Method for manufacturing squirrel cage rotor

Abstract

Ceramic adhesive is applied inside the slots of an electric machine squirrel cage rotor prior to casting or insertion of rotor conductors. This heat-resistant insulation provides superior electrical insulation that is also mechanically rugged, thus enabling casting of squirrel cage rotors by die or cost-effective consumable pattern casting. The consumable pattern cast copper rotors are electrically well insulated, thus minimizing rotor losses. The ceramic insulation in cast rotors enables longer conductors to be cast by thermally insulating the molten metal from the cooler laminations. The ceramic insulation in fabricated rotors withstands the high temperatures of end ring attachment. This method is applicable to both conventional induction machine rotors and brushless doubly-fed induction machine rotors and provides the degree of electrical isolation of the bars from the laminations needed to avoid excessive parasitic torques in doubly-fed machines, while ensuring adequate thermal conductivity to dissipate conductive bar heat to the laminations.

1 . A squirrel cage rotor for use in an electrical machine, comprising: 
 (a) at least one stack of metallic laminations affixed to one of a shaft and a mandrel, a plurality of spaced-apart openings being formed in each metallic lamination around a periphery of the lamination and aligned in the at least one stack, so as to define slots that extend generally axially along the at least one stack;    (b) a ceramic insulating coating applied to at least a portion of an internal surface of each of the slots, said ceramic insulating coating having both a thermal insulating characteristic and an electrical insulation characteristic that are substantially greater than the thermal and electrical insulating characteristics of the metallic laminations;    (c) at least one electrical conductor extending axially within each of at least a majority of the slots; and    (d) at least one electrically conductive component at each axial end of each of the at least one stack, for interconnecting electrical conductors that extend axially from the slots in the at least one stack.    
   
   
       2 . The squirrel cage rotor of  claim 1 , wherein the ceramic insulating coating comprises a binder and a ceramic powder.  
   
   
       3 . The squirrel cage rotor of  claim 2 , wherein the ceramic powder includes at least one of an alumina, a berylia, a magnesia, a thoria, a zirconia, a zircon, a spinel, and a mullite powder.  
   
   
       4 . The squirrel cage rotor of  claim 2 , wherein the ceramic powder most preferably includes an alumina powder.  
   
   
       5 . The squirrel cage rotor of  claim 2 , wherein the binder of the ceramic insulating coating comprises an inorganic binder.  
   
   
       6 . The squirrel cage rotor of  claim 2 , wherein the binder of the ceramic insulating coating comprises an organic-based material that is processed after being applied, so as to substantially eliminate an organic content.  
   
   
       7 . The squirrel cage rotor of  claim 1 , wherein the at least one conductor extending through each of the at least the majority of the slots comprises a cast-in-place conductor that was formed in the slot by casting a molten metal.  
   
   
       8 . The squirrel cage rotor of  claim 7 , wherein the at least one electrically conductive component at an axial end of the at least one stack comprises a cast-in-place end cap that is formed using a molten metal when the cast-in-place conductors are formed in the at least the majority of the slots.  
   
   
       9 . The squirrel cage rotor of  claim 8 , wherein the metal used for the cast-in-place conductors and the cast-in-place end cap comprises copper.  
   
   
       10 . The squirrel cage rotor of  claim 1 , wherein the plurality of slots are open at a peripheral edge of the at least one stack of the metallic laminations.  
   
   
       11 . The squirrel cage rotor of  claim 1 , wherein the at least one conductor extending through each of the at least the majority of slots comprises at least one metallic bar that has been inserted axially through the slot and which extends generally from one end of the stack to an opposite end of the stack.  
   
   
       12 . The squirrel cage rotor of  claim 1 , wherein the squirrel cage rotor is adapted to be energized by one of: 
 (a) a singly-fed stator;    (b) a doubly-fed stator having two sets of windings with a different number of poles on a common lamination stack; and    (c) a doubly-fed stator having one set of windings on each of two lamination stacks.    
   
   
       13 . The squirrel cage rotor of  claim 1 , wherein the ceramic insulating coating is applied over substantially an entire internal surface of each slot.  
   
   
       14 . The squirrel cage rotor of  claim 13 , wherein the ceramic insulating coating is formed as a plurality of generally axially extending ridges that are spaced apart around the internal surface of the plurality of slots and are substantially thicker than the ceramic insulating coating covering the internal surface of the plurality of slots between said ridges.  
   
   
       15 . A method for fabricating a squirrel cage rotor for use in an electrical machine, comprising the steps of: 
 (a) forming at least one stack of metallic laminations affixed to one of a shaft and a mandrel, each metallic lamination including a plurality of spaced-apart openings disposed around a periphery of the lamination and generally aligned in the at least one stack, so as to define slots that extend generally axially along the at least one stack;    (b) applying a ceramic insulating coating to at least a portion of an internal surface of each of the slots, said ceramic insulating coating having both a thermal insulating characteristic and an electrical insulation characteristic that are substantially greater than thermal and electrical insulating characteristics of the metallic laminations;    (c) providing at least one electrical conductor extending axially within each of at least a majority of the slots; and    (d) electrically interconnecting the electrical conductors extending axially within the slots in the at least one stack.    
   
