FEA Based Thermal Analysis of Various Topologies for Integrated Motor Drives (IMD)

Transcription

1 IECON2015-Yokohama November 9-12, 2015 FEA Based Thermal Analysis of Various Topologies for Integrated Motor Drives (IMD) Robert Abebe, Gaurang Vakil, Giovanni Lo Calzo, Tom Cox, Chris Gerada, Mark Johnson Department of Electrical and Electronic Engineering University of Nottingham Nottingham, UK Abstract Potential benefits of Integrated Motor Drives (IMD) include increase in power density, reliability and efficiency. These benefits are particularly valuable in Aerospace and Automotive applications. However, close physical integration of the drives and the machine may also lead to an increase in component temperature. This requires careful thermal analysis and coupled design of the IMD system. This paper reviews different integration topologies and compares them using FEA based thermal analysis. The effects of the power electronics (PE) position on the IMD system as well as thermal management concepts and cooling systems are also investigated. Thermal models of these IMDs are created using ANSYS and the temperature distributions of these models are analysed. Keywords FEA; integrated motor drives, IMD, cooling, thermal analysis, heat transfer coefficients, CFD I. INTRODUCTION Over the past few years, development and production of Integrated Motor Drives (IMD) has increased due to the machines manufacturers recognising the potential benefits they offer. Most important of all the benefits that an IMD offers are high power density and high efficiency. The elimination of transmission cables also brings with it economic advantages as well as increased reliability due to the removal of the output filter commonly used to reduce the voltage doubling effect caused by long cables. Other benefits include - reduced electromagnetic interference (EMI) and reduced size compared to a traditional separated solution [1]. IMD also makes it possible to reduce the total drive volume by eliminating the need of separate housings for the motor and drive controller electronics; and potentially use a singular cooling system [2]. Various integration approaches for power electronic have been proposed in literature ranging from a crude mounting of the converter on the machine housing to a modular structural and functional integration of the power electronics inside the machine housing [1-4]. To take advantage of the benefits a true IMD offers it is important to define what an IMD is. An IMD is the result of the functional and structural integration of the power electronics converter with the machine as a single unit taking into consideration the thermal, electrical and mechanical impacts both components have on each other and the system as a whole. All IMD units fulfil the functional integration of the power electronics with the machine as this is a key factor for the machine to operate - thus solving the electrical problem. However, the structural integration is a design problem which requires careful mechanical and thermal analysis of the converter and machine as a single unit. This is a challenge for designing and manufacturing an IMD. Development of converter technology, more efficient and robust passive components (especially capacitors), reliability and packaging as well as advancements in research of hightemperature power semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN), will enable power electronics to meet the harsh and rigorous environmental conditions the converter faces in an IMD. Careful design analysis has to be performed in order to successfully integrate the power electronics and machine in the same housing envelope. Thermal management problems arise if the power electronics is placed close in the machine housing due to the close proximity to heat sources in the machine - copper windings, back iron and rotor; which may cause irreparable damage to the less robust converter unit. Vibration problems might also exist if the converter is in or around the machine housing leading to damages in electrical connections and a decrease in the expected lifetime of the converter. Adapting the converter to the electromagnetic field stresses inside the motor housing due to lack of available space for mounting also poses major problems [5]. This paper investigates existing IMD configurations based on their mounting position around the machine housing. Thermal analysis of various integration concepts are analysed using FEA with the aim of investigating the effects of close proximity integration on the individual IMD components and system as a whole. The advantages and disadvantages the analysed configurations offer are also investigated based on the temperature distribution of their individual components. II. PE MOUNTING OPTIONS FOR IMD IMDs offer a number of attractive benefits ranging from increased power density to reduced EMI. However, very little work exists on physical integration of a converter and machine in a single housing despite extensive research on IMDs. Reviews on current IMD technologies and market perspective [6-9], thermal analysis of cooling systems [10-12] and functional integration [13] form the majority of existing literature and research on IMDs /15/$ IEEE

