Investigating Mechanisms for an Evaporative Cooled In-Wheel AFPM Machine
Electric personal urban mobility vehicles that can be better supported within an urban infrastructure has been suggested for some time. In order to maximize on passenger space in-wheel AFPM motors have been suggested. Whereas high power on-board electric motors typically rely on liquid cooling and utilize heat exchangers to expel heat to ambient, this cooling technique for in-wheel motors will increase the plumbing and the fragility of the vehicle. Further more sealed air-cooled AFPM machines for mobility application, suffer from thermal limitations. As air-cooling is heavily dependent on rotor speeds, high speed geared machines can be better cooled than low speed direct drive machines. This article briefly discusses an evaporative cooling mechanism that can be applied for an in wheel motor with a cooling system that is independent of any subsystem within the vehicle.
By introducing a fluid with a low boiling point into the machine, the working liquid can absorb heat from the pole pieces it vaporizes and moves within the motor from the hot stator surfaces to the cooler rotor surfaces. Upon contact with the cooler surfaces it condenses back to liquid droplets, which trickle back to the liquid pool either by gravity or by centrifugal forces. The design of this cooling concept is in many aspects similar to a heat pipe as shown in Figure 1.
Figure 1: Similarities in operating principles between a) heat pipe and b) evaporative cooled motor
The design of heat pipes and the theoretical prediction of its limits have been thoroughly investigated in the literature. These are many times associated with the standard pipe geometry. Conversely, this work takes an experimental approach to determining the limits of an evaporative cooled motor. The properties of several working fluids were investigated and candidates for the working fluid and wicking materials were identified. As the machine is required to be entirely designed around a mechanism that keeps the stator windings wet, immersion cooling and wick assisted cooling were both explored. The experiments were set up was shown in Figure 2.
Figure 2: Experimental setup for a) immersion cooling b) cooling through a wicking material. Sensor reference numbers: 1-ambient temperature, 2-winding temperature, 3-iron temperature, 4-test box condensing temperature, 5-liquid temperature
Test results shown in Figure 3 demonstrate that both immersion cooling as well as wick assisted cooling can be applied for an evaporative cooled motor.
Figure 3: Test results comparing cooling concept with immersion and wick assisted for methanol and propanol as the working fluid.
However immersion cooling was seen to suffer from a slight temperature overshoot due to the onset of nucleation boiling. Immersion cooling also requires the whole motor to be filled with the working fluid. Conversely the wick assisted cooling mechanism was found to avoid a temperature overshoot and makes use of less working fluid. When applied to a motor geometry, wick-assisted cooling will require an intricate design that enables the wicking mechanism to be integrated in the machine. While a number of tests were performed to identify the correct wicking material, the effect of wick enhancing coatings on fabrics is still to be investigated. Further more the fluid mechanics within a spinning wheel, and the effect this may have on the design of the wick will be explored in a separate work. A full-scale model that will show the final integrated product is still required and will also be demonstrated into a separate work.
The full paper was presented at the 13th UK Heat Transfer Conference (UKHTC2013) at Imperial College, London. To read the paper click here.