US2979432A - Insulating method - Google Patents

Description

April 1961 E. E. THlESSEN 2,979,432

INSULATING METHOD Filed Dec. 15, 1954 3 Sheets-Sheet 1 FOE/W C 0/1 APPL Y DQ/ME (0A 7' A ppL Y lNJUL A T/O/V APPL Y PIQfSSMQE cf FZAS/f l UL CA IV/ZE ATTORNEY If j] 41;

April 1951 E. E. THIESSEN 2,979,432

INSULATING METHOD Filed Dec. 15, 1954 3 Sheets-Sheet 2 ,7 I 4 J7 4 W 4! I F I 0/2567 cuppa/r JOUPC'E A NO (ON TQOL INVENTOR AT'TORNEY April 11, 1961 E. E. THIESSEN INSULATING METHOD 3 Sheets-Sheet 5 Filed Dec. 13, 1954 I N V E N TO R 6222222 6 775252 022 ff/.6040

ATTOQ NEY United States INSULATING METHOD Filed Dec. 13, 1954, Ser. No. 474,775

3 Claims. (Cl. 154-s0) This invention relates to insulating systems for electrical conductors and coils and more particularly to a method of and apparatus for applying such systems. The invention has particular application to coils for use as field or armature coils for dynamoelectric machines.

It is well known that the insulation system of electrical apparatus such as dynamoelectric machines imposes a limit upon the power output rating of the apparatus because of the deterioration of the insulating material at high temperatures. This is particularly true of organic insulating materials. Certain silicone compounds, however, have found widespread use as electrical insulation and because of their excellentthermal stability are considered as a high temperature insulating material along with mica and other inorganic substances. Silicone rubber is admirably Well suited for insulating systems sub-' jected to severe operating conditions such as mechanical stresses, abrasion and chemical action. A most important characteristic of silicone rubber is its ability to withstand indefinitely long exposure to temperatures in the vicinity of 180' degrees C. without significant change in its mechanical or electrical properties. Additionally, silicone rubber is recognized as a good thermal conductor among insulating materials. Thus, silicone rubber insulation contributes to improved performance of'electrical apparatus by permitting a higher temperature rise and by increasing the thermal dissipation.

However, the prior art insulation systems utilizing silicone rubber, particularly built-up jackets of plural layers and the like, are subject to the adverse effects of thermal barriers such as voids or air spaces in the insulation system. This arises because of the failure to achieve a good bond or adhesion between the silicone rubber and the conductor material and between adjacent layers in the system. Consequently, the insulation system or jacket does not dissipate heat readily.

Therefore, it is an object of this invention to provide a method of applying a high temperature insulation system of silicone rubber to an electrical conductor or coil which will afford improved heat dissipation.

Another object is to provide an insulation system of silicone rubber on an electrical coil which is bonded to the coil conductor and characterized by the absence of voids or air spaces and hence improved thermal conductivity.

Another object is to providea method of insulating electrical coils with vulcanized silicone rubber bonded layer to layer and free of air spaces.

A further object is to provide a method of applying glass cloth supported silicone rubber to electrical coils which provides a vulcanized insulation system bonded to the coil conductor and layer to layer.

An additional object of the invention is to provide apparatus for simultaneously vulcanizing and pressing silicone rubber insulation on an electrical coil to produce an insulation of high thermal conductivity.

These and other objects of the invention and the manner in which they are achieved will become apparent from stem a 2,979,432 Pat ented Apr. 11, 1961 ICC the description which follows taken with the accompanying drawings in which:

Figure 1 is a plan view of a typical coil to be insulated by the inventive method.

Figure 2 is a diagrammatic representation of the method steps.

Figure 3 is a cross section taken on line 3-3 of Figure 1.

" Figure 4 is a cross section of a coil conductor showing successive stages of application of the insulating material.

Figure 5 is a plan view of vulcanizing and pressing apparatus for performing a part of the method.

Figure 6 is an elevational view in section of part of the apparatus of Figure 5.

It has been found that an insulation system of silicone rubber having exceedingly good properties of heat dissipation may be produced by bonding the silicone rubber to the electrical conductor and by bonding the insulation layer to layer. According to this invention, the bonding to the electrical conductor is obtained by treating the conductor with a prime coat of varnish and applying the silicone rubber in a semi-vulcanized state. The silicone rubber is then flash-vulcanized by electrical resistance heating and simultaneously pressed against the conductor. Where plural layers of silicone rubber are used, bonding of layer to layer is achieved by the simultaneous flash vulcanizing and application of pressure.

