ABSTRACT
The power output rating of an electric motor is often limited by the ability to provide additional electric current through stator and rotor windings. However, the increase of current in the windings is constrained by temperature limitations imposed on the electric conductorinsulation and PMs (for aPM motor). According to the Arrhenius equation [8.1,8.2], the failure rate of an electric/electronic device is exponentially related to the reciprocal of the operating temperature. Afairly accurate approximation of the Arrhenius equation states that an operating temperature exceeds the thermal limit by 10°C and reduces the lifetime of the device by half. Therefore, effective cooling of stator and rotor windings contributes directly to the output capability, operation reliability, and lifetime of electric motors. This is especially true in motor end-winding regions where the conŽguration of cooling žow paths is extremely complicated and direct forced cooling is very difŽcult [8.3,8.4]. In addition,the relatively poor heat dissipation from the rotor can cause the increase in rotor core losses, leading to an excessive temperature rise in the rotor and magnets. This may result in partially irreversible demagnetization of the magnets, particularly of sintered Nd-Fe-B magnets [8.5] and thus a decline in the performance of the motor.
