Patent Publication Number: US-4546130-A

Title: Molding material for the stator of a submersible motor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates in general to molding materials for protecting the stators of submersible motors, and in particular to a molding material having a lower moisture permeability than conventional molding materials for submersible motors. 
     FIG. 1 shows a simplified longitudinal cross-sectional view of the stator portion of a conventional submersible motor of the type to which the present invention is directed. In the figure, element 1 is a stator core, element 2 is the coil end portion of the stator coils for the stator core 1, element 3 is a stainless steel inner wall of a stainless steel casing (other parts of the casing not shown) which seals off the stator and protects it from moisture, element 4 is a molding material which fills all the empty spaces in the casing, and element 5 is electrical connections for the stator coils. That part of the motor lying inside the inner wall 3 is generally filled with an anti-freeze which serves to lubricate the thrust bearings (not shown) of the rotor (also not shown) of the motor. 
     The inner wall 3 is able to prevent anti-freeze, water, and other liquids from entering, via the rotor, the space around the stator. However, liquids are able to enter the motor from the ends of the casing or from connecting members via gap, between the inner wall 3 and the molding material 4, which form at the time of molding or thereafter. It is therefore important for the molding material to have a low permeability to moisture. 
     Materials commonly used for the molding material 4 include polyester resins and vinyl resins. However, such molding materials have the drawback that they are not sufficiently impermeable to moisture, and water is able to permeate the molding material 4 and reach the stator. This permeation results in an undesirable decrease in the insulating resistance of the molding material 4. 
     There is therefore a need for a molding material for use in the type of submersible motor illustrated in FIG. 1 which has a lower moisture permeability than presently used molding materials. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to overcome the drawbacks of conventional molding materials for the stators of submersible motors and provide a molding material with lower moisture permeability. 
     In order to achieve a low moisture permeability, a molding material according to the present invention has included therein one or more inorganic substances having moisture-absorption characteristics selected from anhydrous gypsum and silica gel. When one or more of these substances is included, any moisture which enters a submersible motor is absorbed and prevented from penetrating to the stator coils. 
     The novel molding material according to the present invention comprises approximately 20-30 parts by weight of an epoxy resin, approximately 1-3 parts by weight of a curing agent, approximately 0.5-1.5 parts by weight of zinc stearate, approximately 2-3 parts by weight of a mechanical reinforcing agent, approximately 7.5-56.5 parts by weight of quartz powder, and one or more inorganic substances having moisture-absorption characteristics selected from anhydrous gypsum and silica gel, with the total amount of said inorganic substance(s) being approximately 55-20 parts by weight, and with the total amount of quartz powder and said inorganic substance(s) being approximately 62.5 to 76.5 parts by weight. 
     In a preferred embodiment, the novel molding material according to the present invention comprises approximately 20-21.5 parts by weight of an epoxy resin, approximately 1-2.5 parts by weight of a curing agent, approximately 0.5-1.5 parts by weight of zinc stearate, approximately 2-3 parts by weight of a mechanical reinforcing agent, approximately 20-55 parts by weight of quartz powder, and one or more inorganic substances having moisture-absorption characteristics selected from anhydrous gypsum and silica gel, with the total amount of said inorganic substance(s) being approximately 55-20 parts by weight, and with the total amount of quartz powder and said inorganic substance(s) being approximately 75 parts by weight. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified longitudinal cross-sectional view of the stator of a conventional submersible motor to which the present invention is directed. 
     FIG. 2 is a graph of the insulation resistance for a conventional molding material (Curve 10) and for a molding material according to the present invention (Curve 11) plotted as a function of time. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A molding material according to the present invention is prepared by mixing an inorganic substance having moisture-absorption characteristics, an epoxy resin, a curing agent, zinc stearate, a mechanical reinforcing agent, and quartz powder in a kneader at ordinary temperatures until a uniform mixture is achieved. 
     An inorganic substance having moisture-absorption characteristics is included in the molding material for the purpose of absorbing any moisture which enters the casing surrounding the stator of a submersible motor, thereby preventing moisture from reaching the stator. 
     As the inorganic substance, one or more of anhydrous gypsum and silica gel in an amount of 55-20 parts by weight should be used. Although anhydrous gypsum and silica gel may be used in combination, the resistance to moisture of the resulting molding material is superior if either one is added alone. 
     As the epoxy resin, diglycidyl ether of bisphenol A such as Epicote 1001, manufactured by Shell International Chemicals Corporation, or Edolite EM 100, manufactured by Fudo Chemical Industries Inc. of Japan, in an amount of approximately 20 to 30 parts by weight, and preferably 20 to 21.5 parts by weight, is suitable. Approximately 1 to 3, and preferably 1 to 2.5, parts by weight of 4,4&#39;-diaminodiphenyl methane (DAM) is suitable as the curing agent. 
     Zinc stearate is added in order to prevent the oxidation of the stator core and to increase the binding of the epoxy resin. Approximately 0.5 to 1.5 parts by weight of zinc stearate is appropriate. 
     As the mechanical reinforcing agent, approximately 2-3 parts by weight of a material such as glass fibers is appropriate. 
     Quartz powder serves as a filler in the present invention. In addition, it increases the thermal conductivity and the mechanical strength of the molding material. The amount of quartz powder may be approximately 7.5 to 56.5 parts by weight, with the total of quartz powder and the inorganic substance(s) having moisture-absorption characteristics being approximately 62.5 to 76.5 parts by weight. Preferably, however, the amount of quartz powder is approximately 20-55 parts by weight and the total of quartz powder and the inorganic substance(s) is approximately 75 parts by weight. 
     Three examples of a molding material will now be described. Using the conventional methods described above, three different examples of molding materials according to the present invention were prepared. The composition of each example is shown in the following table. In all three examples, anhydrous gypsum was used as the moisture-absorbing inorganic material. Diglycidyl ether of bisphenol A was used as the epoxy resin, diaminodiphenyl methane (DAM) was used as the curing agent, Silica 3H, manufactured by Tatumori Trading Company of Japan was used as quartz powder, and glass fibers were used as the mechanical reinforcing agent. For the purpose of comparison, a conventional molding material containing no anhydrous gypsum was also prepared by the same method. 
     The physical and electrical properties of all of the examples are shown in the same table. Permeability was determined using a JIS Z0208 test. The permeability to moisture of the examples of the present invention was significantly lower than that of the Comparative Example, while the other characteristics of the examples according to the present invention were not significantly different from those of the Comparative Example. 
     
