End Windings Research Articles

Electric Traction Motor Spray Cooling—Empirical Model Development and Experimental Validation

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.

Analysis of PMSM NVH performance in electric propulsion systems using FE models

The stator of the PMSM is the main transfer path for the Electro Magnetic (EM) force from stator teeth to the drive unit (DU) through its mounting ears. First, a FE model of full winding stator model along with connection ring is analyzed for modal behaviors. Time domain EM force excitations are converted into frequency domain and applied at the stator teeth. This enables stator mounting forces analysis at all critical NVH orders. Next, the detailed FE modelling approach is compared with response calculated form a simplified modelling approach, with stator end windings modelled as lumped mass and inertia to reduce computational time. The comparison study of two approaches clearly shows the benefits of using simplified FE models for studying motor whine with reduced modelling and computational costs. Furthermore, the frequency-dependent stator damping measured from modal test is applied in electric DU analysis to evaluate NVH performance excited by motor EM force excitations using simplified stator model. The sound power level radiated from the DU and the mount vibration response show negligible influence of stator damping at DU unit level NVH response in the low frequency range. Analysis and test results correlate reasonably well at the dominant motor whine orders for the DU, which validates the proposed method.

Study on high-speed electric motor cooling with oil spray

The transportation sector is experiencing quick electrification to pave the way to carbon-neutral society, which poses great challenges to the thermal management technologies for the motor as they are expected to be more and more powerful. Advanced cooling with oil spray on the end winding was reported experimentally and theoretically in this work. It was suggested that with the oil spray cooling, the temperature of winding was well within the safety range, indicating it is an efficient way to dissipate the heat for high-speed motors. Furthermore, a lumped-parameter model for motor was also introduced. It was found that the main heat transfer thermal resistance of winding is the resistance between the yoke and winding. Meanwhile, the main influencing parameters of spray system are heat transfer coefficient and wetting ratios of spray. The paper is expected to lay a solid foundation for the application of oil spray cooling in high-speed motor thermal management.

Correlation of single-phase convective heat transfer on spray cooled plain surfaces with high Prandtl number liquids

Spray cooling with transmission oils is an innovative cooling concept for electrical machines, whereby the coolant wets the end windings directly. The thermal design process of such machines requires reliable correlations of the heat transfer coefficients obtainable. Thereby, the majority of the literature only focuses on low Prandtl number coolants at ideal nozzle distances. The transferability of the existing heat transfer models to highly viscous coolants above and below the optimal nozzle distance has yet to be investigated. Therefore, the heat transfer coefficients of spray cooling on plain surfaces with aqueous glycerol solutions (model fluid) and a transmission oil of type ATF VI was investigated. The model fluid serves the purpose of covering the entire range of material properties of commercial transmission oils with only one liquid. Prandtl numbers ranging from 80 to 340 were investigated. Various cases of spray coverage were researched by varying the nozzle distance (2mm−100mm), the spray angle (20∘−80∘) and the size of the spray cooled surface (6.35×6.35mm2;12.7×12.7mm2). Regarding the case of the optimal nozzle distance, where the spray impact area just inscribes the cooled surface, the experimental data was validated with models from the literature. Large deviations of up to 450% resulted at nozzle distances above and below the optimum. Therefore, a dimensionless correlation of the heat transfer coefficient is proposed which includes all cases of spray coverage investigated, exceeding the applicability range of the existing models. The model describes the experimental data with a mean absolute percentage error of 15%.

Design of a Spoke Type PMSM With SMC Stator Core for Traction Applications

This article presents four different slot/pole configurations to select the best slot/pole for a traction application with a wide speed range. The electromagnetic performances are improved for the selected slot/pole configuration. This paper presents a modified spoke type rotor that significantly reduces the torque ripple, back electromagnetic force (EMF) harmonics, and cogging torque from conventional spoke type designs. The effectiveness of the modified spoke type rotor is proved with state-of-the-art designs such as Toyota Prius and Honda Accord motors. Along with the modified spoke type rotor it focuses on a segmented soft magnetic composite (SMC) stator core to improve the copper fill factor and improve torque density by reducing the tooth length to allow additional space for the end windings within the stack length of the motor. A comparison between the laminated steel stator and SMC stator is presented. The laminated motor is capable of producing higher torque than the SMC motor when both machines are designed to operate with the same temperature, though the torque density in terms of volume of SMC motor is higher than the laminated motor. In all cases, the machines are designed to fit into the same frame. A cost comparison has been presented between the SMC and laminated motors, which proves that the SMC motors can be manufactured much cheaper than the laminated motors. Also, SMC stator design presents axial magnet segmentation, optimal flux weakening, thermal analysis, stator stress analysis due to electromagnetic forces, and a rotor mechanical stress analysis.

