More efficient cooling systems are an enabler for the increase in power density in an electric traction motor. Contemplating to existing reviews, this article presents a comprehensive collection of heat transfer mechanisms for the different heat removal techniques used in electric traction motors. In this article, an overview of various cooling concepts in existing and future traction motors is presented, and the cooling approaches are compared. The following literature review compiles geometry-based calculation formulas of the different mechanisms of cooling heat transfer applied in motor technology. Furthermore, general heat transfer phenomena appearing in electric traction motors are reviewed and compiled for easy access. Various specifically relevant aspects, including rotor shaft cooling, different spray cooling concepts, different air convection phenomena, bearing heat transfer, and the stator-housing contact, are examined in more detail. For validation of the review result, based on a well-known motor design, thermal analysis and comparison of the different cooling methods are carried out. Modeling is done with a lumped parameter thermal network (LPTN). This article concludes by demonstrating the advantages and disadvantages of the different cooling concepts based on the collection of modeling data.

The growing interest in electric vehicles leads to new developments of efficient electric traction motors with high power density. Since a large part of the losses in an electric motor can occur in the windings, it is important to cool the windings to reduce the temperature and protect the winding insulation from thermal aging. Oil spray cooling systems are becoming more and more relevant for cooling the windings. In this work, an approach for heat transfer modeling of electric traction motors (power classes #2-4 with 60 to 180kW) with hairpin end windings is developed and tested. The model is developed with a testing rig for different flat jet spray nozzle arrangements for a hairpin electric traction motor. The model approach showed a good agreement with the measured temperatures of the end windings in the testing rig. A transient thermal simulation of the testing rig confirms the good agreement. In a second step the model approach is transferred to another electric traction motor with a different spray cooling system. The simulation results of the second electric traction motor also show good agreement with thermal quasi-static operating points and transient measurements at low and medium speed. At high speeds the deviations increased. Possible causes for this behavior are discussed.

For identifying new improvement potentials for electric traction motors, accurate models are needed. In this paper, a geometry-based 2D lumped parameter thermal network model for different electric traction motor and cooling concepts is studied and validated. In the second section, the design and functionality of the thermal model is explained. In the third section, the best fit of the literature correlations for describing the different heat transfer mechanisms was identified and a parameter study of the heat transfer coefficients was carried out and discussed. In the last section, the model is validated with measurement results from six different electric traction motors and drives units. For validation measurement results of stationary operating points, peak operating points and drive cycles are used. Based on the validation results, a model error of less than 10% is achieved for the most motor components in the different cooling concepts and traction motor designs. Inaccuracies and deviations are discussed and suggestions for improvement are made.