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               Comparative                                                
               Example                                                    
                      Example 1                                           
                            Example 2                                     
                                  Example 3                               
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COMPOSITION                                                               
Moisture-absorbing                                                        
               --     20    40    55                                      
inorganic material                                                        
Quartz powder  75     55    35    20                                      
Epoxy resin    20     20    20    20                                      
Curing agent   2      2     2     2                                       
Zinc stearate  1      1     1     1                                       
Reinforcing agent                                                         
               2      2     2     2                                       
CHARACTERISTICS                                                           
Spiral Flow (mm)                                                          
               600    560   530   500                                     
Mold Shrinkage Factor (%)                                                 
               0.5    0.5   0.5   0.5                                     
Insulation Resistance:                                                    
               1.7 × 10.sup.10                                      
                      1.5 × 10.sup.10                               
                            1.6 × 10.sup.10                         
                                  1.8 × 10.sup.10                   
Ordinary State (ohm-cm)                                                   
Insulation Resistance:                                                    
               1.5 × 10.sup.9                                       
                      1.3 × 10.sup.9                                
                            1.0 × 10.sup.9                          
                                  0.5 × 10.sup.9                    
After Boiling (ohm-cm)                                                    
Bending Strength (kg/cm.sup.2)                                            
               13.0   13.4  14.0  13.6                                    
Breakdown Strength (kv/mm)                                                
               14.5   14.2  13.5  13.0                                    
Permeability (g/m.sup.2 * 24 hr)                                          
               0.7    0.5   0.4   0.3                                     
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 Note:                                                                    
 All amounts given as parts by weight.                                    
 
    
     To test the characteristics during use of a motor employing the molding material of the present invention, submersible motors like the one illustrated in FIG. 1 were prepared using either a conventional molding material or the present material. The motors were submerged in water having an average temperature of 25° C. (maximum temperature: 35° C.) under a water pressure of 10 kg/cm 2 . The motors were run continuously and the electrical resistance between earth and the main stator coil was measured. FIG. 2 shows the results of resistance measurements plotted as a function of time. Curve 10 is the result for a submersible motor using a conventional molding material around the stator, and Curve 11 is the result for a motor using a molding material according to the present invention. The insulation resistance of the motor using a conventional molding material stayed constant until approximately 2500 hours, after which it rapidly decreased. In contrast, the insulation resistance of a motor using a molding material according to the present invention stayed constant for the duration of the test. 
     Thus, because a molding material according to the present invention has decreased permeability to moisture, its insulating properties are superior to those of conventional molding materials and can be maintained indefinitely.