HIGH VOLTAGE TRANSFORMER COIL ENCAPSULATION PROCESS

Abstract: A process for encapsulating a high voltage coil with an electrically insulating thermoplastic resin such that the electrically insulating thermoplastic resin is of sufficient thickness that the coil is isolated from other electrical surfaces and the electrically insulating thermoplastic resin is substantially void free, includes providing a mold having a cavity, a gate, a vent, and retractable support pins for supporting an object in the mold, placing a high voltage coil in the cavity, moving the pins into contact with the coil to support the coil in the cavity, injecting an electrically insulating thermoplastic resin into the mold through the gate, retracting the pins from the cavity as the electrically insulating thermoplastic resin is injected into the cavity before the resin freezes around the pins to prevent the formation of voids in the electrically insulating thermoplastic resin, allowing air to vent from the cavity through the vent as the resin moves through the mold, and continuing to inject the electrically insulating thermoplastic resin until the mold is filled to form a substantially void free electrically insulating thermoplastic resin layer of sufficient thickness so that the coil is isolated from other electrical surfaces.

Description: TITLE HIGH VOLTAGE TRANSFORMER COIL ENCAPSULATION PROCESS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of encapsulating electrical coils with an insulating thermoplastic. 2. Description of the Related Art It is known in the art to encapsulate a low voltage coil with an insulating thermoplastic resin. As used herein, the term "low voltage coil" means a coil having a voltage below 5.0 kilovolts . An example of a low voltage coils encapsulated with an insulating thermoplastic is a a coil encapsulated with DuPont Zytel® 70G33L nylon resin in a new world clutch for automotive air conditioners manufactured by Warner Electric Division of Dana Corp., of South Beloit, IL. When energized, the encapsulated coil acts like a solenoid, causing the clutch to engage a compressor. Low voltage coils are typically encapsulated by one of two methods. In the first method, the low voltage coil is encapsulated with a thermoset such as an epoxy, phenolic or thermoset polyester where the thermoset is heated until it is in a liquid state, and the coil is vacuum impregnated with the liquid thermoset. In the second method, the low voltage coil is encapsulated with a thermoplastic by providing a coil wound on a coil bobbin in a mold and injecting a thermoplastic resin into the mold until the entire coil is encapsulated with the resin to a desired thickness, and then removing the encapsulated coil from the mold. The thickness of the thermoplastic resin is such that it is sufficient to isolate the coil from other electrical surfaces. Alternatively, the low voltage coil may be a self supporting coil formed by winding a bondable magnet wire on a mandrel, then charging the coil electrically to melt an adhesive coating on the coil, and then cooling the coil. The self supporting low voltage coil is made this way so as not to depend on an independent coil form for support, which reduces assembly costs. The coil is then encapsulated in a mold with a thermoplastic as described above. For a low voltage coil the typical thickness of the thermoset or thermoplastic is generally greater from about 30 mils (0.762 mm ) to 80 mils (2.032 mm). The known methods for encapsulating a low voltage coil are not acceptable for encapsulating a high voltage coil. As used herein, "high voltage coil" means a coil having a voltage of at least 5.0 kilovolts. The fact that the low voltage coil has a voltage of only up to 5.0 kilovolts means that the thermoplastic that encapsulates the low voltage coil can be relatively thin because not much dielectric strength is needed to insulate the coil. Further, because the voltage in the coil is low, the presence of voids in the thermoplastic resin encapsulating layer are less of a concern because the voltage in the coil is not sufficiently high to cause corona discharge. Corona discharge is caused by the electrical ionization of gas present in voids in the encapsulating layer which eats away at that layer and eventually causes failure of the encapsulated part. If the conventional encapsulation process used for low voltage coils were used for a high voltage coil there would be voids present in the thermoplastic resin encapsulating layer, and those voids would cause corona discharge and degradation of the insulating resin layer leading to a shorting out of the coil. SUMMARY OF THE INVENTION A process for encapsulating a high voltage coil with an electrically insulating thermoplastic resin such that the electrically insulating thermoplastic resin is of sufficient thickness that the coil is isolated from other electrical surfaces and the electrically insulating thermoplastic resin is substantially void free, includes providing a mold having a cavity, a gate, a vent, and retractable support pins for supporting an object in the mold, placing a high voltage coil in the cavity, moving the pins into contact with the coil to support the coil in the cavity, injecting an electrically insulating thermoplastic resin into the mold through the gate, retracting the pins from the cavity as the electrically insulating thermoplastic resin is injected into the cavity before the resin freezes around the pins to prevent the formation of voids in the electrically insulating thermoplastic resin, allowing air to vent from the cavity through the vent as the resin moves through the mold, and continuing to inject the electrically insulating thermoplastic resin until the mold is filled to form a substantially void free electrically insulating thermoplastic resin layer of sufficient thickness so that the coil is isolated from other electrical surfaces. