bldc ripple torque reduction via modified sinusoidal pwm · pdf file1 1 bldc ripple torque...
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www.fairchildsemi.com
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BLDC Ripple Torque Reduction via
Modified Sinusoidal PWM
Neil Wang
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Why BLDC Motors?
Brushless DC (BLDC) Motor Characteristics
• High Torque Density, Compact
• Very Low Maintenance
• Lower EMI than Brush DC
• Low Rotor Inertia
• Better Cooling Design - Can Easily be Sealed
• Safer in Explosive Environments (No Brushes)
• Costs More than PMDC – Magnets & Controls
• Cost Getting More Competitive - Copper $$$ Rising
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3
BLDC Torque-Speed Curve
Note high torque @ zero speed(very important for many applications!)
4
BLDC Motor Construction
Hall-Effect
Sensor (x 3)
A, B, C
Windings
PM Rotor
Stator
3
5
Typical BLDC Power Electronics
Controller
rs
eb
ea
ec
n-
-
-
++
+
Ls rsLs
Lsrs
a
b
c
BLDC motor
Hall Sensor
Smart Power Module
(SPM®)BLDC Motor Windings
MCU, DSC,
DSP,
or ASIC
Hall-Effect Sensors
6
BLDC Motor Back-EMF Types
Bδ
Bδ
rθ
rθ
• Skewed Stator Slots
• Distributed Stator Windings
• Sinusoidal Field Distribution
• Concentrated Windings
• Shorter End Windings
• Surface Mounted Magnets
Note: Can be viewed by rotating shaft externally with scope on motor leads!
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7
Torque Ripple – Go Away
Torque
(N*m)
Speed
(rpm)
PWMa
(V)
ea (V)
ia (A)
Time (50ms / div)
Time (50ms / div)
Time (50ms / div)
Time (50ms / div)
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Torque Ripple – Go Away, Please
Torque Ripple is Problematic
• Audible & Noisy
• Mechanical Resonance Issues
• Mechanical Fatigue
• Precision Speed Regulation Difficult
Torque Ripple Reduction Ideas
• Use Sinusoidal PWM (SPWM)
• Acquire Motor With More Poles
• Implement Modified SPWM
• Use Permanent Magnet Sync Motor + SPWM
• Tip: BLDC with sinusoidal winding pattern
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9
Trapezoidal / 6-Step
Modulation
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Trapezoidal Modulation System
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11
Trapezoidal Control Diagram
dcV
1−−nnθθ
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Trapezoidal Sim Waveforms 1
Torque
(N*m)
Speed
(rpm)
PWMa
(V)
ea (V)
ia (A)
Time (100ms / div)
Time (100ms / div)
Time (100ms / div)
Time (100ms / div)
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13
Trapezoidal Sim Waveforms 2
Torque
(N*m)
Speed
(rpm)
PWMa
(V)
ea (V)
ia (A)
Time (50ms / div)
Time (50ms / div)
Time (50ms / div)
Time (50ms / div)
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Sinusoidal PWM
(SPWM) Modulation
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15
SPWM Control Diagram
PIFilter
PWMGenerator
SinusoidalLookupTable
BLDC Motor
HallSensors
PWMGenerator
PWMGenerator
Phase A
Phase B
Phase C
TorqueRequest
PIFilter
+ -
+ -
Cmd A Error
Current Feedback
Cmd B
C = A + B
Sinusoidal Currents
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SPWM Sim Waveforms 1
Time (1s / div)
Ref, Rotor
Freq
(Hz)
Torque
(N*m)
ia (A)
ea (V)
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17
SPWM Sim Waveforms 2
Time (10ms / div)
