HEAT TRANSFER IN ELECTRIC MACHINES Overview of cooling and simulation techniques in electric machines JANDAUD Pierre-Olivier LE BESNERAIS Jean 20th September 2017 www.eomys.com [email protected] 1 PRESENTATION OF EOMYS • Innovative Company created in may 2013 in Lille, North of France (1 h from Paris) • Activity: engineering consultancy / applied research • R&D Engineers in electrical engineering, vibro-acoustics, heat transfer, scientific computing • 80% of export turnover in transportation (railway, automotive, marine, aeronautics), energy (wind, hydro), home appliances, industry 2 EOMYS SERVICES & PRODUCTS • Diagnosis and problem solving including both simulation & measurements • Multi-physical design optimization of electrical systems • Technical trainings on vibroacoustics of electrical systems • MANATEE fast simulation software for the electromagnetic, vibro-acoustic and heat transfer design optimization of electric machines EOMYS can be involved both at design stage & after manufacturing of electric machines 3 WEBINAR SUMMARY • INTRODUCTION • TYPES OF COOLING TOPOLOGIES • SIMULATION TECHNIQUES • CONCLUSION 4 INTRODUCTION • Why is heat management important in an electric machine? • General introduction to Heat Transfer & Fluid Mechanics • Types of Losses 5 Why is heat management important? • Temperature levels impact directly on the lifetime of a machine • High temperature increases the fatigue of a material • Each machine has an insulation class for its windings based on the nature of the insulation material • Basic rule of thumb: lifetime divided by two for each 10°C over the rated temperature, multiplied by two for each 10°C below. • Temperature levels are also important to avoid demagnetization of the permanent magnets and efficiency reduction • Heat Management is important for reliable and robust machines Demagnetization and characteristic curves of a Overheated windings (Reinap, 2015) PM (Neorec53B magnet) 6 Why is heat management important? • Temperature levels impact directly on the efficiency of the machine • High temperatures increase linearly the electric resistance of conductors: ( ) = 1 + ( ) • Higher temperatures higherJoule losses − ⇒ Torque vs Temperature in a PM motor (Lungoci, 2008) • Several studies show the impact of temperature on efficiency of PM machines • From 25°C to 100°C, the efficiency can decrease up to 5% • Investing in the cooling system optimization at the design stage of the machine can give significant long-term cost savings Efficiency vs Temperature for different PM (Wang 2008) 7 General introduction to heat transfer in electric machines • An electric machine is a complex system in terms of heat transfers • The three kind of heat transfers interact (Conduction, Convection, Radiation) • Heat is generated by losses in the machine • Heat always flow from the hottest temperature to the lowest From Techniques de l’Ingénieur (Bertin, 1999) 8 General Introduction: Conductive heat transfer • Conduction occurs inside a body, depends on the thermal conductivity ( in . ) −1 −1 Material ( /m/K) • In a homogeneous body, heat flux ( in / ) respects a simple Air 0.026 PDE the Fourier’s Law, fundamental law for conduction:2 = . PVC 0.15 Epoxy 0.25 • For an equivalent heat flux, a higher− thermal conductivity means a lower temperature gradient i.e. lower temperature levels Water 0.6 Stainless Steel 30 Cast Iron 50 • Electric analogy: Ohm’s Law, Temperature is Voltage, thermal conductivity is equivalent to electric conductivity Aluminum 230 Copper 390 • Electric insulators are most of the time good thermal insulators. Thermal conductivities of common • Air is one of the best insulator if it’s not moving; if there is air motion, materials at 20°C convective heat transfer appears Ex: thermal effect of Vaccum Pressure Impregnation (VPI) when air replaced by resin 9 General Introduction: Convective heat transfer • Convective heat transfer occurs in case of a moving fluid on a solid body Material (W/m²/K) • The convective heat flux between a solid and a fluid body is given by Air (natural ℎ 5-10 Newton’s Law: convection) = . ( ) Air (forced 10-300 convection) • is the convective Heat Transfer ℎ Coefficient− (HTC) in / / Water (forced 500 – 10000 2 convection) • The fluid can be a gas (e.g. air), or a liquid (e.g. water, oil) ℎ Range of convective HTC for air and • Natural convection: fluid motion due to thermal gradients (e.g. hot air water balloon, ocean currents) • Forced convection: fluid motion due to an external source (e.g. pump, fan)- main method to cool electric machines Ex: effect of relative wind on the cooling of outer rotor wind turbine generator 10 General Introduction: Radiative heat transfer • Each body emits electromagnetic radiations depending on