Bldc motor drive system

BLDC motor drive system
Improving efficiency a perspective on electric vehicles

www.controltrix.com

copyright 2011 controltrix corp www. 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


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


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


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


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


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


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


Losses
• Most type of losses are related to current
• (Motor + inverter )Resistive losses
• Inverter Switching losses
• Motor magnetic losses
• To reduce losses reduce current !!


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


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


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


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

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)


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%


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%


Energy budget
Motoring + Regeneration gain

Motoring efficiency 86

Regenerating efficiency 86

FOM 1.79
Range gain on baseline +18.9%


Simulation results @ 300 rpm
Motoring / Regeneration current wave Rapid rise and fall of
shape current


Simulation results @ 70rpm
Small commutation spike
Flat current profile


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


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


Thank You
consulting@controltrix.com


Bldc motor drive system

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