- The silicone rubber employed in the inventive method is suitably in tape or sheet form. It preferably comprises a glass cloth backing of approximately 0.007 inch thickness impregnated and coated on one side with iron oxide filled, partially vulcanized silicone rubber to a total thickness of approximately 0.015 to 0.025 inch. The chemistry of the silicones including silicone rubber is treated at some length in the literature and will not be discussed herein. See for example, Rochow, Chemistry of the Silicones, 2nd ed.,-Wiley and Sons, Inc, 1951, and the literature cited therein. Very briefly, silicone rubber is a synthetic material, the molecules of which are long chains of silicon-oxygen 'units with organic groups attached to each silicon atom.

A specific silicone rubber tape and sheet which is preferred for the inventive method is a polymeric organosiloxane on glass cloth backing sold under the trademark Silastic R by the Dow-Corning Corporation. The physical properties of this material are very well suited for coil insulating jackets which must Withstand high operating temperatures, mechanical stresses, abrasion and exposure to chemicals. Typical properties are as follows:

Tensile strength unvulcanized, pounds per inch Width (15 mil thickness) Water absorption (24 hrs. immersion at 25 degrees C.) 0.5 to 1.0

Thermal stability:

Dielectric strength, volts per mil inch electrode)- c No aging 863 Aged 7 days at 523 degrees F. 793 Compression set percent 4 to 6 humidity) 2 1o To insure superior bonding of the silicone rubber in the process to bedescribed subsequently, the silicone rubber should meet certain initial conditions. The rubber should be partially vulcanized or cured only sufficiently to prevent adhesion between layers on the rolls or adjacent sheets. The rubber surface and glass backing must be free from dirt and other foreign material.

The inventive method as practiced in a typical application for the insulation of a motor coil will be described in detail with reference to the drawings. In Figure 1 there is shown an interpole coil 1 for a traction motor which is to be provided with an insulating system or jacket in accordance with the present invention. The coil 1 is formed from a coil conductor of rectangular, metallic copper strap 3 and is provided with leads 4 and 5. The method steps performed in this specific application are enumerated in the diagram of Figure 2.

Referring now to Figure 3, after the coil has been wound from the copper strap 3 and the leads have been formed, the individual coil conductor turns 6 are insulated from each other by inserting any suitable insulating material preferably of the high temperature class such as mica plates 7. The mica plates 7 are of a'width such that they are flush with the inside walls 8 and outside Walls 9 of the coil 1.

After the coil is formed and insulated turn to turn as described above, all exposed surfaces including inside surface 8, outside surface 9, upper axial surface 10 and lower axial surface 11 of the coil conductor are given a prime coat of shellac varnish 12. The coil is then baked until the resoftening temperature of the varnish 12 exceeds the temperatures of subsequent processing or operation which will appear hereinafter. In the specific example a suitable baking time is 5 hours until the varnish is cured such that it is not thermoplastic at 190 degrees C. The varnish found to be most suitable for obtaining a good bond of the subsequently applied layers of silicone rubber is prepared as a solution or cut of dry lac resin and denatured alcohol or other suitable solvent. Preferably, the shellac varnish is that known in the art as grade A, orange, of light body and approximately a four pound cut. The shellac portion of the solution should constitute approximately 34.5 percent of the solution and contain substantially no rosin, a maximum of 1.25 percent of insoluble matter, 5.5 percent of wax, and 1 percent of ash.

Following the baking of the varnish on the coil conductor surfaces, silicone rubber tape 13 is applied as shown in Figure 4 which represents a plan view in section of a coil turn 6. The tape is applied with the silicone rubber coating adjacent the varnish coat and is suitably wrapped in half-lapped fashion providing a double layer for each wrapping. Multiple layers may be provided by an'additional wrapping of silicone rubber tape 13 if desired. A tight wrapping of the tape is very desirable and may be insured by compressing the coil turns 6 as the taping progresses. To provide an additional layer of silicone rubber insulation on the inside wall 8 of the coil 1 a silicone rubber sheet 14 is wrapped on the inside wall 8 and folded over the outside wall 9. An adhesive tape 15 of any suitable kind such as glass cloth tape is then wrapped tightly about the coil 1 to complete the insulating jacket 16.

