US 20020142161 A1
A magnet wire having a boron nitride filled enamel. The enamel can be a resin material having boron nitride particles in an amount effective to impart corona resistance to the magnet wire. A method for manufacturing the magnet wire is also disclosed.
1. A magnet wire having corona resistant properties, said magnet wire comprising:
a conductor strand; and
an enamel disposed on said conductor strand, said wire enamel comprising a resinous material and boron nitride in an amount effective to increase corona resistance of the magnet wire.
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10. A method of manufacturing corona resistant magnet wire comprising the steps of:
coating a conductor strand with an enamel comprising a resinous material and boron nitride in an amount effective to increase the corona resistance of the magnet wire;
curing said enamel onto said conductor strand.
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19. A method for increasing corona resistance of a magnet wire comprising applying to a conductor strand an enamel comprising a resinous material and boron nitride in an amount effective to increase corona resistance of the magnet wire.
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 The invention relates to corona resistant wire enamel compositions and conductors insulated therewith. More specifically, the invention relates to a corona resistant magnet wire having an enamel with a boron nitride filler and a method of manufacturing such a magnet wire.
 Dielectric materials used as insulators for electrical conductors may fail as a result of corona discharge occurring when the conductors and dielectrics are subjected to voltages above the corona starting voltage. This type of failure may occur for example in certain electric motor applications. Corona induced failure is particularly likely when the insulator material is a solid organic polymer. Dielectric materials having resistance to corona discharge-induced deterioration are therefore highly desirable. For some applications, mica-based insulation systems have been used because mica exhibits corona resistance. However, mica inherently has poor physical properties.
 Solid, corona-resistant dielectric materials are particularly needed for high-voltage wire, for example magnet wire, used in applications in which corona discharge can occur. The phrase “magnet wire” as used herein refers to a solid single strand conductor wire insulated with a polymer based enamel coating to provide a uniform dielectric coating while taking up a minimum amount of space.
 Resins containing a minor amount of an organometallic compound of either silicon, germanium, tin, lead, phosphorus, arsenic, antimony, bismuth, iron, ruthenium or nickel are disclosed in U.S. Pat. No. 3,577,346 as having improved corona resistance. A composition having corona resistant properties disclosed in U.S. Pat. No. 3,228,883, consists of a mixture of ethylene-alpha-olefin copolymer, a homo- or copolymer co-vulcanizable therewith and a non-hydroscopic mineral filler, such as zinc, iron, aluminum or silicon oxide.
 U.S. Pat. Nos. 4,493,873, 4,503,124, and 4,537,804, all issued to Keane et al. and incorporated herein by reference, disclose a corona resistant wire enamel having alumina particles dispersed therein. The use of alumina as an enamel filler has become quite popular to increase corona resistance. However, in some cases, alumina fillers present practical problems that limit their use. Also, alumina fillers are relatively difficult, and thus expensive, to implement. For example, the molecular structure of alumina causes a tendency of agglomeration and thus it is difficult to properly disperse alumina particles in a polymer resin or other material. Also, the manufacturing process of alumina particles yields a chlorine residue and thus wire must be pretreated prior to being coated with the alumina filled resin. For example, a polyimide base coat is often used to protect the wire from the residual chlorine in aluminum oxide. Further, alumina filled enamels are relatively susceptible to damage due to moisture. For these reasons, alumina filled wire enamels are difficult, and thus expensive, to manufacture and apply, and thus are unsuitable for certain applications.
 The use of boron nitride to increase the thermal conductivity of insulation for stator bars and the like is known. For example, U.S. Pat. No. 5,710,475, issued to Irwin et al., discloses stator bars coated with a high temperature thermoplastic resin having boron nitride therein. This patent discloses that the use of boron nitride increases thermal conductivity of the insulation. In particular, the boron nitride is present in a proportion effective to produce a thermal conductivity in the composition of at least 0.35 W/m° K at 180° C. Irwin et al. does not relate to a magnet wire. Also, commercially available boron nitride has a particle size that is too large to permit extrusion onto magnet wire. In fact, Irwin et al. teaches that the boron nitride particles should have an average particle size of 1-100 microns. This size is far too large to be applied to magnet wire enamel. More particularly, when the particles are too large, they do not stay suspended in the liquid enamel matrix without constant agitation.
 It is also known to use boron nitride in mica sheets which are used as a main insulation for coils or the like. Japanese patent application No. 127564 is exemplary of references disclosing such a sheet. The sheets are wrapped around a bundle of wire strands or the entirety of plural windings of a single wire strand. However, such sheets are not suitable for magnet wire insulation because they are too thick and would be impractical to apply to a single magnet wire strand. Mica sheets do not provide protection between adjacent strands of magnet wire which is where the failure will occur.
 Accordingly, there is a continuing need for corona resistant materials which are easily fabricated and suitable for use as a filler in magnet wire enamels.
 The invention relates to a corona resistant magnet wire. A first aspect of the invention is a magnet wire having corona resistant properties comprising a conductor and a wire enamel disposed on the conductor and comprising a resinous material and boron nitride in an amount effective to increase corona resistance of the magnet wire.
 A second aspect of the invention is a method of manufacturing corona resistant magnet wire comprising the steps of coating a conductor with an enamel comprising a resinous material and boron nitride in the resinous material in an amount effective to increase the corona resistance of the magnet wire and then curing the enamel onto the conductor strand.
 A final aspect of the present invention is a method for increasing corona resistance of a magnet wire comprising applying to a conductor strand an enamel comprising a resinous material and boron nitride in an amount effective to increase corona resistance of the magnet wire.