2 Based on available space for physical integration of converter and machine in the housing envelope, there are three major categories of IMD configurations possible Surface Mount Integration, End Plate Mount Integration and Stator Iron Mount Integration. A. Surface Mount Integration Surface Mount Integration is the most popular and easiest to implement of the three choices available. This involves the physical mounting of the power converter on the motor housing. Variations of this concept exist from a simple mount of the converter on the machine housing to mounting modularised converter units on the housing. Fig. 1 shows simple addition of power electronics on the motor while Fig. 1 shows the modularized converter units mounted on the motor housing [14]. The cooling system in this concept can also be shared (by the converter and motor) [15] or separate. adopting this configuration is the machine components might suffer due to the lack of ventilation from the fan. Also, since the power electronic is cooled before the machine, the fluid temperature increases before reaching the machine components further increasing the machine s overall temperature. At high torque and low voltage operation, an extra cooling system is required due to the increased temperature of the IMD system [15]. Fig. 2 Axial view of PE mounted on endplate and Redesigned finned endplate with converter loss simulation resistors [15] Fig. 1 Simple addition of PE on the motor [5] and Modularised converter units mounted on the motor casing [14]. The low cost of manufacturing, ease of implementation and simplicity are some of the attractive benefits this configuration offers. The machine housing can provide thermal isolation between the machine and converter by acting as a thermal barrier if a parallel cooling path system is adopted. With a series cooling path system adopted, the machine housing can act as a heat sink for the converter by providing a large surface area for heat dissipation. In high power density machines where temperatures sometimes exceed 200 C, extra thermal management and cooling systems can be adopted. Water pipes through the machine housing is utilised in [12] and [14] employs forced air cooling using fins embedded in the machine housing. B. End Plate Mount Integration In the example published in literature, this specialised configuration is dependent on the cooling system adopted and available space for mounting. The configuration aims to protect the power electronics from the machine heat sources by placing it closer to the fan. It is assumed that the machine is robust enough to withstand increased temperature compared to the converter unit hence the endplate is used as a thermal barrier between converter and machine. The endplate is redesigned as a suitable mounting platform and heatsink for the converter. Extra cooling fins added to aid heat dissipation for the converter. The obvious drawback of C. Stator Iron Mount Integration The power converter is mounted on the stator back iron as shown in Fig. 3. The available space between the endplate and stator back iron is utilised for mounting hence it is imperative the height of the converter - especially the otherwise large DClink capacitors, fits into that space. Fig. 3 PE mounted on stator back iron [1] and Stator pole module consisting of stator iron pole piece and a power converter module [3] GaN semiconductor devices is adopted in [1] for stator back iron mount integration; for high temperature operation. The power converter is modularised around the back iron and the individual modules are connected in series. The modular segments energise independent machine windings similar to a Switched Reluctance Machine (SRM) mode of operation. A similar approach is adopted in [3] but with silicon based devices instead of GaN devices. The high temperature operation capability of GaN is absent in this approach hence the heat sink is used on each module to alleviate the increased temperature in module. This leads to an increase in the overall volume of the IMD system. III. INTEGRATED MODULAR MOTOR DRIVES The converter integration concepts discussed in the previous section were proposed as a result of available space for mounting. They were based on the possible positions were the power electronics could exist, on or inside the machine housing. Modular integration involves partitioning the converter into smaller units that control specific stator winding sections. These

