The present invention relates generally to the art of electric motors. More particularly, the invention relates to an improved rotor assembly for use in an electric motor and the casting process by which the rotor assembly is made.
Typically, the rotor assembly of an AC induction motor will be constructed as a "squirrel cage" in which a plurality of conductor bars are located in respective conductor channels defined in a magnetically permeable core. The magnetically permeable core, in turn, is generally formed from a multiplicity of individual laminations arranged in a stack. The conductor bars are electrically connected together by conductive end rings located at respective ends of the magnetically permeable core.
It is often specified that an electric motor not exceed a certain current draw in the lock rotor condition. The stator windings and various other parameters of the motor's construction are adjusted to ensure that the lock rotor current will not exceed the specified maximum. Thus, while stronger stator windings may be otherwise desirable, weaker stator windings are sometimes used to comply with the lock rotor current specification.
To manufacture the rotor, end ring casting molds are typically affixed to each end of the lamination stack. The lamination stack, with the casting molds affixed thereto, is oriented vertically to receive a quantity of molten metal (typically aluminum). Generally, the molten metal is injected into the bottom mold until all voids are filled. After the molten metal has cooled to rigidity, the casting molds are removed to yield the rotor assembly. The outer diameter of the resulting rotor may be machined to achieve the desired air gap when positioned inside of the stator core.
In the past, the common practice has been to form the end rings such that they have generally flat end surfaces. In other words, the end surface of the end ring is located in a plane transverse to the axis of the rotor. One problem with such an arrangement has been the occurrence of air bubbles in the end ring formed by the upper casting mold. In particular, air forced upward when the molten metal is injected into the lower casting mold tends to collect in the upper casting mold. While air escape orifices are provided in the upper casting mold, the shape of the end ring cavity does not facilitate the flow of trapped air toward those cavities.