Stator winding hotspot temperature rise characteristic study of an axially forced air-cooled motor with air deflector

A ventilation system with an air deflector is suggested to enhance the cooling of the stator windings in an axially forced air-cooled motor. The global fluid and thermal evaluations are carried out using the numerical analysis method. The simulation results are verified through experiments, and the relative error between the two sets of data is 4.4%. Then, the fluid flow and heat exchange behavior within the motor are studied in relation to the placement of the air deflector. When an air deflector is installed at the air outlet side end region, the air is directed to collide with the stator end winding, thereby promoting convective heat exchange on the stator end winding surface. Additionally, numerical analysis is employed to investigate how the position and geometric features of the air deflector affect the variation in the stator winding temperature rise and overall pressure drop. Shortening the distance between the air deflector and the cooling duct and the stator end winding is conducive to further improving the cooling. Furthermore, it is established that the maximum and average temperature rise of the stator winding may be reduced by 8.9 K and 8.2 K, respectively, when using the proposed air deflector.

Thermal Effects in the End-Winding Region of Electrical Machines

The main heat transfer mechanism in the end-winding region of electrical machines is convection. In order to increase the air motion, the rotor is equipped with a series of blades. Their geometry is reflected in the fanning factor, i.e., the ratio between the rotor peripheral speed and air velocity. An accurate calculation procedure for the fanning factor has not yet been given. Knowing its value is crucial for the determination of air velocity and heat transfer coefficient (HTC), as the latter describes the end-winding heat removal capability. In this study, the convective heat transfer phenomena between the end winding and air inside the end-winding region were analyzed, with the heat generated only in the end winding, mimicked with a custom designed coil, and air moved by the blades. The analysis was performed by experimental testing and computational fluid dynamics (CFD) modeling. Measurements data were used to build a reliable CFD model. Further on, CFD results were used to derive a generalized analytical equation for calculation of the end-winding HTC, related to blade geometry and rotor rotational speed. The developed analytical model significantly improves the quality of real-time lumped circuit thermal modeling of electrical machines and, thus, enriches this field of science.

Investigation of Ferrofluid Cooling for High Power Density Permanent Magnet Machines

This article investigates an advanced thermal management method adopting ferrofluid (FF) for improving the end-winding cooling of permanent magnet (PM) machines. An oil-based liquid with nano-sized ferromagnetic particles (which is known as FF) is used to fill in the cavity around the end windings. This is to establish an effective heat flux path between the end winding that is often regarded as hot spot in electrical machines and the external cooling system, i.e., water jacket, to improve the cooling performance of the PM machines. This improvement does not only result from the higher thermal conductivity and thermal expansion of the nanofluid with metal particles but also from strong thermomagnetic convection generated by the magnetic body force of the ferromagnetic particles within the FF. Multiphysics models considering the interaction between the electromagnetic field, the heat transfer, and the fluid dynamics have been built to study the thermal performances of a PM machine under different load conditions. Several factors affecting the thermomagnetic convection, such as the temperature-dependent magnetization curve of the FF, the concentration, and different ferromagnetic materials as well as different current densities, have been investigated to analyze their influences on cooling performance. One major finding is that, compared with other coolant without magnetic body force, the FF can significantly reduce machine peak temperature, e.g., by around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${36.4}~^{\circ }\text{C}$ </tex-math></inline-formula> when the current density is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {22.1}~\text {A} / {\text {mm}}^{ \boldsymbol {2}}$ </tex-math></inline-formula> .

