Durable Coreplate For Improved Electrical Resistance In Electric Equipment And A Process Thereof
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An improved inter-laminar coating for steel laminations used in electrical equipment such as electric motors, generators and transformers. The invention further includes a process for creating, restoring, and improving the condition of insulation of such equipment using an aqueous solution of phosphoric acid with additives including zinc and/or manganese.

Yung, Charles A. (Florissant, MO, US)
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Attorney, Agent or Firm:
Sandberg Phoenix & von Gontard, PC (St. Louis, MO, US)
What is claimed is:

1. A solution for coating, or restoring the coating of an interlaminar insulation of electrical steel laminations, comprising a mixture of: a) an aqueous solution of phosphoric acid with or without nitric acid, applied to a steel lamination; b) zinc and/or manganese in any combination, which enhances the electrical resistance of the conversion coating so formed; and, c) about 0.01% by weight of iron, based on the weight of a) and b), dissolved into suspension, use of the mixture reducing core losses of the laminations when used in transformers and rotating electric machines including motors and generators.

2. The solution process of claim 1 in which Calcium (3.35e-8), Strontium (1.31e-7), or Barium (3.32e-7) is substituted for the manganese.

3. The solution of claim 2 in which Cadmium (7.24e-8) is substituted for the zinc.

4. The solution of claim 1 further including substituting any combination of Calcium, Strontium, Barium, and Cadmium for the manganese or zinc.

5. A process of applying the coating of claim 1 wherein the laminations are preheated to approximately 160°-210° F. (71° C.-≈100° C.) before the mixture is applied.

6. The process of claim 5 in which the mixture is applied by immersion of the preheated laminations into the mixture.

7. The process of claim 5 in which the mixture is applied by spraying the mixture onto the preheated laminations.

8. The process of claim 5 wherein the solution is applied by a continuous process using a spray or a mist, or applying the solution using a roller.

9. The process of claim 5 in which the aqueous solution is applied to the steel laminations either individually, in combination, or when assembled into one or more cores.



This claims priority to Provisional patent application 60/726,138 filed Oct. 13, 2005 which is incorporated herein by reference.




This invention relates to electrical equipment such as electric motors, generators and transformers, and more particularly, to a durable coreplate for improved electrical resistance in such equipment and a process for applying the coreplate.

As is well-known in the electric motor art, steel laminations are used to form the stator core of an electric motor or generator. The laminations are also used to make electrical transformers. The laminations, which are electrically insulated from each other, are formed into a stack. The present invention relates to an improvement over previously used interlaminar insulation (aka, coreplate) such as that described in U.S. Pat. Nos. 3,908,066 and 3,839,256. These patents describe compositions, methods for coating, and coated electrical or magnetic grade steel for use in the magnetic cores of transformers, motors and generators. The coatings so described have been widely used in the industry.

There are, however, numerous problems with present coreplate processes. For example, use of sodium silicate is hampered by the fact it is water-soluble so the coating formed is not durable. Further, the process employed to form the Fe3O4 widely used as an interlaminar insulation in electrical equipment to develop coreplate, requires careful control of temperature, oxygen and humidity, as well as extended exposure at temperatures above 400° F. (204° C.). Because of this, the energy required to develop Fe3O4 often accounts for one-third of the lamination cost.

During operation of a motor or generator, the laminated core vibrates at twice the line frequency. Over time, this results in interlaminar insulation abrading and deteriorating, resulting in increased operating temperatures on portions of the laminations (localized hotspots) that reduce performance of the equipment, and sometimes cause winding failure. This is especially problematic in large generators (several megawatts and larger) used to produce power. The cost to repair the core damage, using traditional repair methods, is often beyond economic feasibility. While a small amount of coreplate deterioration does not significantly affect the performance of electric motors, generators or transformers, it does increase core losses and thereby reduces efficiency of the machine. And, the effect of such damage is cumulative; so, over time, the operating costs of a motor or generator may increase. Evidence of this degradation can be found in reports leading up to the Energy Policy Act of 1992, and numerous studies before and since.

When coreplate deteriorates, the operating temperature of the motor, generator, or transformer increases, reducing the insulation life of the windings. When used as a part of the original manufacturing process for this equipment, the improved coreplate and process of the invention extends the life of the lamination insulation. Further, it results in more efficient repairs when used as part of the repair process.


The present invention is directed toward an improved inter-laminar coating for steel laminations used in electrical equipment such as electric motors, generators and transformers. The invention further includes a process for creating, restoring, and improving the condition of insulation of such equipment using an aqueous solution of phosphoric acid with additives including zinc and/or manganese.

The invention includes a single-step process for eliminating interlaminar shorts and developing coreplate. The coreplate can withstand burnout at high oven temperatures, and the coreplate is relatively safe, free from harsh chemicals or residue, and it does not degrade or contaminate varnishes or resins. The coreplate is also highly durable. Use of the process and coreplate of the invention both reduce energy costs during manufacture of a motor, generator, or transformer, and during subsequent operation of the machine.

