanying sheets of drawings, in which:
Fig. 1 is a transverse sectional view of an induction furnace for flat products, including two flat-surfaced sliding-field inductors, Fig. 2 is the basic diagram of the bridge branch-circuit of the two inductors:
Fig. 3 is the symbolic representation of the branch-circuit diagram of Fig. 2, Fig. 4 is the branch-circuit diagram of the bars of an inductor.
In Fig. 1 there has been shown in transverse section a furnace for reheating steel slab by induction. The structure and the operation of this furnace have already been described in detail in French patent no. 2,339,316. The only difference is the electrical branch-circuit o~ the current conductors of the indu~tors, according to the presenk invention.

A ~teel slab bloom 1 acting as an armature is placed on one end in a heating space 2 delimited by insulating refrac-tory walls 3 which protect the electromagnetic inductors 4~ In the magnetic mass of each inductor are made twenty-four slots -such as 5 arranged horizontally and in which are placed twenty-four Roebel bars cooled by circulation of water, which con3ti-tute the conductors of the inductor windin~s. The inductors are supported in welded casings 6 which may be displaced late-rally on rolling tracks 7, by means o-f rollers 8, when they are set in motion by jacks 9. The slab stands in the furnace on an insulating refractory support 10 covered by a refractory steel cap llo The cooling of the magnetic ,f,~

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- mass of the inductors is provided for by ducts 12 through which -Elows fluid which carries off heat.
Each inductor is connected in two-phase. It comprises twenty-four current bars, that is twelve per phase, in an inter-laced wlnding series, usual in electrotechnics. Each turn of the windings is formed by a bar of rowJI and a bar of row (~ + 12) connected in series so that the same current circulates therein, in the reverse direction.
Fig. 2 shows the basic diagram of the bridge branch-circuit according to the invention. It is a question of a simpli-fied diagram in which the two inductors are presupposed to be brought down on a plane parallel to the plane of Fig. 1, with symmetrical rotation with respect to the vertical median plane of -the furnace (perpendicular to the plane of Fig. 1).
In Fig. 2, where each inductor is represented by a broken line rectangle, each horizontal bar in fact represents -six current bars connected either in parallel or in series, pre-;~ ferably in series for the high powers. The four phase windings thus constituted are connected as a Wheatstone bridge as shown in Fig. 2 and symbolized in Fig. 3.
It can be realized that the system is wholly symmetric-;; al. The four phase windings therefore form the four arms of a Wheatstone bridge, the impedances of which are complex. The two phases PH.l and PH.2 of the two-phase generator are respectively connected across a diagonal of the bridge. Each arm of this bridge has its own impedance Z and mutual impedances with the three other arms, respectively Z~, Z~, Z~, shown in Fig. 3. It is the combination of these different complex impedances which is why the bridge is balanced and that neither of the two diagon-als discharges into the other.
It can indeed be shown by a complex calculation that the phase currents Il and I~ are given by the relations:

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Il = V
Zl I2 = V2 with Zl = Z + Z~ + Z~ + Z
Z2 Z Z~ + Z~ - Z~
These results show that the current in each phase de-pends only on the voltage of this phase alone, that is to say that each phase is entirely disassociated from the other. The result of this is that if the voltages Vl and V2 are out-of-phase by 90 on no load (two-phase supply), they remain so under load, although the power factors (cos ~ ) are different in the two dia-gonals and the transformer reactance is not negligible. More-over, as the real parts of Zl and Z2 are different, there is a difference in load (active power) between the two phases.
The voltages at the terminals of the bridge diagonals are given, respectively, by the following vectorial relations: -~

BA 1 2 and ~CB ~1 ~ ~2

2 2 These ralations show that for the two-phase supply voltages (i.e. ~ out of phase) the vectors ~1 and V~2 being or-thogonal, the amplitudes of VBA and ~CB are equal and the voltages are balanced over the four arms. However, the power factors (cos ~ ) being different in the two diagonals, the currents in ; the arms IAB and IAC are not quite orthogonal together and do not have quite the same amplitude, but this out-of-balance remains ~ `
` slight, less than 10%. -Fig. 4 shows the connection diagram of the twenty-four current bars of an inductor. The two inductors are identical.
Each of the twelve first bars of row ~ is connected in series with the bar of row (a -~ 12), the latter coming back in series -~

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with the bar (~ + 1). The phase AB is therefore made up of the bars 1 to 6 and 13 to 18, the phase BC being made up of the bars 7 to 12 and 19 to 24.
In an experimental materialization of this furnace put-ting the invention into practice for the heating of steel slab of 250 x 1200 x 4000 mm and of about nine tons in weight, each in-ductor is made up of twenty-four RoEsEL type bars of 80 x 27 mm disposed in slots 5 of corresponding dimensions, separated by teeth of 23 mm of the magnetic circuit. The pole-pitch is equal to 0.6 m~ The effective intensity in each bar is 12,000 amperes at 50 Hz. The air gap between the inductors and the slab is equal to 110 mm. In these conditions, the working power trans-mitted to the slab by the two inductors is 2.8 megawatts. ~`
The advantage of the invention is clearly evident with regard to the electrical balance of the system. With an inductor supplied in a standard manner, the respective impedances of the input phase and the output phase are in the ratio about 3, while the power factors are respectively of the order o~ cos ~1 = 0.14 and cos ~2 = 0 03 In the system according to the invention, the following figures are found:
COS ~1 = - 10 cos ~2 = 0 07 For the impedances:
; Zl = 0-74 Z2 = 0.84 It is therefore seen that the out-of-balance is consi- -~
derably reduced, since the ratio of the cos ~s goes from almost 5 to less than 1.5 and the ratio of the impedances goes from 3 to a value close to 1. From the electrical supply point of view, it can be acknowledged that such a system is almost balanced.
The arrangement of the present invention may be used . . .

;2 whatever the power of the inductors may be. Of course, this arrangement, described,in the case o-f two inductors, may be ex-tended to any even number whatsoever without departing from the scope of the invention.

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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Inductive electromagnetic device comprising two sliding-field flat inductors arranged face-to-face which delimits a sub-stantially parallelepipedal space in which can fit an electric-ally-conductive member acting as an armature, characterised in that each inductor is provided on its working face with an even number of associated rectilinear electrical conductors in order to form two standard interlaced two-phase windings, themselves connected in series to the middle portion of each inductor, the input conductor of each of the two inductors being connected to the output conductor of the opposite inductor, so as to form a closed aggregate Wheatstone bridge winding, the supply of elec-trical energy being effected by a two-phase generator one phase of which is connected between the junction points of the two-phase windings of one and the same inductor and the other phase of which is connected between the junction points of the two in-ductors.

2. Device according to claim 1, characterised in that the conductors of each inductor are connected in order to form inter-laced two-phase windings series with divided poles.

3. Device according to claim 1, characterised in that it includes several pairs of inductors associated two by two in Wheatstone bridge form.