Following the application of the insulating material to the coil, the entire surface of the insulating jacket 16 is pressed tightly against the coil conductors 6 and is subjected to what is termed herein as flash vulcanizing. The flash vulcanizing is effected by the application of heat in such manner that a very high rate of temperature rise is produced. The temperature for vulcanizing, suitably in excess of 120 degrees C. and preferably about 175 degrees (3., must be reached in a short period of time, preferably less than 7 minutes to achieve the desired result of an intimately bonded, void free, flexible and tough insulating jacket. If the rate of heating is less than a certain critical minimum the rubber becomes a gummy or viscous, fluid-like mass. Prior to or simultaneously with the application of vulcanizing heat, pressure must be applied to the insulating material. Although the value of the pressure is not critical, it has been found that approximately 50 pounds per square inch is quite satisfactory. The pressure is effective to insure good heat transfer during vulcanization and elimination of all air spaces in the completed insulating jacket.

At this point in the process, the silicone rubber tape 13 and 13 and sheet 14 are bonded intimately together and the inner layer of tape 13- is bonded intimately to the coil conductor 6. It has been found that the interposition of the organic varnish coat 12 between the coil conductor 6 and the silicone rubber is of great importance to superior bonding. Even though the organic material, in a high temperature insulation system, eventually deteriorates from thermal aging, no detrimental effect on the bond results. An exact explanation of this behavior and the manner in which the organic varnish contributes to the bonding is presently unknown. It has been advanced as a theory that shellac varnish during the baking period becomes somewhat porous and forms an acid, both of which conditions contribute to the bond.

Following the flash vulcanizing and simultaneous pressing of the insulation jacket 16 the insulated coil is subjected to an after-cure by baking to develop the optimum electrical and mechanical properties of the insulation. This baking may be suitable carried out in an air circulating oven at a temperature higher than subsequent operating temperatures of the coil. In the specific application described herein baking at degrees C. for a period of 16 hours is most satisfactory although it will be understood that the baking time will vary with the size and shape of the coil and the insulation thickness. Subsequent coatings of protective resins or the like may be applied if desired.

Simultaneous application of pressure to all surfaces of the insulating jacket and flash vulcanizing of the silicone rubber may be most suitably accomplished in the vulcanizing and pressing apparatus of Figures 5 and 6. The vulcanizing and pressing apparatus comprises a table 20 adapted to be raised and lowered on shafts 21 by a hydraulic actuator 22 (Figure 6). A lower press 23 is mounted on a slidable tray 24 provided with operating handle 25. The lower press 23 has a resilient facing 26, suitably of rubber, which is adapted to support the coil 1 thereon in engagement with the insulating jacket 16 at the axial surface 11 of the coil. In axial alignment with lower press 23 is upper press 27 depending from plate 28. Upper press 27 is provided with a resilient facing 29 adapted to engage the insulating jacket 16 at the axial surface 10 of coil 1 when the press 23 is moved with table 20 from the lowered or open position to the raised or closed position.

A compressible block of resilient material 30, suitably of rubber, is mounted on lower press 23 by any suitable securing means such as bolt 31 extending through washer 32 and sleeve 33. A resilient plug 30' is fitted over bolt 32 to provide a continuous surface on block 30. As shown in Figure 5 the rubber block 30 has a cross sectional configuration conforming to the inside surface 8 of the coil 1. The rubber block is adapted to extend axially through and protrude beyond the coil 1 and with the presses 23 and 27 in the open position the block terminates at 34. With the presses 23 and 27 in the closed position the rubber block 30 is compressed therebetween and terminates at 34'. Axial compression of block 39 causes it to expand laterally and to exert pressure against the insulating jacket 26 at the inside surface 8 of coil 1.

