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

2. 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 corp www. controltrix.com

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

4. Six Step BLDC Control
HALL A
60 o
Red Winding
HALL B
Q1 Q2 Q3
Green Winding R G B
HALL C
Q4 Q5 Q6
Blue Winding
Q3,Q5 Q1,Q5 Q1,Q6 Q2,Q6 Q2,Q4 Q3,Q4 Q3,Q5 Q1,Q5 Q1,Q6
+TORQUE FIRING
Sector 5 0 1 2 3 4 5 0 1
Hall States 5 4 6 2 3 1 5 4 6
copyright 2011 controltrix corp www. controltrix.com

5. Electric Vehicle Specific
• 250 W , 24 V, 12 A direct drive system
• 350 rpm @
• 80% drive +motor efficiency(baseline) @ 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 corp www. controltrix.com

6. 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: http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles
copyright 2011 controltrix corp www. controltrix.com

7. Example
• 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 =
100 + 34 + 34*0.34 + 34*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
copyright 2011 controltrix corp www. controltrix.com

8. Automobile standard..
• = 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
copyright 2011 controltrix corp www. controltrix.com

9. 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 corp www. controltrix.com

10. 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 corp www. controltrix.com

11. Other sources.
Motor inductance
• Current lags voltage
• Derates motor constant @ 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 corp www. controltrix.com

12. 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 corp www. controltrix.com

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

14. 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 corp www. controltrix.com

15. Energy budget
• 100 units supplied by battery
• 80 converted to kinetic energy
• 20 lost due to current flow
• 10% reduction in current reduces losses(I2R) by 20 %
• Only 16 are now lost
• becomes 84%
copyright 2011 controltrix corp www. controltrix.com

16. Strategies and efficiency
Motoring + Regeneration gain
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 corp www. controltrix.com

17. Energy budget
Motoring + Regeneration gain
Motoring efficiency 86
Regenerating efficiency 86
FOM 1.79
Range gain on baseline +18.9%
copyright 2011 controltrix corp www. controltrix.com

18. Simulation results @ 300 rpm
Motoring / Regeneration current wave Rapid rise and fall of
shape current
copyright 2011 controltrix corp www. controltrix.com

19. Simulation results @ 70rpm
Small commutation spike
Flat current profile
copyright 2011 controltrix corp www. controltrix.com

20. 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 corp www. controltrix.com

21. 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 corp www. controltrix.com

22. Thank You
consulting@controltrix.com
copyright 2011 controltrix corp www. controltrix.com