PERMANENT MAGNET (PM) ELECTRIC MACHINE INCLUDING PERMANENT MAGNETS PROVIDED WITH A SACRIFICAL COATING HAVING A THERMAL INTERFACE MATERIAL (TIM)

A permanent magnet electric machine including a housing, a stator mounted within the housing, and a rotor assembly rotatably mounted within the housing relative to the stator. The rotor assembly includes a plurality of laminations. Each of the plurality of laminations includes a plurality of slots and one or more permanent magnets mounted within respective ones of the plurality of slots. Each of the plurality of permanent magnets includes a sacrificial coating formed from a thermal interface material (TIM).

DETAILED DESCRIPTION OF THE INVENTION

A permanent magnet electric machine in accordance with an exemplary embodiment is indicated generally at2inFIG. 1. Electric machine2includes a housing4having first and second side walls6and7that are joined by a first end wall8and a second end wall or cover10to collectively define an interior portion12. First side wall6includes a first inner surface16and second side wall7includes a second inner surface17. At this point it should be understood that housing4could also be constructed to include a single side wall having a continuous inner surface. Electric machine2is further shown to include a stator24arranged at first and second inner surfaces16and17of first and second side walls6and7. Stator24includes a body28, having a first end portion29that extends to a second end portion30, which supports a plurality of windings36. Windings36include a first end turn portion40and a second end turn portion41.

Electric machine2is also shown to include a shaft54rotatably supported within housing4. Shaft54includes a first end56that extends to a second end57through an intermediate portion59. Shaft54supports a rotor assembly70. Rotor assembly70includes a hub72including a first bearing74that supports first end56relative to second end wall10, and a second bearing75that supports second end57relative to first end wall8. Rotor assembly70includes a rotor body79formed from a plurality of rotor laminations, one of which is indicated at84. Each rotor lamination84includes a plurality of slots, one of which is indicated at94inFIG. 2. Rotor laminations84are stacked and slots94are aligned prior to undergoing a bonding process that forms rotor body79. A plurality of permanent magnets (PM)100,101, and102are provided in rotor body79in slots94.

Reference will now be made toFIG. 3in describing PM100with an understanding that PM101and PM102include similar structure. PM100includes a body114having an outer surface117. In the exemplary embodiment shown, PM100is encased in a sacrificial coating124. Sacrificial coating124includes a thermal interface material (TIM)134that facilitates heat exchange from PM100. In accordance with one aspect of the exemplary embodiment TIM134possess a thermal conductivity of at least about 0.3 W/mK. In accordance with another aspect of the exemplary embodiment, TIM134is formed from a material having a cohesive shear strength that is less than an adhesive bonding strength. In accordance still another aspect of the exemplary embodiment, TIM134takes the form of a thermally conductive resin144. The thermally conductive resin144may take the form of a B-stage resin. A B-stage resin is a resin in which a limited reaction between resin and hardener has been allowed to take place. The reaction is arrested while the resin remains flexible and soluble. The B-stage resin includes sufficient hardener that allows for subsequent hardening upon exposure to a hardening input such as heat or light of a particular wavelength. Thermally conductive resin144may also take the form of an epoxy based resin and/or a silicon based resin.

A portion of sacrificial coating124is removed upon insertion to, for example, slot94. As best shown inFIG. 4, sacrificial coating124establishes an outer dimension of PM100that is greater than an inner dimension of slot94. Upon insertion, a portion of sacrificial coating124is removed, shaved, broached, or scraped from PM100. The partial removal of sacrificial coating124results from an interaction with rotor body79. In this manner sacrificial coating124establishes an interference fit between PM100and rotor body79. Accordingly, PM100is positively retained within rotor body79despite variations in dimension of slot94resulting from manufacturing tolerances. If/when using a B-stage coating, thermally conductive resin144is finally cured and hardened. For example, heat generated during operation of electric machine2may provide the desired hardening input required to fully harden thermally conductive resin144.

At this point it should be understood that the exemplary embodiments provide permanent magnets in a PM electric machine with a sacrificial coating that not only establishes a desired retention between the permanent magnets and a rotor, but also facilitates heat removal. In addition, while described as being encapsulated in the thermally conductive resin, it should be understood that axial end portions of the permanent magnets may be devoid of any coating material. It should be further understood that the type and chemical make-up of the sacrificial coating may vary. Further, while shown and described as providing multiple permanent magnets in each slot, it should be understood that each slot may also be provided with a single permanent magnet. Further, it should be understood that there may exist slots that are not provided with magnets.