Effect of core losses on thermal analysis of 3ɸ- Squirrel Cage Induction Motor using lumped parameter thermal model

The proposed Lumped Parameter Thermal Model (LPTM) is complex enough to predict temperatures in most parts of the 3ɸ- Squirrel Cage Induction Motor (3ɸ-SCIM), including the end winding and rotor surface temperatures. It is easily adaptable to a variety of frame sizes because it is formulated using dimensional information and constant thermal coefficients. The solution of linear differential equations adequately describes the thermal behavior of the 3ɸ-SCIM. The application of the thermal model on 15 kW 3ɸ-SCIM is discussed. The rise in temperature at different parts of motor is estimated considering copper loss, constant and variable core loss. The maximum flux density derived from Finite Element Method (FEM) analysis, used to calculate core loss at various loads of a 3ɸ-SCIM. Variable core loss causes deviation in temperature rise, from the results, it is observed that, the relative percentage error considering variable core loss to constant core loss is about 19 %.

Optimal Designs of Wound Field Switched Flux Machines with Different DC Windings Configurations

Wound field switched flux (WFSF) machines exhibits characteristics of the simple robust rotor, flexible flux-adjustable capability, and no risk of demagnetization. However, they suffer from a poor torque density compared with permanent magnet machines due to the saturation. Therefore, in this paper, two WFSF machines with single- and double-layer DC windings, respectively, are optimized for the maximum torque. The end-winding (EW) lengths differ in these two machines, which can affect the optimal design. Design parameters including the DC to armature winding copper loss ratio, slot area ratio and split ratio are optimized when two machines have the same copper loss and overall sizes. In addition, the influence of the flux density ratio, total copper loss, air-gap length and aspect ratio on the optimal split ratio is investigated using the finite element method and results are explained through the analytical model accounting for the saturation. It is discovered that the EWs have no effect on the optimal copper loss ratio, which is unity. In terms of the slot area ratio, the machine with single-layer DC windings prefers smaller DC slot areas than armature slot areas. In the WFSF machine with longer EWs, the optimal split ratio becomes smaller. Moreover, compared with other parameters, the flux density ratio can significantly affect the optimal split ratio.

A New Analytical Thermal Model of Distributed Winding Wheel Machine for Electric Vehicles

Due to the complex boundary conditions of the thermal analysis method (AM), the error of the calculation results is large, and the lumped-parameter thermal network (LPTN) mostly focuses on the solution of the average temperature. This paper proposes a thermal analytical model for wheel-motor windings, which includes LPTN and AM to solve the problems of boundary condition extraction and maximum temperature solution in AM. The heat transfers between the end caps of electric vehicle wheels and the outside air are considered by the model. The node temperature of LPTN can be adopted as the boundary condition of the proposed analytical method to predict and calculate the maximum temperature and average temperature of the wheel motor and improve the accuracy of temperature rise calculation. To further analyze the winding temperature, the armature winding is divided into active winding and end winding for calculation respectively. In the calculation of active winding, based on the equivalent area method, the active winding and other materials are layered into a series of equal-width rectangular rings through a novel layered strategy. The compensated thermal resistance (TR) is introduced into the analytical model of end winding, and the accuracy of the end temperature solution is improved by the iteration of compensated TR. Finally, the effectiveness and correctness of the proposed analytical thermal model for motor temperature rise prediction are verified on a 2kW distributed double-layer winding wheel motor.

Study on AC Copper Losses in an Air-Cored Axial Flux Permanent Magnet Electrical Machine With Flat Wires

In this article, we propose the calculation method of ac copper losses of air-cored axial flux permanent magnet (AFPM) electrical machine with flat wires fed by pulsewidth modulation (PWM) inverters. The ac copper loss due to armature currents in air-cored electrical machines is significant when these machines are driven by PWM inverters. Besides, the exposure of windings to the inhomogeneous alternating rotor magnetic field causes ac copper loss. Analytical formulas are efficient tools to evaluate these losses but few pieces of literature have reported the derivation in such machines. To address this issue, the formulas of aforementioned ac copper losses are derived based on Maxwell's equations and Poynting theorem. As for end windings, the ac copper loss caused by armature current is calculated analytically, while the rotor-induced ac copper loss is calculated by aid of static three-dimensional finite-element analysis (FEA). The interaction of load current and rotor-induced eddy current is also studied. A 12-slot-16-pole air-cored AFPM machine is analyzed with the proposed method, and the analysis is validated by time-transient FEA simulations and prototype experiments. The influence of modulation ratio on ac copper loss due to armature current of the AFPM machine is also discussed.

