Numerical Approach to Determining Windings' Thermal Conductivity

International audience Abstract-This study concerns the characterization of windings' thermal conductivity, which is of primordial importance for thermal management of a high power density electrical machine. Indeed, such machines exhibit high heat dissipation in the windings' area. A precise knowledge of this conductivity value is therefore important for good thermal predictions. Unfortunately, the heterogeneity of the windings and the heat production repartition make it difficult to calculate easily winding's equivalent thermal conductivity. The influence of four thermal and geometrical parameters on the windings' equivalent thermal conductivity is numerically studied on a Finite Element Analysis software, allowing determining more precisely this characteristic. The impregnation's thermal conductivity, the conductors' number, the fill factor and the configuration of the copper wires inside the slot are investigated. The equivalent thermal conductivity, principally controlled by the impregnation thermal conductivity, the slot fill factor and the conductors' disposition, is expressed in terms of the first two parameters in its worst-case configuration. Index Terms-Thermal management, windings, thermal conductivity, Finite Element Modeling (FEM

The rebirth of the current source inverter: advantages for aerospace motor design

It is well known and widely accepted that the voltage source inverter (VSI) now dominates the world of electrical drives. Its success is probably due to its simplicity, high efficiency, and the widespread availability of VSs. This popularity has, in turn, influenced the evolution of the semiconductor industry, which has focused in recent years on devices tailored for VSIs. Thus, products such as depletion devices (normally off) and those without reverse voltage blocking have been widely marketed and used

Additive-Subtractive Process Chain for Highly Functional Polymer Components

Additive manufacturing processes offer the possibility of producing components without using tools. Especially in mobility, new technologies are needed to make geometrically complex, functionally integrated and highly precise components. The fused filament fabrication (FFF) process is an additive manufacturing technique that offers easy handling and a large range of materials. However, the FFF process has a considerable shortcoming in dimensional accuracy. A process hybridization consisting of additive and subtractive steps was developed to eliminate this shortcoming. Applying subtractive work steps enables the precise integration of inserts and, thus, the production of highly functional polymer components. For this purpose, suitable demonstrators are derived from an example of a stator of a double-sided axial flux machine and the manufacturing process with the different working steps (additive & subtractive) is demonstrated. The focus is on increasing the dimensional accuracy and more precise integration of the inserts with the help of subtractive steps. Furthermore, non-planar overprinting during the additive manufacturing steps was investigated. The advantages of the combination of subtractive processing and non-planar printing were concluded

Thermal modeling of hollow conductors for direct cooling of electrical machines

A direct cooling design using hollow conductors with the coolant flowing inside can significantly improve the heat dissipation in an electrical machine. To predict the thermal performances of an electrical machine with such cooling configuration, this paper proposes a computationally efficient thermal model of hollow conductors with direct cooling features. The hollow conductor is modeled using four equivalent solid cuboidal elements with a three-dimensional thermal network and internal heat generation. The heat transfer coefficient between the coolant and conductors is determined by an empirical model considering fluid dynamics behaviors. Axial discretization is performed to take into account the nonuniform temperature distribution along the axial direction. Experimental validation is performed with a U-shaped hollow conductor test rig. Compared to computational fluid dynamics analysis, the proposed thermal model is much more computationally efficient, and thus can be incorporated into design optimization process and electrothermal simulations of the electrical machine over a driving cycle

Back-iron extension thermal benefits for electrical machines with concentrated windings

This paper proposes a novel, low-cost, effective way to improve the thermal performance of electrical machines by extending a part of the back-iron into the slot. This modification helps in reducing the thermal resistance path from the center of the slot to the coolant, however its thermal benefits must be clearly evaluated in conjunction with the electromagnetic aspects, due to the higher iron losses and flux-leakage, and furthermore such an extension occupies space which would otherwise be allocated to the copper itself. Taking a case study involving an existing 75kW electric vehicle (EV) traction motor, the tradeoffs involving the losses, fluxleakage, output torque, torque-quality and the peak winding temperature with back-iron extension (BIE) and without are compared. Finally, experimental segments of the aforesaid motor are tested, verifying a significant 26.7% peak winding temperature reduction for the same output power with the proposed modification

Electrical Machine Slot Thermal Condition Effects on Back Iron Extension Thermal Benefits

The slot thermal condition is critical for thermal management of high performance electrical machines, due to the high heat losses and poor heat transfer ability within the slot. With a part of the back-iron projected radially downwards into the slot, back-iron extension (BIE) shortens the heat dissipation path from the slot coil to the back-iron and was proven to be an effective thermal improvement technique. The relationship between BIE thermal benefits and various electrical machines’ parameters remains to be investigated. Based on an existing concentrated-wound machine, the relationship between the equivalent slot thermal conductivity (ESTC) and the back-iron extension effectiveness is researched in this paper. Utilizing a developed 3D thermal model, the equivalent slot thermal conductivity effects on the temperature reduction with BIE are indicated with simulation results and verified with experimental tests. BIE is reported to provide temperature reductions ranging from 48°C down to 18°C across the plausible range of ESTC values considered. Guidelines are given in the final part to suggest the situations under which BIE is more effective

Improved Thermal Modelling and Experimental Validation of Oil-Flooded High Performance Machines with Slot-Channel Cooling

Thermal management is often considered a bottleneck in the pursuit of the next generation electrical machines for electrified transportation with a step change in power density. Slot-channel cooling is considered to be an effective cooling technique, either as an independent method or as a secondary heat transfer path which compliments traditional cooling systems. The slot-channel specific geometry and position effects on the thermal benefits are not thoroughly investigated in literature, while previous work focuses on passing fluid through the un-used space left in between coils forming concentrated windings. In this paper, slot-channel cooling is implemented within an oil-flooded cooling system for a high power density motor used as a pump. A flexible and detailed lumped parameter thermal network (LPTN) is proposed for the cooling system, with the LPTN used to optimize the slot-channel dimensions and location for obtaining maximum thermal benefits. Finally, a surface-mount permanent magnet (SPM) machine with the optimized slot channel geometry is built and tested to validate the thermal model, experimentally achieving an armature continuous current density in excess of 30A/mm2