 As used throughout the specification and claims, the term “magnet wire” means a continuous strand of a conductor wire, such as copper or aluminum, coated with an insulating layer of enamel.
 By “conductor” is meant a conductive wire, and preferably a round or rectangular strand of copper or aluminum capable of transmitting electricity.
 By “resinous material” is meant an organic polymeric material such as poly(ester)(amide)(imide) which becomes an enamel when cured and provides the insulating layer.
 By “amount effective to increase the corona resistance of the wire” is meant that there is sufficient boron nitride present in the enamel so as to increase the corona resistance of the magnet wire as compared to the same enamel which does not contain the boron nitride.
 The conductor employed according to the present invention is in a form adapted for a magnet wire application. Generally, it will be of a diameter ranging between 0.0070 in. and 0.1825 in. (0.0178 and 0.4636 cm.), and preferably between 0.0201 in. and 0.0641 in. (0.0511 and 0.1628 cm.) for round conductor and any combination of thicknesses from 0.020 to 1 inch (0.051 and 2.54 cm.).
 Various resins can be used in wire enamels of the preferred embodiment. For example, polyimide, esterimide, polyamide, or polyamideimide resins can be used. For example, an esterimide available under the tradename IMIDEX-E™ from General Electric Company can be used.
 The particular choice of the boron nitride is not critical to the present invention. Generally, the particle size of the boron nitride incorporated into the resins should be large enough so as to provide a barrier for electrical discharges but not so large as to cause settling of the material in the absence of agitation or cause loss of flexibility in the cured film. This generally corresponds to particle sizes <0.16 μm. The shape of the boron nitride incorporated into the resin is also not critical. Generally, a hexagonal platelet shape is preferred. The boron nitride of the preferred embodiment can be Grade 6058 acid washed boron nitride available from Advanced Ceramics and preferably is processed to have an average particle diameter of less than about 0.16 μm. The boron nitride is admixed with the resin so as to form a final cured enamel product which resists corona discharge. This amount is typically in the range of about 1% to about 35% by weight and preferably about 15% by weight of the resulting enamel.
 To prepare the magnet wire of the present invention, the boron nitride, admixed with a resin capable of forming an enamel, is applied to the conductive wire. Generally, the resin will be in the form of a liquid or a suspension into which the boron nitride is added. Generally, the proportion of resin and boron nitride is can be readily determined by a person skilled in the art.
 A suspension of the boron nitride particles in resin can be prepared by high shear mixing, using for example a ball mill mixer, and the resin can be coated on a conductor strand to form magnet wire. The conductor is coated with the resin/boron nitride mixture employing techniques well known in the art for wire coating.
 After the conductive wire has been coated, the coating is cured to form the enamel layer which provides the resistance to corona discharge. Generally, the curing is carried out using techniques well known in the art. Typically, curing involves heating the coated conductive wire at a temperature between 330 and 380° C., preferably between 350 and 370° C. for a period between approximately 0.45 and 0.65 minutes for average wire.
 To obtain the smooth continuous coating that is required to produce an insulating film in the minimal thickness required in producing commercial electrically insulated magnet wire, the boron nitride can be dispersed in the resin by means of high shear mixing, in, for example, a high energy mixing device such as a differential speed rolling mill or by high speed agitation (for example, in a Cowles unit). The resulting composition can be applied to the wire coating with wiping dies at wire speeds of from 2 to 120 ft/min (0.61 to 36.58 m/min), or more, depending on the type of conductor strand being coated. Curing temperatures between about 330° C. and 370° C. can be used to cure the enamel as indicated above. The build-up of the enamel on the wire can be about 0.002 to 0.010 in. (0.005 to 0.025 cm.) and preferably is about 0.003 in. (3 mils) (0.0076 cm).
 The enamel of the preferred embodiment yields magnet wire which exhibits greatly enhanced resistance to corona-induced deterioration. Additionally, magnet wire having a boron nitride filled enamel has superior thermal conductivity. Further, the platelet structure of boron nitride yields reduced agglomeration tendencies and thus a boron nitride enamel is more readily coated on wire. Boron nitride is less susceptible to moisture damage because it is more stable than alumina because aluminum can oxidize to different levels. Thus, its use as a filler is advantageous in applications where moisture may be present. Also, the manufacturing process of boron nitride, unlike alumina, does not yield chlorine and thus enamels using boron nitride fillers do not require pretreating of the conductor. Finally, the lower density of boron nitride, as compared to alumina, permits filler loading to be at a lower weight percentage for comparable corona resistance.
 The following example depicts in more detail the preparation and use of a representative composition. A suitable polyesterimide wire enamel may be made according to the following procedure.
 A suitably sized flask is charged with the following ingredients:
 The ingredients were heated for about 2 hours to reach a temperature of about 215° C. and held at this temperature for about 8 to 10 hours. Then enough cresylic acid was added to reduce the solids content to 27% by weight and the mixture was maintained at about 200° C. for 8 hours, until it was completely homogeneous. Boron nitride particles having an average diameter of 0.16 micron were dispersed in the enamel solution by high speed agitation in a Cowles unit or by rolling on a 3 mil paint roll for 12 hours to provide high sheer mixing.
 The invention provides new and improved corona-resistant insulating materials which comprise wire enamels for magnet wire based on polyimides, polyesters, polyesterimides, polyamideimides, polyetherimides, and the like which are formulated to include about 1% to about 35% of submicron or microscopic particles of boron nitride. When applied to an electrical conductor of magnet wire, the enamel provides a continuous coating which exhibits high corona resistance.
 Although the invention has been described with reference to a particular preferred embodiment and example, it is apparent that modifications may be made without departing from the scope and spirit of the subject invention, as defined by the appended claims.