Other objects and features will be in part apparent and in part pointed out hereinafter.


The following detailed description illustrates the invention by way of example and not by way of limitation. This description clearly enables one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. Additionally, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it will be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

This present invention comprises the use of an aqueous solution of phosphoric acid, with or without a small quantity of nitric acid, and with certain additives, to improve the formation of a coating for steel laminations used in motors, generators, transformers, etc., and to provide the laminations with excellent properties of electrical resistance. Further, previously published research (EASA AEMT study) indicates that conventional organic coreplate deteriorates at temperatures in excess of 680° F. In accordance with the invention, the combination of phosphoric acid, to etch the surface of the laminations, and the chemical reaction of the solution with the steel, forms a phosphate conversion coating on the surface of the laminations. The additives, which include zinc and/or manganese, enhances the electrical resistance of the lamination surface exposed to the solution, and reduces eddy-current losses therein. A phosphoric acid solution, with zinc in amounts ranging from approximately 0.05% to approximately 5.0%, together with manganese is amounts ranging from approximately 0.02% to approximately 2%, is applied to the metal substrate of the laminations as follows:

A solution of dispersion in an aqueous medium, as set forth Table 1 below, was used for testing. A preferred formulation includes a 20% concentration of phosphoric acid and 1% nitric acid, together with a zinc content between 0.01% and 1%, and less than 2% manganese.

Parts by Weight
Phosphoric acid,200
Nitric acid45
Zinc Phosphorate53
Manganese iron oxide, Fe2O3, and
manganese dioxide, MnO2,

Samples of some solutions contain traces of zinc salts as impurities.

Small cores (less than 5 kW), when being rewound, have been treated by immersion in the solution for 15 minutes. Larger cores (100-200 kW) have been treated by a method described hereinafter. Core losses in the electrical equipment in which these cores are installed were reduced by over 50%. Still larger magnetic cores (up to 1500 kW thus far) have been treated using similar methods, with similar results.

On the basis of these observations, and tests summarized in Table II below, eddy-current losses in magnetic cores are considerably reduced which improves the electric equipment's performance by reducing losses and thereby increasing operating efficiency and reducing operating cost, and by reducing the equipment's operating temperature and thereby increasing the life of its windings. This latter is in accordance with the widely accepted 10° C. rule; i.e., a 10° C. increase in winding temperature reduces insulation life by one-half.

Test #Core loss (watts/lb)Power factorHot spot temp. (F.)
14.21 to 2.340.67 to 0.58128 to 83
23.75 to 1.960.70 to 0.50109 to 74
311.89 to 1.82 187 to 79

Reduction in core loss and hot spot temperature for before and after treatment.


The solution is preheated to between 160° F. (71° C.) and 210° F. (≈100° C.). The part to be treated can be immersed or sprayed with the solution. Smaller cores are immersed in the solution while at ambient temperature when immersed. Here, reliance is placed on heat from the solution to heat the core. For large cores, the core is preheated to between 160° F. and 210° F. before immersion into the solution. For larger cores it is also more practical to apply the solution topically, except in instances of large volume production. One way of doing this is to use a spray bottle to apply the solution. Regardless of the method of application, the surface of the core lamination should be in contact with the solution for 15 minutes to achieve the optimal coating. The benefits of the solution and its application are achieved with either method of application.

The chemical process is a conversion coating; that is, the surface of the steel reacts chemically with the phosphoric acid solution to coat the surface of the steel laminations with a phosphate coating. Presence of zinc and/or manganese further improves the electrical resistance of the coating so formed. After 15 minutes of exposure to the solution, the treated core is rinsed in cold water to remove traces of any zinc salts.

The micro-porosity of the conversion surface has additional benefits. One is the improvement of bond strength of winding treatments to the core after the wound core is dipped, or VPI processed. This affords a better seal and improves moisture resistance.

Overall, the results show an average reduction in core-loss greater than 50% of the before-treatment value for cores being rewound. In instances where surface shorting of laminations was noticeable, the interlaminar shorts were corrected by treatment with the solution, and the reduction in core loss was even greater. Long-term benefits from applying the solution to the cores did not appreciably lessen over the course of tests to which the equipment was subjected. Motors treated with the solution during the research and development stage of the product have been running successfully in industrial applications for over one year, without failure. Comparable results are expected for new manufacture of cores using this process.

Finally, in preparing the solution, Calcium (3.35e-8), Strontium (1.31e-7), or Barium (3.32e-7) may be substituted for the manganese, and/or Cadmium (7.24e-8) substituted for the zinc. Or, any combination of Calcium, Strontium, Barium, and Cadmium can be substituted for the manganese or zinc.

In view of the above, it will be seen that the several objects and advantages of the present disclosure have been achieved and other advantageous results have been obtained.