Ref, Rotor
Freq
(Hz)
Torque
(N*m)
ia (A)
ea (V)
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Modified Sinusoidal PWM
(Modified SPWM) Modulation
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19
Normal PWM
SPWM vs. Modified SPWM
Low loss modified SPWM
−=
−=
−=
),,min(
),,min(
),,min(
'
'
'
cbacc
cbabb
cbaaa
vvvvv
vvvvv
vvvvv
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Modified SPWM Control Diagram
dcV
1−−nnθθ
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21
Modified SPWM Sim Waveforms 1
Torque
(N*m)
Speed
(rpm)
PWMa
(V)
ea (V)
ia (A)
Time (100ms / div)
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Modified SPWM Sim Waveforms 2
Time (50ms / div)
Torque
(N*m)
Speed
(rpm)
PWMa
(V)
ea (V)
ia (A)
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Modified SPWM Application
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Modified SPWM Application Photos
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25
VM
GNDVCC
VSPFG
C422uF/25V
C161uF/450V
VMVCC
C5103/630V
VCC1
P-GND2
Idc3
Z4
Y5
X6
W7
V8
U9
Td10
RES11
OS12
S-GND13
Xin14
Xout15
REV16
FG17
CW/CCW18
HW19
HV20
HU21
Ve22
LA23
Vref24
IC2 TB6551F
R9 1.2
1K
R10
1.2
W
V
U
VCB
C3 103
C2 103
C1 103
GND2
Vs1
Q3
HuEW632
GND2
Vs1
Q3
HvEW632
GND2
Vs1
Q3
HwEW632
C10 105
C9
102
R11
100R
FG
C6105
Vref
Vref
R14.7K
R3 1.2K
R8 1.2K
R13 1.2K
R15
10K
R1410K
Vref
C14105
CU1
CU2
CU3
123456
CON5 6PIN
REV
Q29013
R4 100RREV
R12
1KC7102
VSP
NetC3_1
VSP_IN
XTAL1
4.19M
COM1
VB(U)2
VCC(U)3
IN(UH)4
IN(UL)5
VS(U)6
VB(V)7
VCC(V)8
IN(VH)9
IN(VL)10
VS(V)11
VB(W)12
VCC(W)13
IN(WH)14
IN(WL)15
VS(W)16
P17
U18
NU19
NV20
V21
NW22
W23
IC1
FSB5045
R16 100RR17 100RR18
100R
R19 100RR20 100RR21
100R
C32105
C33105
C34105
C31105
C35105
C36105
D1
S1J
D2
S1J
D3
S1J
VCCVBU
VSU
VBV
VSV
VBW
VSW
VBUVBVVBW
VSUVSVVSW
UVW
C11
105
C8
104
VM
Z210V
R7 500RVCC VCB
CCW
VCC
R31 15R
R33 15R
R35 15R
LA
R2 1.8k
C12102
J010RVCC VCC
J02 0RVCC VCC
J03 0RVCC VCC
J04 0R
V_Shunt
V_Shunt V_Shunt
J05 0RV_Shunt V_Shunt
J06 0RV_Rev V_Rev
J070R
V_Rev
V_Fg
Q19013
Modified SPWM Schematic
SCH Design Tips• Bootstrap
• Shunt Resistor :
0.5V/(1.5Rated Current)
• Signal part:R*C = 1.8us
FSB50450
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Bottom Test-Point
Bottom Test-Point
Modified SPWM Layout
Advantages
• Compact the system (make the PCB board built-in design possible)
• High density integration
• Low thermal resistance
• High reliability
Smart Power Module
(SPM®)
PCB Design Tips
• Isolation
• Signal GND & Power GND
• Signal part
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Modified SPWM Waveforms 1
ia
(0.5A / div)
PWMa
(100V / div)
Time (400ms / div)
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Modified SPWM Waveforms 2
ia
(0.5A / div)
PWMa
(100V / div)
Time (4ms / div)
PWMb
(100V / div)
PWMc
(100V / div)
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Modified SPWM Summary
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Where From Here
Modified SPWM Considerations
• BLDC Less Expensive than PMSM
• Lower Switching Losses
• Decreased Ripple Torque, 50%
• When compared to SPWM on BLDC
• Maybe Not Ideal Where Application
• Requires speed precision
• Has very dynamic loads
• Runs at high pole frequency
See the Fairchild White Paper
• “BLDC Ripple Torque Reduction via Modified Sinusoidal PWM”