   
       16 . The method of  claim 15 , wherein the step of providing at least one electrical conductor comprises the steps of: 
 (a) heating an electrically conductive metal to a molten state;    (b) casting a conductor within each of the at least the majority of the slots using the electrically conductive metal that has been heated to the molten state and a consumable pattern; and    (c) enabling the electrically conductive metal cast as the conductor within each of the at least the majority of the slots to cool to an ambient temperature.    
   
   
       17 . The method of  claim 16 , wherein the step of casting comprises the step of employing one of: 
 (a) a lost foam casting process in which a plastic foam comprises the consumable pattern; and    (b) a lost wax casting process in which a wax material comprises the consumable pattern.    
   
   
       18 . The method of  claim 16 , wherein the electrically conductive metal comprises copper.  
   
   
       19 . The method of  claim 15 , wherein the step of applying the ceramic insulating coating comprises the steps of: 
 (a) cleaning the interior surface of the slots;    (b) filling any gaps between the metallic laminations comprising the stack;    (c) coating the interior surface of the slots with a ceramic adhesive; and    (d) heat treating the stack at a substantially elevated temperature, to substantially increase a coating strength for the ceramic adhesive.    
   
   
       20 . The method of  claim 19 , wherein the step of applying the ceramic insulating coating further comprises the steps of: 
 (a) air drying the ceramic adhesive coating;    (b) applying another coat of the ceramic adhesive over the ceramic adhesive already coating the interior surface of the slots; and    (c) air drying the other coat of the ceramic adhesive before the step of heat treating.    
   
   
       21 . The method of  claim 19 , wherein the step of coating the interior surface of the slots comprises the step of applying a relatively thicker coating of the ceramic adhesive to form ridges that extend axially through the slots and are spaced apart on the internal surface of the slots, with a relatively thinner coating of the ceramic adhesive on the internal surface of the slots between the ridges.  
   
   
       22 . The method of  claim 15 , wherein the ceramic insulating coating comprises a binder and a ceramic powder.  
   
   
       23 . The method of  claim 22 , wherein the ceramic powder includes at least one of an alumina, a berylia, a magnesia, a thoria, a zirconia, a zircon, a spinel, and a mullite powder.  
   
   
       24 . The method of  claim 22 , wherein the ceramic powder most preferably comprises an alumina powder.  
   
   
       25 . The method of  claim 22 , wherein the binder of the ceramic insulating coating comprises an inorganic binder.  
   
   
       26 . The method of  claim 22 , wherein the binder of the ceramic insulating coating comprises an organic-based material that is processed after being applied to the interior surface to substantially eliminate an organic content.  
   
   
       27 . The method of  claim 15 , wherein the step of providing at least one electrical conductor extending axially within each of the at least the majority of the slots comprises the step of inserting a conductive metal bar into each such slot so that the conductive metal bar extends axially through the slot from opposite ends thereof, the conductive metal bar comprising the at least one electrical conductor.  
   
   
       28 . The method of  claim 15 , wherein the step of electrically interconnecting the electrical conductors comprises the steps of: 
 (a) providing a consumable pattern defining a shape of at least one end cap adapted to fit an end of the squirrel cage rotor;    (b) heating an electrically conductive metal to form a molten metal;    (c) using the consumable pattern, casting at least one end cap from the molten metal; and    (d) enabling the at least one end cap to cool substantially to an ambient temperature, said at least one end cap electrically interconnecting the electrical conductors extending through the slots.    
   
   
       29 . The method of  claim 15 , wherein the step of electrically interconnecting the electrical conductors comprises the steps of: 
 (a) forming a consumable pattern defining an end cap for each end of the stack, said consumable pattern also defining fan blades, vents extending into the slots, and balancing posts for each end cap;    (b) assembling the consumable patterns to at least one end of the stack; and    (c) casting the end cap for at least one end of the at least one stack and the electrical conductor extending axially through the at least the majority of the slots, in place, with a molten metal.    
   
   
       30 . The method of  claim 28 , wherein the step of casting comprises the step of casting the at least one end cap in place on the at least one stack by employing one of: 
 (a) a lost foam casting process, in which a plastic foam comprises the consumable pattern; and    (b) a lost wax casting process in which a wax material comprises the consumable pattern.    
   
   
       31 . The method of  claim 28 , further comprising the step of vaporizing the consumable pattern with one of the steps of: 
 (a) vaporizing the consumable pattern with the molten metal; and    (b) preheating an assembly that includes the consumable pattern before the step of casting with the molten metal, so that heat thus applied vaporizes the consumable pattern, yielding a void into which the molten metal subsequently flows during the step of casting.    
   
   
       32 . The method of  claim 31 , wherein before the step of preheating the assembly, further comprising the step of coating an assembly that includes the at least one stack and the consumable pattern with a ceramic slurry, said ceramic slurry drying to form a shell that defines the void after the step of preheating has vaporized the consumable pattern, and before the step of casting that fills the void with the molten metal.  
   
   
       33 . The method of  claim 28 , wherein the electrical conductor comprises copper.  
   
   
       34 . The method of  claim 28 , wherein before the step of casting, further comprising the steps of: 
 (a) creating an assembly of components that includes a sprue, at least one foam gate, at least one vent, the at least one stack in which the at least the portion of the internal surface of the slots is coated with the ceramic insulating coating, and the consumable pattern for at least one end cap;    (b) coating the assembly with a gas permeable layer, producing a coated assembly;    (c) placing the coated assembly into a container; and    (d) adding particulate material that is heat resistant and conforms around the coated assembly within the container to provide support during the step of casting.