Effectiveness of active cooling on torque performance for prosthestic applications

Abstract

Motors used to actuate powered prostheses generally under-utilize their torque capacity due to thermal limitations of the windings. This thesis investigates the effectiveness of increasing the rate of heat transfer away from the windings in order to enable running motors at higher currents than their rated maximum levels, thus raising this torque saturation limit. Simulation models and physical prototypes based on the RCTiger 100KV U8 brushless outrunner motor were built to observe the temperature of the windings as constant current was applied to the motor. The addition of a fan-based active cooling system allowed the motor to run at 142% of its maximum continuous current rating for up to 56 seconds before winding temperatures exceeded 550 C, and limited temperature increase in simulation to 26' above ambient temperature when the full 35A stall current was applied for one second. Although the simplified circuit model was not able to fully capture nonlinear thermal behavior of the motor, simulations were able to predict approximate heating time constants and time duration before windings reached threshold temperature for current ranges 5- 15A. Experimental and simulation results support the hypothesis that active cooling enables motors to run at their full torque potential for short periods of time, which holds promise for the use of cooling mechanisms in prosthetic applications.