Understanding Motor and Controller Efficiency in Low Voltage AC Vehicles

System Overview ■Traction System Block Diagram ■Sources of power loss (decreased efficiency) ■Battery (Internal Resistance) ■Battery to controller wiring ■Controller ■Controller to motor wiring ■Will combine with motor losses for this study ■Motor ■Mechanical (gearbox, tires, drag, etc.) BatteryController Mot or Mechanical Losses Wiring Losses Wiring Losses

Battery and Wiring ■Causes of losses ■Battery internal resistance ■Wire losses ■Crimps ■Bolted joints ■Equations to calculate efficiency Power Loss = I 2 ∙ R Efficiency = Power In - Loss Power In

AC Controller ■Controller power circuitry consists of a MOSFET three phase bridge

AC Controller Motor voltage waveforms, showing PWM generation of sine waves, and 3-phase relationships

AC Controller Causes of Losses MOSFET Conduction Losses – Accounts for most of the loss Switching Losses – Not Dominant Conduction Loss = 3 ∙ I RMSphase 2 ∙ R ds(on) Controller Efficiency = P out = 3 ∙ (V LL /√3) ∙ I RMS ∙ cos θ P in I B ∙ V B

AC Motor ■Causes of Losses ■Stator losses ■Induced rotor current losses ■Leakage inductances contribute to non unity power factor ■Windage losses – Not dominant ■Motor Efficiency Calculation Efficiency = P out = Torque(N-m) ∙ Speed(RPM) ∙ 2π / 60 P in 3 ∙ (V LL /√3) ∙ I RMS ∙ cos θ

AC Motor Efficiency Base Speed and above – Maximum Efficiency Low Efficiency – Losses are the same, but doing less mechanical work Typical AC Motor Efficiency Curve Speed (RPM) Motor Efficiency [%]

Motor Model Steinmetz model example for a low voltage motor (at one load point) =1.5 m  =7.1  =9.2  =0.15 m 