Bldc motor drive system
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Transcript of Bldc motor drive system
copyright 2011 controltrix corp www. controltrix.com
Improving efficiency a perspective on electric vehicles
BLDC motor drive system
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
copyright 2011 controltrix corp www. controltrix.com
• High Frequency Carrier• Duty Cycle Varied Over Time to Generate a Lower Frequency
Signal
+V
PWM1H
PWM1L
3 PhaseBLDCPWM2H
PWM2L
PWM3H
PWM3L
PWM with Inverter
copyright 2011 controltrix corp www. controltrix.com
Six Step BLDC Control
+TORQUE FIRING
BR G
Q1 Q3Q2
Q4 Q6Q5Green Winding
Q1,Q5 Q1,Q6 Q2,Q6 Q2,Q4 Q3,Q4 Q3,Q5
60o
HALL A
HALL B
HALL C
Q1,Q5 Q1,Q6Q3,Q5
Sector 5
Hall States
0 1 2
5 4 6 2
3
3
4
1
5
5
0
4
1
6
Blue Winding
Red Winding
copyright 2011 controltrix corp www. controltrix.com
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
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)• h 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
• 100 units from battery• 80 units converted to kinetic ( = 80%)h• 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
Example
copyright 2011 controltrix corp www. controltrix.com
• h= 85% => FOM = 1.77=> +18% range• h= 90% => FOM = 1.95=> +29% range• h= 95% => FOM = 2.19=> +45% range
Summary : • Small change in h large range change• Imperative to explore ways to improve h
Automobile standard..
copyright 2011 controltrix corp www. 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 corp www. 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 corp www. controltrix.com
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
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.comKinkRising 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 corp www. 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 corp www. controltrix.com
• 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• h becomes 84%
Energy budget
copyright 2011 controltrix corp www. controltrix.com
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
Energy budgetMotoring + Regeneration gain
Motoring efficiency 86
Regenerating efficiency 86
FOM 1.79
Range gain on baseline +18.9%
copyright 2011 controltrix corp www. controltrix.com
Simulation results @ 300 rpmRapid rise and fall of current
Motoring / Regeneration current wave shape
copyright 2011 controltrix corp www. controltrix.com
Flat current profileSmall commutation spike
Simulation results @ 70rpm
copyright 2011 controltrix corp www. 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 corp www. 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 corp www. controltrix.com
Thank [email protected]