Mounted on table 20 are side presses. 35 and 35' for compressing the insulating jacket 16 against the outside lateral surface 9 of the coil 1. For purposes of illustration only side press 35 is shown in the open position and side press 35 is shown in the closed position. Since the'two side presses are identical, the description will be given with reference to press 35 although the same reference numerals with prime symbols affixed will apply to corresponding parts of press 35'. A continuous resilient facing 36 is mounted on pressing shoes 37, 38 and 39 which have a configuration conforming to that of the adjacent outside surface 9 of the coil. The pressing shoes 37, 38 and 39 are connected by plungers 40, 41, and 42 to hydraulic actuators 43, 44 and 45, respectively. The shoes 37 and 39 are preferably pivotally connected to the respective plungers 40 and 42 to permit the necessary angular displacement between the open and closed position of the press 35. Mounted on the tray 24 are end presses 46 and 46'. End press 46 comprises resilient facing 47 mounted on pressing shoe 48 which is con- ,nected by plunger 49 to hydraulic actuator 50 for movement from open to closed positions. Similarly end press 46' includes resilient facing 47, mounted on pressing shoe 48 which is connected by plunger 49' to hydraulic actuator 50.

The hydraulic actuators are of any suitable type, although preferably provided with double acting pistons energized through conduits 51 and 52 to provide controlled closing and opening of the presses. The conduits are connected to a hydraulic source and control system (not shown) which will not be described since it forms no part of the present invention and suitable arrangements are well known to those skilled in the art.

The coil leads 4 and extend outwardly from the coil 1 to a position adjacent end presses 46 and are connected through conductors 53 and 54 to an electrical source 55 provided with suitable control arrangements to regulate the current through the coil 1. The electrical source is preferably of direct current and since the detailed structure thereof forms no part of this invention, it is represented in block diagram in the interest of clarity.

The electrical source must be capable of delivering a suflicient current to insure the proper rate of temperature rise in the coil to produce flash vulcanization of the silicone rubber. The current required is, of course, dependent upon numerous factors including the size, shape, insulation thickness and conductor material of the coil being processed. For coils of large mass such as traction motor coils, electrical resistance heating is exceptionally well suited to attain the required heating rate and such coils necessarily require a high value of heating current. As a typical example, an interpole coil for traction motors of 14 turns of copper strap having a cross section of approximately 0.75 inch by 0.34 inch and approximate total length of 500 inches in an insulating jacket substantially that shown in Figure 4 requires approximately 1600 amperes to raise the coil temperature to 175 degrees C. in 7 minutes or less.

The operation of the vulcanizing apparatus of Figures 5 and 6 will now be apparent. The coil 1 provided with the insulating jacket 16 of silicone rubber is positioned on lower press 23 coaxially of rubber block 36. Leads 4 and 5 are connected to source 55 by conductors 53 and 54. Pressure is admitted to the hydraulic actuators and the lower press 23 is raised by hydraulic actuator 22 to press the upper and lower surfaces of the coil insulating jacket 16 between resilient facings 29 and 26. The rubber block 30 is compressed axially between presses 23 and 27 and is expanded laterally thereby exerting pressure on the inside surface of the insulating jacket. At the same time, the presses 35, 35', 46 and 46 are closed by the associated hydraulic actuators and exert pressure on the outside lateral surface of the coil insulating jacket. Substantially. simultaneously with the closing of the presses, the coil is electrically energized from source 55 with the requisite heating current. After the heating current and pressure have been applied for a predetermined period of time, the current is interrupted and the presses are opened. The coil 1 is removed for the subsequent processing operation of baking and the vulcanizing apparatus is ready for a new cycle.

The advantages achieved from the inventive process and apparatus will now be apparent. The process of application is comparatively simple and permits the use of commercially available insulating materials. The insulated coil is capable of withstanding operation at high temperature and readily dissipates internally generated heat. The high rate of heat dissipation is attributed to the absence of voids or air spaces in the insulating jacket and the intimate bonds between the silicone rubber layers and coil conductor. The heat dissipation is further enhanced by the relatively high thermal conductivity of the silicon rubber itself. The finished insulating jacket exhibits excellent properties of mechanical toughness and resilience and thus is capable of withstanding severe mechanical stress imposed by vibration, thermal expansion and high currents in the coil. Additionally the insulating jacket is resistant to abrasion, water absorption and chemical action.

This invention has been described with respect to a particular embodiment for illustrative purposes only and such embodiment is not to be construed in a limiting sense. Numerous modifications and variations within the spirit and scope of the invention will now occur to those skilled in the art. For a definition of the invention, reference is made to the appended claims.