Heat dissipation design of end winding of permanent magnet synchronous motor for electric vehicle

With the continuous improvement of the requirements for motor accuracy, power and weight the heat of the motor is greatly increased while the heat dissipation space is becoming smaller and smaller. In this paper, using potting materials and heat pipes to build an efficient heat path from the coolant jackets to the winding head is to improve the cooling efficiency of the motor. Three motors are manufactured and installed the same casing outside the stator. The end windings of the motor A are directly exposed to air. The end windings of the motor B are filled with potting materials. The end windings of the motor C are installed with heat pipes and filled potting materials in gaps. Under the same heat source and cooling conditions, the final temperature rises of motor A, motor B and motor C are 114 °C, 98.4 °C and 91.6 °C respectively. The experimental results show that the design of improving the heat transfer at the ends of the coil reduces the motor temperature rising by 13.7%–19.6%.

Enhanced Weighting Factor Eliminated Predictive Torque Control of an Open End Winding Induction Motor Drive

Predictive torque control (PTC) of induction motor drives is one of the best suitable alternatives for direct torque control (DTC). In this article, the weighting factor eliminated PTC for four-level inversion fed open end winding induction motor drive (OEWIMD) has been introduced. The major limitations of classical PTC are as follows: assignment of weighting factor due to dissimilar control objectives, tuning of weighing factors and high computational burden. To circumvent these problems, this article introduces an effective weighting factor elimination-based PTC scheme to an OEWIMD. The proposed algorithm eliminates the complex tuning process of weighting factors by classifying the cost function. The proposed PTC algorithm is simple and provides less computational burden on the controller. In the proposed PTC, the cost function is segregated and ranked accordingly to reduce flux and torque ripples. Cost function-I is formulated to obtain optimum voltage vectors to reduce flux ripples. The same voltage vectors obtained from cost function-I are used in cost function-II to maintain minimum torque ripple. Therefore, the proposed PTC reduces torque and flux ripples with a limited number of voltage vectors and maintains minimum computational burden. The effectiveness of the proposed PTC is verified through MATLAB/SIMULINK and experimentation.

Thermal Benefits of a Cooling Guide Vane for an Electrical Machine in an Electric Ducted Fan

Equipped with a distributed electric propulsion system, eVTOL is the primary vehicle for future urban air mobility. The electrical machine determines the weight of the distributed electric propulsion system, and the cooling capability determines the electrical machine’s weight. The electrical machine with air cooling is preferred for eVTOL due to its simplicity and light weight. This paper presents a new air-cooling system for an electrical machine in an electric ducted fan propulsion system. The novel cooling system consists of a cooling guide vane with a heat pipe (CGVHP), which is located downstream of the ducted fan rotating blade. The heated end of the heat pipe is inserted into the stator end windings and the cold end is inserted into the cooling guide vane. The heat generated in the windings is transmitted to the cooling guide vane by the heat pipe and dissipated by the forced convection using low-temperature air passing through the ducted fan. The cooling performance of the cooling guide vane with the heat pipe was numerically simulated, considering several critical design factors. The simulation results showed that the proposed CGVHP can be helpful for reducing the winding temperature of the electrical machine by over 25.5 °C at the peak power condition of 15.7 kW.

Multi-objective optimum design of five-phase squirrel cage induction motor by differential evolution algorithm

Compared with the traditional induction motors, five-phase squirrel cage induction motor (FSCIM) have the advantages of lower torque ripple and single-phase power, flexible control strategy, higher power density and fault-tolerant operation, which has been widely applied on electrical vehicles, rail transit, underwater vehicles and so on. This paper presents an analytical design optimization method based on the coupling of electromagnetic equivalent circuit (EEC) and differential evolution algorithm (DEA). Furthermore, the difference between three and five-phase winding on harmonic specific leakage permeance is investigated in detail. The nonlinear and oeolotropic influences of design geometrical parameters (slots openings, main dimension ratio, yoke height, airgap length, etc.) on the performances of the basic FSCIM model are evaluated with analytical model. For more accurate results, the harmonic characteristics of five-phase winding structure, iron core saturation, rotor slot skew width, stator end winding, slot leakage inductance and stator and rotor slot structure are considered. Meanwhile, the efficiency, power factor, maximum torque, slot fill factor and material consumption of FSCIM are calculated as the comprehensive objective function. The transient finite element analysis (TFEA) and experimental test verify the accuracy of the proposed optimum model. In addition, the feasibility of the multi objective optimization design scheme provided by the analytical model is verified by TFEA.

