copyright 2011 controltrix corpwww. controltrix.com Improving efficiency a perspective on electric vehicles BLDC motor drive system

copyright 2011 controltrix corpwww. controltrix.com BLDC drive basic Standard 6 step hall effect sensored drive 3 Hall Sensors used to determine the sector At any time 2 of the phases energized Only single top side switch is PWMed for variable speed Ref: App Note AN957 microchip.com

copyright 2011 controltrix corpwww. controltrix.com High Frequency Carrier Duty Cycle Varied Over Time to Generate a Lower Frequency Signal +V PWM1H PWM1L 3 Phase BLDC PWM2H PWM2L PWM3H PWM3L PWM with Inverter

copyright 2011 controltrix corpwww. controltrix.com Six Step BLDC Control +TORQUE FIRING B Green Winding Q1,Q5Q1,Q6Q2,Q6Q2,Q4Q3,Q4Q3,Q5 60 o HALL A HALL B HALL C Q1,Q5Q1,Q6Q3,Q5 Sector 5 Hall States Blue Winding Red Winding

copyright 2011 controltrix corpwww. controltrix.com Electric Vehicle Specific 250 W, 24 V, 12 A direct drive system % drive +motor 10A,300 rpm Regenerative efficiency (3 bottom PWM) 0.70 Regenerative braking by PWMing the 3 bottom switches Target use - Stop and go city traffic Limited Range ~50-70 km/charge Average Indian urban vehicle speed < 25 Km/hr Battery round trip efficiency 0.90

copyright 2011 controltrix corpwww. controltrix.com Automobile standard Energy wasted in braking : Energy used in rolling = 3:2 Indian urban braking losses much more (ratio = 2:1) Aerodynamic losses negligible at low urban speeds Rapid accelerating phase (hi torque / hi current) Large i2R losses and low output power (low speed)  is even lower ~ 50 % e.g. Stop and go traffic conditions Ref:

copyright 2011 controltrix corpwww. controltrix.com 100 units from battery 80 units converted to kinetic (  26 lost in rolling 54 remaining in vehicle KE 54*0.7 = 37.8 returned back to battery For next cycle 37.8*0.9 = 34 reusable total usable energy over lifetime = * *0.34^2+….. = 100/(1-0.34) = 151 Figure of merit (FOM) = 151/100 = 1.51 (base line case) FOM directly co relates to range and time between charging Example

copyright 2011 controltrix corpwww. controltrix.com  = 85% => FOM = 1.77=> +18% range  = 90% => FOM = 1.95=> +29% range  = 95% => FOM = 2.19=> +45% range Summary : Small change in   large range change Imperative to explore ways to improve  Automobile standard..

copyright 2011 controltrix corpwww. controltrix.com Losses Most type of losses are related to current (Motor + inverter )Resistive losses Inverter Switching losses Motor magnetic losses To reduce losses reduce current !!

copyright 2011 controltrix corpwww. controltrix.com Other sources Non trapezoidal current shape Spikes, kinks on commutation instants Motor dynamics Increases RMS current More losses Commutation pattern and duty control addresses this problem

copyright 2011 controltrix corpwww. controltrix.com Other sources. Motor inductance Current lags voltage Derates motor higher speed Increase in current for given torque Increase in losses for given torque ~ 12% Proper dynamic phase advance removes this problem

copyright 2011 controltrix corpwww. controltrix.com Other sources. Asynchronous vs. synchronous switching Asynchronous Bottom diode conducts during off time Diode conduction losses are higher Synchronous Complementary mode PWM Bottom MOSFET conducts during off time Loss reduction ~ 10 W

copyright 2011 controltrix corpwww. controltrix.com Kink Rising gradual slopes Commutaion kickback current Commutation kickback current Commutation kink due to finite inductance in current waveform… leads to increase in RMS current and thus losses

copyright 2011 controltrix corpwww. controltrix.com Other sources.. Regeneration strategy 3 bottom switches are PWMed Large Diode conduction losses ~24W Non ideal current waveshape (with peaks) 2 leg switching Low losses ~ 10% efficiency gain Slightly more logic/math computation Proper implementation else noise, current spikes Not to be confused with phase reversal (causes enormous jerk, potentially destructive)

copyright 2011 controltrix corpwww. controltrix.com 100 units supplied by battery 80 converted to kinetic energy 20 lost due to current flow 10% reduction in current reduces losses(I 2 R) by 20 % Only 16 are now lost  becomes 84% Energy budget

copyright 2011 controltrix corpwww. controltrix.com Strategies and efficiency Motoring + Regenerationgain Synchronous switching+1% Torque mode or current mode control+2% Proper calculated phase advance+2% Reduce commutation kinks and spikes+1% Only Regeneration Proper 2 leg regenerative braking+10%

copyright 2011 controltrix corpwww. controltrix.com Energy budget Motoring + Regenerationgain Motoring efficiency86 Regenerating efficiency86 FOM1.79 Range gain on baseline+18.9%

copyright 2011 controltrix corpwww. controltrix.com Simulation 300 rpm Rapid rise and fall of current Motoring / Regeneration current wave shape

copyright 2011 controltrix corpwww. controltrix.com Flat current profile Small commutation spike Simulation 70rpm

copyright 2011 controltrix corpwww. controltrix.com Reliability issues Hall Sensor State change use change notification Interrupt (CNI) Improper Hall state determination leads to improper commutation Cause of possible accidents Controller failure/ reliability problems PWM switching causes noise causes spurious CNI  failure

copyright 2011 controltrix corpwww. controltrix.com Improving Reliability Do not use CNI poll Hall IO lines Polling triggered using ADC variable trigger Trigger away from PWM switching instants Improves reliability many fold Cycle by cycle current limiting

copyright 2011 controltrix corpwww. controltrix.com Thank You