GB2051491A - Magnetic core for a capped core shunt reactor - Google Patents

Abstract

A spirally wound core for a gapped core shunt reactor is manufactured by winding a strip of magnetic steel in a spiral until the desired diameter is reached. Ventilation spacers 12 may be included in the spiralled construction. When the winding process is complete, the core is annealed and the vacuum impregnated with a thermosetting resin to solidify the structure. The core is then machined, and a radial air gap is cut. The gap filled with an insulating spacer held by epoxy resin. A resin-glass bond 16 is then applied and cured in situ. <IMAGE>

Description

SPECIFICATION Magnetic core for a Gapped core shunt reactor BACKGROUND OF THE INVENTION Field of the Invention: Compared to other types of shunt reactor, it is well known that gapped core reactors may be made smaller than other types of reactors because of the increased flux density which can be employed in the air gaps thus reducing both core and winding diameter. The reduction in size of the reactor is only advantageous if the manufactured cost of the gapped core are kept low. Thus it is desirable to keep the number of iron packets to a miniumum, and the diameter of the packets as small as possible. Thus the resulting air gaps between the magnetic packets tend to be large, causing severe flux fringing which can be lead to high ion and winding losses unless suitable precautions are taken.

Previous core designs have taken a variety of forms, and this disclosure will illustrate one of the prior art types which although possessing excellent mechanical and electric properties, the known method of manufacture are so labour intensive as to make the core uneconomic to build. Thus in the construction of the core it is very important to keep the manufacturing costs low while providing suitable mechanical and electric properties.

Description of the Prior Art Canadian Patent Application Serial No.

225,129 filed April 22, 1975, in the name of Selwyn Palmer, entitled "Gapped Core Reac tor SUMMARY OF THE INVENTION The core of the reactor of this invention is fabricated of packets of suitably constructed magnetic material separated by a suitable non-magnetic material such as MYCALEX (Registered Trade Mark), porcelain, or slate as is common practice in the construction of a gapped core reactor.

The magnetic packet is formed by spirally winding a strip of suitable magnetic material into a circular core of the required diameter. If the magnetic packets are large enough, it wll be necessary to form one or more cooling ducts in the core, which cooling ducts may be formed by winding a special dimpled or corrugated strip into the core where ventilation is desired. The balance of the winding is completed with the suitable magnetic strip. After the winding operation is completed the core packet may be banded if it is deemed necessary and it is annealed to relieve stresses therein.

The core packet is then vacuum impregnated with a suitable binding agent such as thermosetting resin which solidfies the core into a single unit so that suitable machining may proceed on the core.

Two machining operations are next carried out on the core. The four sharp corners which are formed during the winding operation are rounded to desired radii, and then a radial slot is cut through the core packet.

The machined core is now etched to remove any bridging of the iron laminations which might have occurred during the machining and cutting operations performed on the core. After etching, a suitable insulating strip is placed in the radial cut and it is secured in the slot with a suitable adhesive.

Finally a "B" stage resin-glass tape is wound around the outside of the completed packet and cured in situ as an added precaution against the subsequent loosening of the iron in the packet.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a prior art core; Figure 2 is a perspective view of a finished core packet; Figure 3 is a partial sectional perspective of a partially completed core.

DESCRIPTION OF THE PREFERRED EMBODI MENTS Referring now to Fig. 1, a prior art core is shown which is ideally suited for the iron core portion of a gapped core reactor. In this core device, the laminations are generally stacked in a radial fashion to yield a shallow cylindrical core. It is noted that the laminations making up the core are generally of different length, and hence the task of stacking the laminations in the core is quite a complex matter. Finally the completed core must be a unitary body and about the only method of achieving this criterion, is to vacuum impregnate the assembled core with epoxy or polyester resin and cure the core in its assembled state to lock all the laminations permanently in the state in which they appear in Fig. 1.

Because cores such as the one shown in Fig. 1 are so labour intensive, i.e. the laminations are cut in a large variety of lengths and stacked by hand, this type of core although ideally suited to the magnetic conditions existing in reactor cores is seldom used because of the fabrication cost.

Referring now to Fig. 2, a core of the instant invention will be described. Core 10 basically comprises an assembly formed by winding a strip of electrical grade transformer steel in a spiral around a mandrel to form a shallow cylinder. The thickness of the steel strip is typically of the same thickness as the laminations forming the core of most power transformers or reactors.

The spiral strip is wound around a mandrel or form until the desired outer diameter is reached, at which time winding ceases. This winding operation may be carried out at high speed due to the simplicity of the operation.

