Directly Cooled Windings - Conjugate heat transfer assessment of air-cooled hollow conductor

The torque capabilities of electrical machines are today limited by magnetic saturation and the ability to transfer thermal loads away from the machine. A solution to thermal management in power turbo-generators involves hollow conductors allowing direct cooling of the heat source thus increasing the power density significantly. The work done in this master thesis investigates the pos- sibilities of implementing such cooling techniques in smaller electrical machines and predicts the current capabilities of directly cooled conductors. Experiments have been made on hollow conductors using compressed air as cooling fluid and mathematical models are used to predict the capabilities of using oil as coolant. Results show current density capabilities in excess of 30A/mm2. Results from experiments are compared to finite element studies and mathematical models to assess validity.The torque capabilities of electrical machines are today limited by magnetic saturation and the ability to transfer thermal loads away from the machine. Using hollow conductors the cooling capabilities are drastically increased allowing for a significant increase in current densities. Results show continuous current density capabilities well in excess of 30 A/mm2. INTRODUCTION The torque capability of an electrical machine is highly dependent on the maximum allowed current through its windings. This maximum current is in turn limited by the heat generated as the current flows through the copper conductors inside the machine. As the copper is heated the insulation between the conductors deteriorates and eventually fails, short circuiting the windings. Modern electrical machines uses water-cooled jackets around the outside of the machine to allow higher currents and thus higher torque. A problem with this method is that the cooling takes place on the outside and not where the heat is generated thus relying heavily on good thermal connection to the inside the machine. A solution to this problem widely used in larger electrical machines is found in the use of a hollow conductors where a cooling fluid passes through interior channels. This cooling method allows for direct cooling of the heat source with a significant increase in current capabilities. The work done as part of this master thesis attempts therefore to predict the cooling performance of hollow conductors intended for use in a smaller electrical machine. COMPONENTS EXAMINED Four different components have been examined using compressed air as cooling fluid. By measuring the current flowing through the copper and the inlet pressure of the airflow a relationship between the power needed to cool the machine and the maximum possible current density is obtained. These results can then be used to assess which method is the best and what levels of current density can be achieved. RESULTS As a general reference the uncooled hollow conductor will overheat at a current density of 12-15A/mm2, with temperatures rising above 180◦ C. By flowing compressed air through the parallel flow winding it is possible to more than triple the current flowing through it while still keeping a maximum temperature below 100◦C. This is achieved using a theoretical pumping power of approximately 100W per cooling channel. The results also show that a significant increase in current capabilities can be achieved already at 10W per cooling channel if a higher maximum temperature is allowed. CONCLUSION Experiments using compressed air show great potential for direct cooled windings: the use of hollow conductors allows for a significant increase in current capabilities, thereby opening up possibilities for a new generation of electrical machine cooling