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view in elevation of a high voltage coil; Figure 2 is a view in elevation of a high voltage coil encapsulated with an electrically insulating thermoplastic resin; Figure 3 is a view in elevation of a number of coils that have been vibrationally welded together in a stack; and Figure 4 is a top plan view of the stack of Figure 3. DETAILED DESCRIPTION This invention relates to a process for encapsulating a high voltage coil with a thermoplastic resin. The high voltage coil, in order to function properly, must be encapsulated with a sufficiently thick layer of an electrically insulating thermoplastic resin to isolate the coil from other electrical surfaces. Furthermore, the electrically insulating thermoplastic resin must be substantially void free because the voltage in the high voltage coil is sufficiently high that voids in the electrically insulating thermoplastic resin will cause corona discharge and lead to shorts in the coil. As used herein the term "high voltage coil" means a coil having a voltage of at least 5.0 kilovolts. Most thermoplastic resins are molded into a desired shape or around a desired article by a process known as injection molding. In this process, a resin is preheated in a chamber with a screw to a temperature at which the resin will flow, and then the resin is forced under high pressure into a relatively cold, closed mold cavity. The high pressures may be applied hydraulically or mechanically such as through a plunger or a ram. The screw rotates to pick up the resin in a particulate form, compacts and melts the resin, mixes the resin melt, and delivers the resin melt to the entrance to the mold. The screw then acts as a ram and moves forward to force a fixed volume of the resin melt into the closed mold. After the resin melt has solidified in the cool mold, the screw rotates and moves backward to ready the charge of resin in the next cycle. Meanwhile, the mold is opened and the molded article is removed. The rate of injection and the pressure achieved in the mold are controlled by a machine hydraulic or electronic system. In conventional injection molding processes, the pressures used usually range from 35-138 MPa (5,000-20,000 psi), and melt temperatures used usually vary from about 200C to about 350C for crystalline diermoplastic. The molten resin flows into a mold from the screw through a sprue and/or runner and then through a gate to the cavity in the mold. There are various types of gates, such as rectangular, fan, slit or flash, diaphragm, pinpoint or tunnel gates. The design of the mold used to encapsulate the high voltage coils is an important part of the present invention and significant features in the design of the mold include sprues and runners, gates, the mold cavity, and vents. In the present invention, it is preferred to use a sprue and a flash or diaphragm gate, in which the resin melt flows from the screw through a sprue to the gate, and then into the mold cavity. The entrance diameters of the sprue is in the range of from 0.15 to 0.28 inches (3.80 to 6.4 mm). The diaphragm or flash gate is the preferred type of gate because it minimizes the weld line and improves the circularity of the molded part. The thickness of the gate is greater than 1 mm. The mold cavity is shaped to accommodate the coil to be encapsulated and is sized to permit a desired thickness of thermoplastic resin to be formed around the coil. There are a plurality of vents formed in the mold at ends distant from the gate so as to allow easier filling of the cavity. A high voltage coil 1 1 shown in Figure 1 is supported in an annular mold cavity on a plurality of movable pins. The pins support coil 1 1 in both the axial and radial direction. Typically, there are four support pins on both sides of coil 11 providing axial support, and there are four support pins on the outside of coil 11 that support coil 1 1 in the radial direction. The pins are preferably moved hydraulically, and have sufficient hydraulic pressure that the pins may remain stationary or move even when under the pressure in the cavity. A typical pressure may be 500 psi. The diameter of the pins depends on the size of the coil to be encapsulated. For coil 1 1, the diameter of the pins is typically about 1/8 of an inch. Coil 11 may be made out of any electrically conductive material, such as copper or aluminum enameled wire. The wire should be wound into a tightly packed precision winding or a self-standing coil, and there should be minimal voids within the winding of the wire that forms coil 11, although coil 11 is not substantially void free. Further, the difference in voltage between adjacent turns in the winding of coil 1 1 should be low enough that a minute void in coil 1 1 does not cause corona discharge. The shape of coil 1 1 is not critical - it may be rectangular, square, oval or circular. Self-standing coil 11 may be formed by winding a bondable magnet wire on a mandrel, charging the wire electrically to melt an adhesive coating on the coil, compressing and then cooling the coil, or alternatively using a heated mandrel and external heat. Coil 11 is suspended with four pins that extend in both axial