If the finished core is of large enough size to require cooling ducts, the winding operation may be interrupted at a suitable stage to wind on a "dimpled" steel strip as shown at 1 2 where the duct is desired. A "dimpled" or "corrugated" ventilation strip may be wound into the core at the desired location in a similar manner to winding the iron strip. Although only one cooling duct has been shown, more than one cooling duct may be wound into the core at diverse locations if the core is sufficiently large. Only one variation in the shaping of the ventilation strip has been shown, other variations are no doubt possible.

When the ventilation strip is wound in place, the winding of the thin steel lamination strip may be resumed.

Upon completion of the winding operation the wound core is clamped in its wound condition and placed in an annealing furnace to relieve any stresses in the iron which would adversely affect its magnetic characteristics.

The coil is then vacuum impregnated with a suitable thermosetting resin and the impregnated core is heated to cure the resin impregnating the core to gelation. The resin serves two purposes: it fills all the interstices between successive turns of the iron strip so that any fluids used in subsequent processing operations may not enter these intersticies, and the cured resin causes the bore to become a unitary structure.

Further processing of the core is a must in order to prevent excessive eddy current losses. Two machining operations are required. First a radius is formed at all four corners of the packet. The radius chosen is preferably between 0.5 and 1 inch in size according to the requirements of a particular design. Secondly a radial slot is cut through the iron core as shown at 14. Because the core has become a unitary structure during the manufacturing operation, both the above machining operations may be successfully carried out using a high speed grinding cutter.

Because the machining operation may cause some of the iron to bridge the insulation between successive layers of laminations an etching fluid is applied to the cut and machined surfades and after a predetermined period of time is removed by rinsing.

A suitable insulating spacer is next inserted into the radial slot 14. The spacer serves to mechanically replace the missing laminations removed during the cutting operation. Suitable spaces include MICARTA (Registered Trade Mark), BAKELITE (Registered Trade Mark), or MYCALEX (Registered Trade Mark) or some other suitable substance. The spacer will preferably set in place in slot 14 in an epoxy resin which when cured holds the spacer securely in slot 14. The core thus becomes a unitary structure.

Finally a "B" stage resin-glass insulating tape is tightly wound round the outside of the completed packet and cured in situ as shown at 1 6. This completes the assembly of the core which is now a smoothy radiused unitary body.

The resulting core is a unitary structure which may be manufactured using non-oriented silicon-iron and which may incorporate a cooling duct (of the core is sufficiently large) is of such construction that during operation in a reactor the fringing flux fluid is smoothed somewhat at the corner radii so that local flux densities are reduces giving reductions in losses in both the core and the winding due to circulating eddy currents.

Claims (7)

1. A reactor core assembly comprising an annular core body formed of a spiral of tightly wound turns of electrical grade insulated silicon steel strip, having a suitable binding agent disposed between adjacent turns of said spiral strip, a gap formed in said reactor core in the form of a slot extending radially from the outermost turn of said core to the inner most turn of said core, said gap having a suitable insulating means therein, and suitable insulating banding means surrounding said core.

2. A reactor core as claimed in claim 1 wherein the four sharp edges of the core are rounded so as to form a doughnut shaped core, and a suitable core cooling spacer means is located between predetermined layers of said steel laminations forming said core.

3. A reactor core assembly as claimed in claim 2 wherein said spacer means comprises a dimpled or corrugated steel strip which is sandwiched between said predetermined layers of core steel strip.

4. A reactor core assembly as claimed in claim 1, wherein the suitable binding agent is a thermosetting epoxy resin.

5. A reactor core assembly as claimed in claim 3, wherein the suitable binding agent is a thermosetting epoxy resin.

6. The method of fabricating a reactor core, comprising, winding a strip of suitable electrical grade transformer steel on a mandrel in a spiral in the shape of an annulus until the desired buildup diameter is reached, and annealing said wound annulus to remove any undesired stresses therefrom, impregnating said wound annulus with a suitable thermosetting resin, and heating said impregnated annulus to cure said resin, and machining said annulus to remove the sharp edges therefrom to form substantially a doughnut shaped core, and machining said core to provide a radial slot in said core which extends from the innermost lamination to the outermost lamination of said core, and suitable insulating banding means in wound around the periphery of said core.

7. The method as claimed in claim 6, wherein the winding process is interrupted when the core buildup has reached a predetermined diameter, and a cooling strip is wound around said partially completed core, and subsequently winding electrical grade transformer steel over said partially completed core and cooling strip to the desired final core diameter, said core being etched subsequent to said machining operations, and suitable insulating means being inserted in said radial slot.