A Temperature and Magnetic Field-Based Approach for Stator Inter-Turn Fault Detection

Effective fault detection techniques are needed to reduce the maintenance and downtime costs of induction motors. This article presents an approach for stator end-winding inter-turn short-circuit fault detection using a metal-coated fiber Bragg grating (FBG) sensor with 0.5-mm diameter and 10-mm length to monitor the temperature and magnetic field around the end winding. The sensing theory, design, manufacturing, and installation of the sensor are detailed in this article. The nickel coating on the surface of the sensor is formed by electroless plating and electroplating, which makes the sensor sensitive to both temperature and magnetic field. The feasibility of the presented approach has been supported by both simulation and experimental results. The temperature and magnetic field signals are decoupled by filtering, and the experimental data are analyzed in the time-frequency domain. The results show that the inter-turn short-circuit fault can be detected at an incipient stage with 5% shorted turns.

Impact of Radial Air-Gap Eccentricity on Stator End Winding Vibration Characteristics in DFIG

In this paper, qualitative theoretical derivations, finite element analysis (FEA) and experiments are used to investigate the electromagnetic force (EF) and vibration characteristics of end windings. In contrast to previous studies, this study focuses not only on end winding EF/vibration under normal and radial static air gap eccentricity (RSAGE) conditions, but also for the cases of radial dynamic air gap eccentricity (RDAGE) and radial dynamic static hybrid air gap eccentricity (RHAGE). Firstly, the magnetic flux density (MFD) is derived for normal and radial air gap eccentricity (RAGE) faults, and detailed EF expressions are obtained before and after the RAGE fault. The finite element analysis (FEA) and experimental studies were performed on a four-pole DFIG at a speed of 1500 rpm to verify the proposed theoretical analysis. It is shown that RSAGE only enlarges the EF/end winding vibration and does not introduce new frequency components. RDAGE not only increases EF/end winding vibration but also introduces new frequency components. RHAGE can be seen as a superimposed effect of RSAGE and RDAGE.

Cooling structure design for an outer-rotor permanent magnet motor based on phase change material

• A cooling structure of an outer-rotor PM motor based on PCM is proposed. • The structure can prolong the operating time by 149 s under 2 times overload. • The thermal conductivity of PCM is enhanced by porous copper foam. • The composite PCM prolongs the operating time by 16.8% compared with pure PCM. For a small-sized outer-rotor permanent magnet (PM) motor during overload operation, the internal heat increases sharply and the effective heat dissipation area is seriously insufficient. Aiming at the heat dissipation for the motor, given the limitation of size, a passive cooling structure based on phase change material (PCM) is proposed in this paper. The copper shell filled with PCM is mounted between the flange and end winding using the thermally conductive silicone. The three-dimensional model of the PM motor and the cooling structure is established and the heat dissipation is simulated by finite element method. The experiment is conducted and the results show that the cooling structure can prolong the operating time of the motor by 149 s under 2 times overload. Additionally, the PCM-copper foam composite consisted of PCM and porous copper foam is adopted to enhance the thermal conductivity. The experimental results show that the operating time of the motor is prolonged by 16.8% compared with pure PCM, and the overload capability is enhanced.

Design optimization of a novel linear transverse flux switching permanent magnet generator for direct drive wave energy conversion

Linear transverse flux permanent magnet machines (LTFPMs) are categorized as high force density topologies, which are suitable candidates for direct drive and low-speed applications. This paper introduces a novel LTFPM topology, which also proposes advantages of the flux-switching machines. This proposed linear transverse flux switching PM machine (LTFSPM) has two long passive translators located at the armature's top and bottom sides. The topology offers a high value of force density, and also, it is a cost-effective topology due to its double-sided passive translator. Furthermore, the PMs are fully utilized in the magnetic circuit, the end winding length is negligible, and the iron cores can be laminated easily. In this paper, the operation principle of the topology is explained and its magnetic circuit is discussed. To reduce the computational time of optimization procedure, a double-layer method is adopted, which provides high robustness, low computational time, and high precision. Results of the finite element method indicate that the proposed machine can be considered a potential candidate for linear motion applications especially direct drive wave energy conversion systems, which require high force density values.