Design of Inverter Driven Induction Machines Daniel M

Design of Inverter Driven Induction Machines Daniel M. Saban, PE Ph. D saban@ieee. org

Overview • The induction machine problem – Stakeholders & design drivers – Analysis & synthesis challenges – Design rules-of-thumb & constraints • Optimization and/or synthesis – Common tools – Selected approaches – Inverter system consideration • Opportunities 2

Induction machine • Stakeholders and their perspectives – – – Customers Sales & Marketing Manufacturing Engineering & Operations Application Engineering Product Development • Opportunities – Materials: improved and exotic – Manufacturing processes and process control – Design, analysis and optimization tools • Size & Topology 3

Induction machine • Temperature “is everything” – Material limits (life) • Insulation system • Bearing system – Material dependencies (performance) – Cooling system – Rules-of-thumb in design • Cost “is everything” – Operating cost: efficiency, power factor – Initial cost: better material, more material • Quality “is everything” • Performance “is everything”? 4

IM analysis challenges • • • Non-linear: saturation, core losses Winding harmonics Rotor/Stator slotting & skewing Material property variation (lot-to-lot) Dimensional variation & shift Manufacturing/assembly variation Rotor resistance End-leakage (consider frame) High-frequency impedance (bearing currents) 5

Proximity & Skin Effect • Fundamental current injected into conductors • 1 turn per coil; 4. 0 k. W loss/pole • 4 turns per coil; 2. 5 k. W loss/pole 6

Slot Ripple Eddy Current • • Current Sheet used to simulate total air-gap flux density No current injected into conductors Loss is due to induced eddy currents Used to analyze effect of wire transposition and aspect ratio 7

IM design synthesis • Clean sheet – Single application – Product family • Existing laminations • Brute Hp vs. finesse 8

IM design synthesis challenges • Knowns – Full stator slots – High conductivity conductors – Small gap? • Unknowns – Rotor & stator aspect ratios – Slot shape details – Discrete values only • • • Pole count Discrete wire sizes, non-linear cost function Winding details: number of turns, coils, pitch Integral numbers of slots, rotor/stator Lamination material, grade, thickness 9

Rules-of-thumb • Stator current density – 620 A/cm 2 to 1 k. A/cm 2 – Highly dependant on cooling system – Revise after thermal modeling • Peak flux density of stator teeth, yoke – ~1. 7 T, ~1. 6 T – Revise upward for more power density – Revise lower for higher efficiency • Rotor current density • Gap flux density: 0. 5 T to 0. 8 T 10

Common Design Constraints • • • Rotor OD Stack length Machine construction Cooling system 11

IM design iteration design constraints mfg constraints matl props LP objectives FE Manual Iteration 12

IM design tools • In-house – Typically only lumped parameter (LP) – May be tied to manufacturing or operations – Some “special” versions of commercial software • Commercial – – – LP: PC-IMD (SPEED), VICA (support? ) LP+FE: PC-IMD/FEA (SPEED), RMxprt (Ansoft) MCM: ? ? FE: Magnet (Infolytica), (Flux, Maxwell) Ansys/Ansoft System simulation: Matlab/Simulink, Simplorer (Ansoft), Easy 5 13

IM design optimization design constraints mfg constraints matl props objectives stand output file stand input file LP geom trans MCM FE addl output files Optimization engine 14

IM design optimization • Inverter driven machines – Pole count is now a free variable – Stator & Rotor lamination design optimization can be decoupled – Skewing penalizes machine • Finesse approach – – – Size machine, ignore details & discrete values Create response surface & narrow search space Optimize rotor and stator separately Second pass takes into account discrete values Requires dedicated code • Key design points: torque corner point, max speed, max torque • Best motor will deliver maximum torque for maximum drive current 15

IM-Inverter system optimization • Max torque-speed envelope (output) – different than constant torque/power/slip – power factor and efficiency variations • Optimal motor leakage – Harmonic ripple current – Chopping frequency – Fundamental AC current – Peak transistor frequency 16

Opportunity • Simple tools – When to apply vs. other technologies (IM vs. PM) – Rough sizing: stack length, stator od, rotor od – Fit of test data for lamination family, or single design • Models of different manufacturing techniques/defects • Stray load loss - rotor/stator harmonic interaction • Stator conductor eddy currents; large copper crosssection, high frequency • Vehicle to adapt academic work into industrial setting – Open source – Widespread use – Extensible framework 17