directions and four pins that extend in a radial direction with four pins being in contact with each of top surface 13, bottom surface 15, and outer surface 19 of coil 1 1 to support coil 1 1 in the mold. Coil 11 is wound axially, but the direction of the coil windings is not critical and the coil windings may also be radial or a combination of radial and axial. Two wires 21 , 23 extend from outer surface 19 of coil 1 1. Wire 21 is an input wire and electricity flows through wire 21 into coil 11, and wire 23 is an output wire and electricity flows through wire 23 out of coil 1 1. However, the flow of electricity may be in either direction. When coil 1 1 is suspended in the mold, wires 21 and 23 are positioned outside the mold and are clamped off to prevent the flow of electrically insulating thermoplastic resin from escaping the cavity while allowing a sufficient build-up of pressure in the mold. One way to cover wires 21 , 23 is with a compressible sleeve such as high temperature elastomer or rubber, so that when the mold closes around the wires, the wires are not deformed or scraped. Alternatively, wires 21, 23 are enameled but are not covered with a separate sleeve but rather are clamped by using a mold that closes to within 1 mil of the surface of wires 21, 23. The mold is formed so that the resin melt enters the mold through a circumferential gate formed in the inner radial surface of the mold. A plurality of vents are formed at the point where the outer radial surface and both axial surfaces of the mold join. In the present invention, there are four vents approximately 0.001 inch (0.025 mm) deep formed at both the junctures of the outer radial surface and both axial surfaces. The size of the gate and the position of the gate and the vents in the mold are critical because when the electrically insulating thermoplastic resin flows into the mold and around coil 11 the thermoplastic resin meets on the side of coil 1 1 and the mold farthest from the gate to form what is commonly referred to as a weld line, and it is an area where voids can occur in an encapsulation process. If the gate is a single continuous opening around the inner radial surface of the mold, then there is formed one single, continuous weld line on surface 19 of coil 1 1 which reduces the chance of voids being formed in the electrically insulating thermoplastic resin encapsulation layer. Further, if the size of the gate is too small, and/or the pressure of the molten resin is too low, and/or the resin freezes too quickly, then the resin may not fully fill the cavity. The gate preferably is positioned close to the middle of coil 1 1 so that equal pressures are applied to both sides of coil 1 1 as the resin is injected into the mold. However, it is often a requirement that coil 1 1 have different thicknesses of resin on surfaces 13, 15, and in that case the gate must be positioned slightly off center from surface 17 of coil 1 1. It is important to position the gate so that the weld line coincides with the vent openings. After coil 1 1 is suspended in the mold and the mold is closed, electrically insulating thermoplastic resin is injected into the mold under high pressure. An example of a suitable electrically insulating thermoplastic resin is Rynite® RE5220 thermoplastic polyester resin, which is a 30% glass reinforced polyethylene terephthalate (PET) polyester available from DuPont of Wilmington, Delaware. Molten electrically insulating thermoplastic resin flows into the mold through the sprue, then through the gate, and then around coil 1 1 to encapsulate coil 1 1 to form encapsulating layer 25 in Figure 2. Layer 25 has a top surface 27, a bottom surface 29, an inner surface 31 and an outer surface 33. As the electrically insulating thermoplastic resin is injected into the mold, the support pins are withdrawn as the electrically insulating thermoplastic resin moves through the mold before the molten resin freezes around the pins. The pins are cooler that the molten resin, and as the resin touches the pins the resin may cool sufficiently to freeze and cause voids in the resin. The pins that support coil 1 1 in the axial direction are removed first, followed by the pins that support coil 11 in the radial direction. If the thickness of the resin is uniform on both sides of coil 1 1, the axial support pins are removed at the same time and at the same speed. If it is desired to have different thicknesses of resin on the top and bottom surface of coil 1 1, the axial support pins may be removed at different speeds. The thickness of layer 25 should be sufficient so as to isolate coil 1 1 from other electrical surfaces. For example, for a maximum difference in voltage from surface 17 of coil 1 1 to surface 31 of layer 25 of about 1 1 kilovolts, the preferred thickness of PET thermoplastic of layer 25 from surface 17 to surface 31 is at least 5 millimeters. Similarly, for a maximum difference in voltage from surface 19 of coil 1 1 to surface 33 of layer 25 of about 1 1 kilovolts, the preferred thickness of PET thermoplastic of layer 25 from surface 19 to surface 33 is at least 5 millimeters. In use, a plurality of encapsulated coils 37, 39, 41, which are also referred to in the art as discs or pancakes, may be stacked together to form a stack 35 as is shown in Figure 3. There may be twelve identical encapsulated coils in a stack 35, six on each end separated by two encapsulated coils of fewer turns in the center or all coils may be identical or different with respect to the number of turns and the relative