Finite element modeling and experimental tests of electromagnetic and thermal behaviors of frameless torque motors for robotic joints

The frameless torque motor, features high torque and efficient power transmission, is widely used in humanoid robots. However, the heat generation and induced temperature increases of frameless torque motor due to joule and eddy current effects pose a significant challenge for its applications. This paper presents a finite element modeling (FEM) to study the electromagnetic and thermal characteristics of frameless torque motors, the thermal effects on motor’s materials are analyzed to obtain the magneto-thermal coupling behavior. Then, a testing platform was built to perform experimental tests on the heat generation of frameless torque motor. Simulation results showed that the temperature increase of the motor can reach up to about 360 °C after 120 mins at rated loading operation, thus can cause insulation failure and mechanical damage, reducing output torque and power transmission efficiency. The results also indicated that heat generation primarily originates from the stator, particularly the windings, which generate 51.32 W accounting for about 80% of total power loss. Then no-loading tests were conducted, results showed that the motor's temperature increases in two stages: a rapid rise followed by a gradual increase, and the operating speeds also have great effects on the temperature changes. The increased temperature distribution of the motor agrees well with that of simulation prediction, verifying the accuracy of our FEM model.