3 modules are usually connected together in series or parallel independent of each other and properly coordinated for the purpose of increasing overall system performance and controllability. Major advantages include fault tolerance, better thermal capabilities due to a delocalized losses spatial distribution, power rating scalability and a possible reduction in the overall size and cost of the PE. Drive modularisation allows for integration on or around the machine housing by mounting the smaller converter modules on the housing [7], stator iron [1, 3] or the end plate [15]. Fault tolerance is another advantage as multiple modules provide system redundancy and increase in the equivalent system reliability. Current and voltage stresses can be reduced in the PE as the total drive power is distributed across multiple subunits. The size of the passive components can also be reduced as well if specific PE topologies and interleaving-based control algorithms are adopted. A centralized control system can exist to control the individual modules or, when higher levels of fault tolerance are mandatory individual modules contain a dedicated control system in order to make the drive resistant to multiple faults. Despite the aforementioned potential benefits modularisation for converter integration, possible drawbacks include the necessity for a complex control system for the individual power modules and the difficulty of physical integration of the modules with the machine. IV. FINITE ELEMENT ANALYSIS OF MOUNTING TOPOLOGIES To compare these mounting topologies, a detailed FE analysis was carried out on each topology. The FEA tool used was ANSYS steady state thermal analysis package. A 'full SiC' module was modelled with the switch ratings of 1200V; 60A and junction temperature 150 C. The full 3- φ power electronic module consists of 12 6x3mm SiC bare dies. The losses inputted for the SiC module was derived from experimental results [17] and was set at 613W. A permanent magnet synchronous motor was modelled as the choice machine. Fig. 4 shows a cross section of the PMSM modelled. The end windings and back iron are forced air cooled through inlet vents and slots machined in the back iron. Table I shows the losses for different components in the machine. Due to the size of the power module and the available space for mounting/integration, in this paper the housing and endplate mount topologies are analysed. The IMD has been designed to be air cooled with an inlet temperature of 65 C. The heat transfer coefficients of forced air cooling on different surfaces were computed by analytical pipe model and the turbulent flow effects were neglected [18-20]. Heat transfer coefficient due to natural convection was calculated to be 4 W/m 2 K and 2W/m 2 K for the machine housing and converter casing respectively. Average heat transfer coefficient for forced air cooling ranges from W/m 2 K. ANSYS FLUENT can be used to model air flow and calculate the heat transfer coefficients of the different motor and converter components provided accurate boundary conditions are set. However, the computational time to implement these boundary conditions for the different integration options analysed would be too great. Instead, FLUENT was used only to model and analyse the air flow through the motor. Fig. 5 shows the air flow through the motor and the air speed at various parts of the motor. The air flow over the end windings is turbulent while the flow through the slots on the outer periphery of back iron is laminar. Using (1), the air speed over the end windings and back iron was used to calculate the Reynolds number Re = DVρ/μ (1) (Where D = Diameter; V = Mean velocity; ρ = Density and μ = Viscosity). Using (2) and (3), the Nusselt and Prandtl numbers were also calculated for both end windings and back iron heat transfer using the thermal properties of air and the machine dimensions. Nu = hl/k (2) Pr = C pμ/k (3) (Where h = heat transfer coefficient; L= characteristic length; k = thermal conductivity; C p = specific heat capacity; μ = viscosity). Using these values, the heat transfer coefficient for the end windings and back iron was 17 W/m 2 K and 9 W/m 2 K respectively. These values were inputted into the ANSYS steady state thermal analysis package. This was to save time in the computational process as the thermal analysis package has a much shorter solving time compared to FLUENT. Also, with the thermal analysis package, the ease of repeatability is greater as it allows for faster analysis of more configurations. However, the accuracy of the results is reduced compared to results from the CFD analysis (software: ANSYS FLUENT). Air Vents Fig 4 Cross section of PMSM modelled The outside ambient temperature was set at 35 C and the ambient temperature inside the module and housing was set at 80 C and 70 C respectively. A part of motor housing was redesigned to accommodate the power module. Fig. 6 shows the

4 full SiC module (peripheral and internal view) used for IMD. The FE models are developed for each topology and are shown in Fig. 7. The end plate mount topology is modelled in two different ways external and internal. For the end plate mount analysis, the end plate was redesigned to accommodate the module. The end plate was extended by 15 mm and the radius increased by 6 mm. Fig. 5: FLUENT model of air flow through the machine Fig. 6 Full Sic module Peripheral view and Internal view (c) (d) Fig. 7 Temperature distribution of the analysed topologies Surface mount integration, End plate mount integration external (c) End plate mount integration internal (d) Modularised end plate mount - internal