positions of the stack of coils. Note that wires 21, 23 of adjacent encapsulated coils are in alignment. Since encapsulated coils 37, 39, 41 are stacked one upon another, the preferred thickness of thermoplastic between the surface 13 of coil 1 1 of encapsulated coil 41 and the surface 15 of coil 1 1 of encapsulated coil 39 is at least 5 millimeters for a voltage of 1 1.5 kV Encapsulated coil 1 1 is removed from the mold and is tested to be sure that layer 25 is substantially void free. This test can be done by any conventional method known in the art. such as by X-ray or by taking partial discharge measurements using an oscilloscope to detect partial discharge, which is caused by voids. As used herein, the term "substantially void free" means that no voids are visible using a transmission electron microscope set to a magnification of 1000 mag. The encapsulation may be performed in either horizontal or vertical clamp molding equipment. Surfaces 27, 29, 31 and 33 of layer 25 of encapsulated coil 1 1 are machined, if desired, into any particular shape. For example it is preferred an inner ridge and an outer ridge be machined into top surface 27 of layer 25. The purpose of the ridges, which are also known as bosses, is to provide a means to vibrationally weld adjacent encapsulated coils 37, 39, 41 together. When adjacent encapsulated coils 37, 39, 41 are stacked together to form stack 35, the ridges on top surface 27 of encapsulated coil 41 are placed in contact with bottom surface 29 of encapsulated coil 39 The adjacent encapsulated coils 39, 41 are welded together using conventional vibrational welding techniques. This process is continued using additional encapsulated coils so as to form a stable stack 35 having a desired number of coils. As an alternative to machining, the ridges may be molded into these surfaces during encapsulation. However, the encapsulated coils 37, 39, 41 may be joined together in a stack 35 by means other than vibrational welding. Examples of other means of bonding together adjacent encapsulated coils 37, 39, 41 include sonic or rotational welding, gluing and other equivalent techniques. As is shown in Figure 4, encapsulated coils 1 1 in stack 35 are positioned so that wires 21, 23 of adjacent coils 1 1 normally are aligned. Stack 35 may be coated with a thermoplastic moldable composite sheet (MCS) material, such as is sold by DuPont, of Wilmington, Delaware so that stack 35 may be used in an electric transformer. EXAMPLES A self-standing coil having minimal voids and an outer diameter of 10 inches was encapsulated with DuPont Rynite® RE5220 thermoplastic polyester resin, a 30% glass reinforced polyethylene terephthalate (PET) polyester, as follows. The coil was supported in a mold cavity by twelve hydraulic pins. Four pins supported the coil in the axial direction on the outer diameter of the coil, and four pins supported the coil in each radial direction. The mold was in a 35 oz. Reed injection molding press, with a general purpose screw, and a mold having a sprue, and a diaphragm gate having an opening of about 85-90 mils. Twelve coils were encapsulated with resin to a thickness of 6.3 millimeters on one axial surface and 5.0 millimeters on the other axial surface with the processing parameters given in Table I. TABLE I TEMPERATURE RANGE (F) 1 Barrel: rear 575 center 570 front 580 nozzle 580 mold (fixed) 260 mold (movable) 255-260 resin melt 575-587 PRESSURE RANGE (psi) injection 1 st stage 8000 injection 2nd 5000-6000 stage CYCLE TIME RANGE (sec) injection 33-45 hold 45 open 0.5 overall 90-140 booster 3.1-3.9 screw retraction 35 PAD (inches) 0.5 SCREW RPM 43 Any electrically insulating thermoplastic resins which may be used in this invention, including 6,6-polyamide, 6-polyamide, 4,6- polyamide, 12, 12-polyamide, 6, 12-polyamide, and polyamides containing aromatic monomers, polybutylene terephthalate, polyethylene terephthalate, polyethylene napththalate, polybutylene napththalate, aromatic polyesters, liquid crystal polymers, polycyclohexane dimethylol terephthalate, copolyetheresters, polyphenylene sulfide, polyacylics, polypropylene, polyethylene, polyacetals, polymethylpentene, polyetherimides, polycarbonate, polysulfone, polyethersulfone, polyphenylene oxide, polystyrene, styrene copolymer, mixtures and graft copolymers of styrene and rubber, and glass reinforced or impact modified versions of such resins. Blends of these polymers such as polyphenylene oxide and polyamide blends, and polycarbonate and polybutylene terephthalate, may also be used. Optionally, such thermoplastic resins may include additives such as flame retardants, reinforcements, fillers, colored pigments, bulk fillers, plasticizers, and heat and light stabilizers. The amount of reinforcements or filler used can vary from about 1 to 70 weight percent based on the weight of the polymer and filler present. The preferred type reinforcement for use is fiberglass, and it is preferred that the fiberglass be present in the amount of about 15 to 55 weight percent based on the total weight of the polymer and filler present. The thermoplastic polymers can be prepared by known methods. When reinforcements or fillers are used, they can be added to the thermoplastic polymers during the preparation of the polymers or compounded in a separate step according to the conventional methods. The dimensions given herein are for the specific coil 1 1 encapsulated in the examples. As one skilled in the art would know, the dimensions specified herein depend on the size of the coil to be encapsulated.