its temperature levels (contactless heat transfer) Material • Aluminum 0.05 Bodies are modelled using the gray body theory. The heat flux (polished) exchanged with a body and its environment is: Aluminum 0.25 = . ( ) (strongly oxidized) 4 4 Black electrical 0.95 • is the Stefan-Boltzmanconstant and − is∞ the emissivity of the body tape Cast iron 0.21 • The emissivity is low for reflective surfaces (polished metals) and (polished) depends strongly on the surface finish Copper (polished) 0.01 • Radiative heat transfer is often neglected inside the machine due to Copper (oxidized) 0.65 relatively low temperature levels Galvanized steel 0.28 • Radiative heat transfer can be important as a boundary condition Ideal Black Body 1 especially in case of natural convection Matt paint (oil) 0.9-0.95 Water 0.98 Ex: alternator in a car exchanging heat with the other parts of the engine Emissivity values for common materials at 20°C (Fluke) 11 General Introduction: Fluid Mechanics considerations • Average velocity of the fluid (m/s) 3 • Volume flow rate ( in m /s) through0 a section S: = . Between 2 points of a circuit, flow rate is constant: 0 = • The pressure of1 the1 fluid2 2( in Pa). Between 2 points of a path line, pressure and average velocities are linked by Bernoulli equation ( is the density of the fluid in kg/m3): + = + + 1 2 1 2 • is the Head Loss1 2 or 1Pressure2 2 drop2 between two points of the circuits. It represents the energy lost due to friction (on walls or due to a singularity). Equation of the hydraulic power: Δ = . P • Hydraulic power is important∆ to evaluate the energy consumption of a cooling system Ex: cost of cooling power consumption over 25 yrs of a wind turbine generator 12 General Introduction: Dimensionless numbers • In Fluid Mechanics and Heat Transfer, most of the phenomena are studied using dimensionless numbers which are used also in correlations • The Reynolds number dimensionless number for the velocity. In a channel, for Re < 1500 flow is laminar. For Re > 3000, flow is turbulent. Laminar (up) and turbulent (down) rotating flow . visualizations at Re=900 and Re=5000 (Bauduin, = 2014) • The Nusselt number is for convective heat transfer. In the scientific literature most of the convection correlations have the form: = . = ℎ • Pressure drop coefficient is given by: = 1 2 • Friction factor in a channel of diameter DΔ and�2 length L is given by: = . For laminar flow, given by an analytical expression: = / . For turbulent flow, the Moody chart must be used. Moody chart for friction factor 13 Heat Sources in a Machine: Losses • Heat in the machine is generated by electromagnetic and mechanical losses • Losses distribution highly depends on machine topology, load and supply conditions • Joule losses are generated by electric currents in the windings • Core losses include hysteresis losses, eddy-current and stray losses, they are located in the laminations of the machine Losses in an 4 poles IM at 50Hz (Yang, 2016) • Magnet losses are due to eddy currents, they can be high in concentrated winding topologies with surface magnets • Mechanical losses include friction and windage losses (friction in bearings, aerodynamic friction and drag) Losses in an IPM machine (Yang, 2016) 14 Heat Sources in a Machine: Joule Losses • Joule Losses are usually the most important sources of losses in an electric machine • Located in windings/end-windings and rotor bars of IM • Usually dissipated with convection on end-windings (for stator) • Temperature dependent: higher temperatures increase electric resistivity • Joule Losses equation with frequency dependent effects: Losses in an 4 poles IM at 50Hz (Yang, 2016) rms phase current = . + + ( ) Phase number 2 DC, Skin and proximity components of phase Losses in an IPM machine (Yang, 2016) resistance 15 Heat Sources in a Machine: Core Losses • Core losses are usually the second sources of losses in a machine • Located in the stator and rotor cores • Combine two phenomena: eddy-current losses and hysteresis losses • Modeling of core losses is more challenging than Joule Losses • Steinmentz equation taking harmonic components into account: Losses in an 4 poles IM at 50Hz (Yang, 2016) Flux density Harmonic rank = , + 1 6 2 2 ℎ � Frequency Hysteresis coeff. Eddy losses coeff. Losses in an IPM machine (Yang, 2016) 16 Heat Sources in a Machine: Magnet Losses • Magnet losses can be critical in some topologies • Magnets can be isolated inside the machine (e.g. IPMSM) -> difficulty to dissipate magnet losses • Magnet Losses equation for SPMSM (Deeb et al, 2012) Losses in an 4 poles IM at 50Hz (Yang, Width 2016) Volume Flux density = 24 2 2 2 2 Harmonic id � Frequency Resistivity Losses in an IPM machine (Yang, 2016) 17 Heat Sources in a Machine: