Thermal Analysis & Design Improvement of an Internal Air-Cooled Electric Machine Dr. James R. Dorris Application Specialist, CD-adapco
Thermal Analysis of Electric Machines Motivation Thermal challenges in electric machines Workflow SPEED & STAR-CCM+ applied to electric machines Project Description Overview of internal fan-cooled induction machine Electromagnetic Modeling SPEED model and computation of losses Thermal modeling STAR-CCM+ CHT model and results of simulations Motors / Cooling systems of particular interest Electric machine applications of particular interest Conclusions
Motivation: Electric Machine Industry Focus The past 10 years shows industry focus on: high torque / power density / high efficiency and reduction in size, weight, cost This combination leads to more performance from a smaller package and a thermal challenge. Increased operating temperatures would result in: Requirement for better insulating materials Reduced lifetime due to higher risk of thermal damages to insulations, bearings, etc Higher risk of demagnetizing permanent magnets Improved cooling systems via CFD thermal simulation provide engineering value. Source graphics: NREL 20 110 ºC 140 200 230 ºC Demagnetization Source graphics: Integrated Magnetics
Electromagnetic Analysis: The SPEED suite of programs The following machine types are available: PC-BDC: Brushless permanent magnet and wound-field AC synchronous PC-IMD: Induction PC-SRD: Switched Reluctance PC-DCM: Direct Current (PM) PC-WFC: Wound-field and commutator PM 4
CD-adapco Tools For Electric Machines SPEED and STAR-CCM+ Workflow Electric machine design solution Template based geometry, analytic tool + models for 3D effects, 2D FEA solver. 1. Create SPEED model based on geometry, parameters, & winding scheme 2. Stability check with static and dynamic analytical analysis 3. FE-analysis and fitting of the analytical model 4. Preparation of the geometry in STAR-CCM+ by reading the xgdf file 7. Solving and post processing in STAR-CCM+ FE-grid SPEED FV-grid STAR-CCM+ Data transfer 6. Mapping process for rotor and stator heat losses is carried out separately and automatically with transfer of the values from neighbor grid node in SPEED to STAR-CCM+ Multi-physics, general purpose simulation solution General geometry, 3D finite volume solvers 5. If stable results, transfer of the heat loss distribution from the FE-analysis to STAR-CCM+ via the sbd-file
Project Description Induction Motor Thermal Performance Challenge: A North American Motor Company s (NAMC) internal fan cooled split-phase induction machine is not sufficiently cooled, expensive to test many design configurations. Solution: Develop a steady-state thermal analysis of a machine using the CDadapco SPEED STAR-CCM+ workflow. Procedure: Compute losses using SPEED Import SPEED geometry, losses, NAMC CAD for non-active components into STAR-CCM+ Define appropriate physics, boundary conditions Solve Conjugate Heat Transfer (CHT) simulation at the specified load point New load point only requires losses to be re-computed and re-imported into STAR-CCM+ - can be ready in ~15min. Geometry changes can be performed by swapping out parts and re-meshing can be ready is less than 1 day.
Electromagnetic Analysis Computing losses using SPEED 2-D Geometry, winding definition Materials Lamination Steel, rotor cage Controller definition, simulation settings Analytic Calculations (< 1s) 2D FEA electromagnetic solution (<1min) Losses Fast computation, computed from can connect FEA B-field with distribution HEEDS optimization via Steinmetz engine Model Heat Loads
Physics Modeling SPEED Geometry: Active Components NAMC Geometry: Non-Active Components Windings modeled as bulk material I 2 R loss slot dependent Anisotropic Thermal Conductivity Rotor Cage I 2 R Losses Uniform distribution over rotor bars I 2 R loss Temperature Dependent SPEED model losses computed at average temperature STAR-CCM+ model adjusts for local temperature dependent resistivity ρ V = Q T (1 + 0.00393 T T ref V Where ρ V is the volumetric heat load, Q T is the total heat load, T is the local region temperature, T ref is the average temperature (80 C) and V is the region volume. End-winding surface roughness Core losses spatially dependent )
Simulation Steady State Temperatures 2 1 2 1 Rotor Bar Avg=116.7 C, End Ring 1 Avg=114.1 C, End Ring 2 Avg=115.9 C Shaft Min Temp=44.8 C, Shaft Max Temp=116.5 C SPEED model with rotor temp @ 116 C requires 52.5 % of copper conductivity for consistent losses and performance at this load point.