Claims: WHAT IS CLAIMED IS: 1. A process for encapsulating a high voltage coil with an electrically insulating thermoplastic resin such that the electrically insulating thermoplastic resin is of sufficient thickness that the coil is isolated from other electrical surfaces and the electrically insulating thermoplastic resin is substantially void free, comprising the steps of providing a mold having a cavity, a gate, a vent, and retractable support pins for supporting an object in the mold, placing a high voltage coil in the cavity, moving the pins into contact with the coil to support the coil in the cavity, injecting an electrically insulating thermoplastic resin into the mold through the gate, retracting the pins from the cavity as the electrically insulating thermoplastic resin is injected into the cavity before the resin freezes around the pins to prevent the formation of voids in the electrically insulating thermoplastic resin, allowing air to vent from the cavity through the vent as the resin moves through the mold, and continuing to inject the electrically insulating thermoplastic resin until the mold is filled to form a substantially void free electrically insulating thermoplastic resin layer of sufficient thickness so that the coil is isolated from other electrical surfaces. 2. The process of claim 1, comprising injecting the electrically insulating thermoplastic resin until a uniform thickness of at least 3 millimeters is obtained on the inside and outside diameters of the coil, and a uniform thickness of at least 2-1/2 millimeters is achieved on the top and bottom of the coil. 3. The process of claim 1, comprising the step of using a glass reinforced polyethylene terephthalate polyester as the resin. 4. The process of claim 1, comprising the step of using a resin selected from the group consisting of 6,6-polyamide, 6-polyamide, 4,6- polyamide, 12, 12- polyamide, 6, 12-polyamide, polyamides containing aromatic monomers, polybutylene terephthalate, polyethylene terephthalate, polyethylene napththalate, polybutylene napththalate, aromatic polyesters, liquid crystal polymers, polycyclohexane dimethylol terephthalate, copolyetheresters, polyphenylene sulfide, polyacylics, polypropylene, polyethylene, polyacetals, polymethylpentene, polyetherimides, polycarbonate, polysulfone, polyethersulfone, polyphenylene oxide, polystyrene, styrene copolymer, mixtures and graft copolymers of styrene and rubber, and blends thereof. 5. The process of claim 1, further comprising the steps of forming at least one concentric ridge on the top surface of the coil to provide means for vibrationally welding adjacent coils together. 6. A process for joining adjacent electric coils encapsulated with an electrically insulating thermoplastic resin comprising the steps of forming at least one ridge on the top surface of a first coil, placing the top surface of the first coil in contact with the bottom surface of a second coil, and providing vibrational force to the coils to vibrationally weld the coils together. 7. The process of claim 6, further comprising the step of encapsulating the welded coils with a thermoplastic moldable composite sheet material.

Inventors: BENARBA ABDELKADER, KOZIELSKI GARY PAUL, PATTERSON JAMES FRANCIS BOYS, SHEER M LANA, BENARBA, ABDELKADER, KOZIELSKI, GARY, PAUL, PATTERSON, JAMES, FRANCIS, BOYS, SHEER, M., LANA
Applicants: BENARBA ABDELKADER, DU PONT, KOZIELSKI GARY PAUL, PATTERSON JAMES FRANCIS BOYS, SHEER M LANA, BENARBA, ABDELKADER, E.I. DU PONT DE NEMOURS AND COMPANY, KOZIELSKI, GARY, PAUL, PATTERSON, JAMES, FRANCIS, BOYS, SHEER, M., LANA
Application year: 1996
Main classification: H01F27
IPC codes: H01F27, H01F41