I claim:

1. The method of insulating an electrical conductor of metallic copper for operation ata high temperature comprising coating said conductor with a shellac varnish, curing said varnish on said conductor until its resoftening temperature exceeds said high temperature, applying a layer of partially vulcanized, glass cloth supported silicone rubber over said varnish, applying pressure to the surface of said rubber, and vulcanizing said rubber by passing a current through said conductor.

2. The method of making a void-free insulating jacket for an electrical coil having plural conductor turns of metallic copper to be operated at a high temperature comprising inserting an insulation material between adjacent turns, coating the exposed surfaces of said conductor turns with shellac varnish, curing said varnish until its resoftening temperature exceeds said high temperature, applying a layer of glass supported partially vulcanized silicone rubber over said varnish, compressing said rubber against said coil, passing a current through said coil to vulcanize said rubber, and baking said coil in an air circulating oven at a temperature exceeding said high temperature to develop optimum properties of said insulating jacket.

3. The method of making a void-free insulating jacket for an electrical coil having plural conductor turns of metallic copper to be operated at a high temperature comprising inserting insulating material between adjacent turns, coating the exposed surfaces of said conductor turns with orange shellac varnish, curing said varnish until its resoftening temperature exceeds said high temperature, applying plural layers of glass supported partially vulcanized silicone rubber over said varnish, compressing said rubber against said coil, passing a current through said coil to effect vulcanization of said rubber at a temperature of degrees C. within seven minutes, and baking said coil in an air circulating oven at a temperature exceeding said high temperature to develop optimum properties of said jacket.

References Cited in the file of this patent UNITED STATES PATENTS 1,874,723 Dawson Aug. 30, 1932 2,454,625 Bondon Nov. 23, 1948 2,473,842 Askey June 21, 1949 (Othefreferences on following page) UNITED STATES PATENTS DOrio Apr. 25, 1950 Coggeshall Apr. 24, 1951 Botts et al. June 24, 1952 Smith-Johannsen June 24, 1952 Smith-Johannsen June 24, 1952 Ford Dec. 30, 1952 Dexter Apr. 13, 1954 8 Bram Nov. 2, 1954' Blaisdell et a1. Jan. 24, 1955 Collings May 17, 1955 Keil Ian. 24, 1956 OTHER REFERENCES October 1951, pages 134137.

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No, 2 979 432 April 11 I961 Elmer E, Thiessen It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 54 for 005 to 1,0 read 005 to 1.0% column 4 line 27 for 'suitable" read suitably column 6, line 14 for "silicon" read silicone Signed and sealed this 5th day of September 1961 Att ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTlFICATlN 0F COEUHN Patent No, ,2 979 432 April ll 1961 Elmer Eo Thiessen It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 54L for "005 to 1.0" read 05 to 1.0% column 4 line 27 for suitable read suitably column 6 line 14 for 'silicon" read silicone e.

Signed and sealed this 5th day of September 19610 Att zs t ERNEST W. SWIDER v DAVID L. LADD Attesting Officer Commissioner of Patents

Claims (1)

1. 3. THE METHOD OF MAKING A VOID-FREE INSULATING JACKET FOR AN ELECTRICAL COIL HAVING PLURAL CONDUCTOR TURNS OF METALLIC COPPER TO BE OPERATED AT A HIGH TEMPERATURE COMPRISING INSERTING INSULATING MATERIAL BETWEEN ADJACENT TURNS, COATING THE EXPOSED SURFACES OF SAID CONDUCTOR TURNS WITH ORANGE SHELLAC VARNISH, CURING SAID VARNISH UNTIL ITS RESOFTENING TEMPERATURE EXCEEDS SAID HIGH TEMPERATURE, APPLYING PLURAL LAYERS OF GLASS SUPPORTED PARTIALLY VULCANIZED SILICONE RUBBER OVER SAID VARNISH, COMPRESSING SAID RUBBER AGAINST SAID COIL, PASSING A CURRENT THROUGH SAID COIL TO EFFECT VULCANIZATION OF SAID RUBBER AT A TEMPERATURE OF 175 DEGREES C. WITHIN SEVEN MINUTES, AND BAKING SAID COIL IN AN AIR CIRCULATING OVEN AT A TEMPERATURE EXCEEDING SAID HIGH TEMPERATURE TO DEVELOP OPTIMUM PROPERTIES OF SAID JACKET.

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