Comparison with Measurements NAMC Measurements on aux and main winding at 2 circumferential locations, both for the fan (cold side) and exhaust (hot side) of the end winding. Do not resolve aux vs. main winding due to NAMC s end-winding geometry. Compare with mean and standard deviation of temperature in outer 5mm of endwinding End Winding 2 (cold side) End Winding 1 (hot side) Measurement Simulation % Error Measurement Simulation % Error Mean 70.6 C 71.6 C 1.40 % STD 3.38 C 1.94 C Mean 88.0 C 88.5 C 0.57 % STD 3.16 C 1.08 C
Heat Flow: Rotor and Stator Easy Reports in STAR-CCM+ Stator: Copper Loss 171 W Aux + Main Winding 84 W 87 W Iron Loss 82 W Air Stator Lams 49 W 120 W Air Housing Rotor: Copper Loss 102 W Rotor Cage 36 W 66 W Iron Loss 17 W Air Rotor Lams 42 W 41 W Air Shaft
CD-adapco Tools For Electric Machines SPEED and STAR-CCM+ Workflow Electric machine design solution Template based geometry, analytic tool + models for 3D effects, 2D FEA solver. 1. Create SPEED model based on geometry, parameters, & winding scheme 2. Stability check with static and dynamic analytical analysis 3. FE-analysis and fitting of the analytical model Modified Original Design 4. Preparation of the geometry in STAR-CCM+ by reading the xgdf file 7. Solving and post processing in STAR-CCM+ FE-grid SPEED FV-grid STAR-CCM+ Data transfer 6. Mapping process for rotor and stator heat losses is carried out separately and automatically with transfer of the values from neighbor grid node in SPEED to STAR-CCM+ Multi-physics, general purpose simulation solution General geometry, 3D finite volume solvers 5. If stable results, transfer of the heat loss distribution from the FE-analysis to STAR-CCM+ via the sbd-file
Phase 2: Vented Stator Design Improvement Geometry of Stator swapped for vented stator design: Approximately 1 day work required by intermediate user to swap geometry New geometry part created in STAR-CCM+ Conformal interfaces rebuilt Entire model re-meshed Physics / boundary conditions reset Expecting Temperatures to drop across entire model: Lower Copper Temp / Heat load Lower Al Cage Temp / Heat load
Simulation Steady State Temperatures Original Vented Stator Design (reference) 2 2 1 1 Rotor Avg=96.7 C End Ring 1 Avg=93.7 C End Ring 2 Avg=96.6 C Shaft Min Temp=36.0 C Shaft Max Temp=96.7 C End Winding Temperatures decreased by nearly 20 C. End Winding 2 (cold side) Orig Design Vented Stator % Mean 70.6 C 59.2 C 16.1 % STD 3.38 C 2.13 C End Winding 1 (hot side) Orig Design Vented Stator % Mean 88.0 C 67.9 C 22.8 % STD 3.16 C 0.97 C
Heat Flow: Rotor and Vented Stator Stator: Copper Loss 156 W 15 W 20º C Aux + Main Winding 62 W 94 W Iron Loss 82 W Air Stator Lams 37 W 86 W 90 W Air Housing Rotor: Copper Loss 93 W 9 W 20º C Rotor Cage 33 W 60 W Iron Loss 17 W Air Rotor Lams 42 W 35 W Air Shaft
Cooling Systems of particular interest for CFD analysis Loss Mechanisms Copper losses, spatially distributed iron losses, friction and windage losses Heat transfer Conduction, Radiation Convection (natural or forced) Simulation of fluids moving in and around objects Liquids and/or gases
Conclusions Computation of losses in SPEED copper (I 2 R) losses on Stator windings and rotor cages Spatial distribution of iron losses (eddy + hysteresis loss in laminations) Efficient workflow form SPEED to STAR-CCM+ Geometry, losses are easily imported New load points or geometric changes are easily studied. Detailed physics easily defined Temperature dependent copper losses Anisotropic thermal conductivity of windings CFD can provide heat-flow analysis that measurements cannot quicker and less expensive leads to better insights for design improvements Workflows also available from other Emag codes Flux 2D/3D, JMAG, or STAR-CCM+ 2D Emag solver