The continuous power to mass ratio (specific power) is an important metric for electric motors, especially for those used to power electric and hybrid vehicles. As this parameter is increased, motor mass can be reduced while maintaining a given level of performance. This provides both direct and indirect economic benefits. Since power is equal to torque times speed (rpm), high specific power may be achieved by the combination of high shaft speed and high torque per unit mass (high specific torque). Since electrical and core magnetic frequencies may be proportionate to shaft speed and since magnetic losses may increase approximately with the square of these frequencies, core losses may increase rapidly with increasing speed. Likewise, since winding losses may be approximately proportionate to the square of the torque, this loss component may increase rapidly with increasing torque. As a result, the operation of high specific power machines may be facilitated by efficient heat rejection for both the core and the winding.
Winding temperature may exceed the core temperature, and elevated winding temperatures may cause increased winding losses. Accordingly, a metric may be defined which involves winding hot-spot temperature and total loss within the stator. This metric, the stator thermal resistance, is defined as the temperature difference between the hottest part of the stator winding and the cooling medium (e.g. inlet coolant) divided by the total stator heat dissipation. As the stator thermal resistance is lowered, the continuous power capability and hence continuous power rating of the overall machine may increase. As such, low stator thermal resistance may be helpful in achieving high specific power.
In a related art liquid-cooled stator, the active core may be contained within a liquid cooled enclosure and the winding may be electrically insulated from the core via slot liners and electrical varnish. Heat produced within the winding may be constrained to flow through a series of elements, such as electrical varnish and slot liners, each adding thermal resistance, before reaching coolant which flows within the enclosure. Both electrical varnish and slot liners may offer significant thermal resistances. Heat is received by the core teeth and flows radially through the back iron and on to the enclosure.
For large diameter machines, both the tooth and back iron thermal resistances may be significant. The interface between the core and the enclosure may present yet another resistance element, as may the material of the enclosure itself. An additional resistance element is associated with heat transfer from the interior surfaces of the enclosure to the coolant. The combination of such thermal resistance elements may limit the performance of a motor. The temperature of the stator, and particularly the temperature of the stator teeth tips, may also affect the rotor, which may exchange heat with the stator by conduction, convection, and radiation heat transfer.
Thus, there is a need for an improved system for cooling a stator of an electric motor.