5 Operation Speed 8000 rpm and Torque Nm Speed rpm and Torque Nm Speed rpm and Torque Nm TABLE I. MACHINE LOSSES FOR PMSM Copper Losses (W) 1 st nd rd st nd rd st nd rd Iron (Stator) Losses (W) Rotor Ohmic Losses (W) Inconel sleeve Inconel sleeve Inconel sleeve Where copper losses are given in following order 1 st - Copper losses without end windings 2 nd - End winding losses 3 rd - Total copper losses Output Power (Txω) (kw) TABLE II. TEMPERATURE DISTRIBUTION OF MACHINE COMPONENTS AND CONVERTER FOR DIFFERENT MOUNTING CONFIGURATIONS Mounting Position / Components Surface Mount Integration Endplate mount integration Internal Endplate mount integration Internal modularised end plate mount Back Iron Housing End Windings Rotor Converter V. RESULTS/DISCUSSION Surface mount topology effectively used the housing as the heat sink for the power module, but proper arrangements are required to be made in order to get an effective contact between the power module and the stator casing. The converter and machine share a common thermal node the machine housing. This allows for adoption of a singular cooling system for both machine and converter which helps increase power density and potentially reduces overall system cost. The curved housing surface makes mounting of the converter slightly complicated. This facilitated the design of a mounting platform for the converter on the housing. However, the mounting platform increases the thermal resistance between the converter and the coolant by providing an extra thermal node between the converter and coolant. The thermal path of the end plate mount external configuration is similar to that of the surface mount configuration. The integration of power module on the end plate is easier and the larger housing of this configuration provides better thermal distribution compared to the housing mount configuration. The mounting platform in the surface mounted configuration is absent in this configuration further reducing the thermal resistance between the converter and coolant. For the end plate mount internal configuration the power module is in direct contact with the coolant aiding a better thermal distribution in the IMD. This increases the heat transfer coefficient between the devices and the coolant resulting in a more efficient heat transfer. The motor housing acts as a heat sink for the converter further increasing temperature distribution. Table II shows the temperature distribution of the machine components and converter for different mounting topologies. The results show that the modularised endplate mount configuration is the most effective for temperature distribution and efficient heat dissipation in the IMD system. Unlike the machine, the converter and associate control electronics are rarely considered either mechanically or thermally robust. This means the converter temperature has to be carefully managed. The overall converter and machine components temperature of the modularised end plate mount configuration is reduced compared to the other configurations. The endplate provides a large contact area for heat dissipation akin to a heat sink in a power module. This reduces the critical thermal path in the system the junction to case thermal resistance. Hot spots in the power device are also reduced because the devices are better spaced out. Configurations where the converter unit is in contact with the coolant also provide power density benefits and potentially low costs compared to the other configurations analysed. This is because heat sources in the IMD systemconverter, end windings and back iron share a singular cooling system thereby decreasing the overall volume of the system and cost of an additional cooling system. Redesigning the end plate by increasing its volume is not required for the modularised end plate mount configuration as the existing endplate profile is sufficient to accommodate the converter unit. The slightly increased temperature of the end windings and rotor for the internal end plate mount and modularised endplate configurations is due to the close proximity of the converter to these components. The converter in contact with the coolant provides the best IMD configuration from the options analysed. However, the end plate was redesigned to accommodate the converter. Effectively modularising the converter in the machine housing offers more integration options by providing more mounting positions and improving temperature distribution in the IMD system. VI. CONCLUSION The simulation results show that the position of the power electronics has a considerable effect on the temperature of the

6 IMD components. However, the cooling mechanism adopted also affects the temperature distribution in the IMD. The IMD design should involve a holistic analysis of the thermal, mechanical and electrical effects of the IMD components and the cooling system as a single unit. The cooling system should be influenced by the PE position as well as the target application. Converter position significantly affects the thermal, electrical and mechanical integration of the PE in the IMD. The endplate mounted power module produces an overall larger IMD volume due to the redesigned endplate. However, the thermal distribution in the IMD improves as the power module is in direct contact with the coolant. The modularised endplate mount configuration is a further improvement on the end plate mount configuration producing better thermal distribution in the system and the initial housing profile is unchanged. Spatial constrictions also determine mounting positions of the power electronics unit. Although the housing periphery offers the only viable mounting option for module integration, by redesigning and modularising the PE module, the stator back iron or end windings could be possible mounting options. Modularisation of the converter into smaller units will also help reduce the thermal stress on the devices and efficiently utilise available space around the machine housing. Despite the numerous potential benefits the modularised configuration offers, questions persist over its manufacturability, initial costs of production and efficient design. Power modules are efficiently designed units and careful design analysis should be performed to ensure the end plate provides a mechanically stable platform for mounting. REFERENCES [1] J. Wang, Y. Li, Y. Han, Evaluation and design for an integrated modular motor drive (IMMD) with GaN devices," Energy Conversion Congress and Exposition (ECCE) IET. pp , [2] N.R. Brown, T.M. Jahns, R.D. 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