BLDC Ripple Torque Reduction via
Modified Sinusoidal PWM
Neil Wang
www.fairchildsemi.com
1
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
2
1
BLDC Torque-Speed Curve
Note high torque @ zero speed
(very important for many applications!)
3
BLDC Motor Construction
Hall-Effect
Sensor (x 3)
PM Rotor
Stator
A, B, C
Windings
4
2
Typical BLDC Power Electronics
Smart Power Module
(SPM®)
BLDC Motor
Windings
BLDC motor
rs
+
Ls
eb
-
b
n
c
Ls
rs
-
a
- ea +
Ls
rs
+
ec
MCU, DSC,
Controller
DSP,
or ASIC
Hall-Effect
Sensors
Hall
Sensor
5
BLDC Motor Back-EMF Types
Bδ
θr
• Concentrated Windings
• Shorter End Windings
• Surface Mounted Magnets
Bδ
θr
• Skewed Stator Slots
• Distributed Stator Windings
• Sinusoidal Field Distribution
Note: Can be viewed by rotating shaft externally with scope on motor leads!
6
3
Torque Ripple – Go Away
Speed
(rpm)
Time (50ms / div)
Torque
(N*m)
Time (50ms / div)
PWMa
(V)
Time (50ms / div)
ea (V)
ia (A)
Time (50ms / div)
7
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
8
4
Trapezoidal / 6-Step
Modulation
9
Trapezoidal Modulation System
10
5
Trapezoidal Control Diagram
Vdc
θ n − θ n −1
11
Trapezoidal Sim Waveforms 1
Speed
(rpm)
Time (100ms / div)
Torque
(N*m)
Time (100ms / div)
PWMa
(V)
Time (100ms / div)
ea (V)
ia (A)
Time (100ms / div)
12
6
Trapezoidal Sim Waveforms 2
Speed
(rpm)
Time (50ms / div)
Torque
(N*m)
Time (50ms / div)
PWMa
(V)
Time (50ms / div)
ea (V)
ia (A)
Time (50ms / div)
13
Sinusoidal PWM
(SPWM) Modulation
14
7
SPWM Control Diagram
Cmd A
Sinusoidal
Lookup
Table
Error
+
-
PI
Filter
PWM
Generator
Phase A
Current Feedback
Cmd B
+
-
PI
Filter
PWM
Generator
BLDC Motor
Phase B
Hall
Sensors
PWM
Generator
Phase C
C=A+B
Torque
Request
Sinusoidal Currents
15
SPWM Sim Waveforms 1
Torque
(N*m)
Ref, Rotor
Freq
(Hz)
ea (V)
ia (A)
Time (1s / div)
16
8
SPWM Sim Waveforms 2
Torque
(N*m)
Ref, Rotor
Freq
(Hz)
ea (V)
ia (A)
Time (10ms / div)
17
Modified Sinusoidal PWM
(Modified SPWM) Modulation
18
9
SPWM vs. Modified SPWM
Low loss modified SPWM
Normal PWM
va' = va − min(va , vb , vc )
'
vb = vb − min(va , vb , vc )
v' = v − min(v , v , v )
a b c
c c
19
Modified SPWM Control Diagram
Vdc
θ n − θ n −1
20
10
Modified SPWM Sim Waveforms 1
Speed
(rpm)
Torque
(N*m)
PWMa
(V)
ea (V)
ia (A)
Time (100ms / div)
21
Modified SPWM Sim Waveforms 2
Speed
(rpm)
Torque
(N*m)
PWMa
(V)
ea (V)
ia (A)
Time (50ms / div)
22
11
Modified SPWM Application
23
Modified SPWM Application Photos
24
12
Modified SPWM Schematic
FSB50450
IC2
R12
VSP
VSP_IN22
1K
Hu EW632
1
Vs
Q
GND
2
3
R3 1.2K
20
19
Hv EW632
1
C14105 C10 105
Vs
Q
2
GND
3
R8 1.2K
24
11
R14
10K
2
Q
GND
3
R13 1.2K
Xin
HV
U
V
W
X
Y
Z
HW
Vref
RES
CW/CCW
LA
R15
10K
HwEW632
Vs
HU
Vref
C2 103
1
TB6551F
1
VCC
15
Xout
23
LA
OS
Td
17
FG
16
REV
NetC3_1
VCB
IC1
4.19M
2
C3 103
Idc
P-GND
S-GND
14
XTAL1
C8
104
9
8
7
6
5
4
18
R16
R17
R18
R19
R20
R21
100R
100R
100R
100R
100R
100R
VM
3
VCC(U)
8
VCC(V)
13
VCC(W)
VCC
21
C1 103
Vref
Ve
C7
102
C11
105 1
4
9
14
5
10
15
COM
IN(UH)
IN(VH)
IN(WH)
IN(UL)
IN(VL)
IN(WL)
P
U
V
W
17
18
U CU1
21
V CU2
23
WCU3
CCW
VBU 2
VB(U)
VBV 7
VB(V)
VBW 12
VB(W)
12
10
R2 1.8k
3
U
V
W
C12
102
13
VSU 6
VS(U)
VSV 11
VS(V)
VSW 16
VS(W)
R9 1.2
19
NU
20
NV
22
NW
1K
R10
1.2
FSB5045
V_Shunt
V_Fg
VCC
VM
VCC
Vref
C4
22uF/25V
C16
1uF/450V
C5
103/630V
FG
D1
R11
R1
4.7K
100R
VCC R7
C31
105
S1J
REV
VM
VCC
GND
VSP
FG
REV
Q2
9013
D2
R33 15R
C32
105 VSU
D3
S1J
VBV
C33
105
S1J
VCC
J02 0R
VCC
J03 0R
VCC
J04 0R
V_Shunt
V_Shunt
V_Rev
C9
102
J01 0R
VCC
VCC
VBU
VCC
R4 100R
Q1
9013
CON5 6PIN
6
500R VCB
5
4
Z2
C6
3
10V
105
2
1
R31 15R
C34
105 VSV
V_Shunt
V_Rev
J05 0R
J06 0R
J070R
R35 15R
VBW
C35
105
V_Shunt
V_Rev
SCH Design Tips
•
•
C36
105VSW
•
Bootstrap
Shunt Resistor :
0.5V/(1.5Rated
Current)
Signal part:R*C = 1.8us
25
Modified SPWM Layout
Smart Power Module
(SPM®)
Advantages
•
•
•
•
Compact the system (make the PCB
board built-in design possible)
High density integration
Low thermal resistance
High reliability
PCB Design Tips
•
•
•
Bottom Test-Point
Isolation
Signal GND & Power GND
Signal part
Bottom Test-Point
26
13
Modified SPWM Waveforms 1
PWMa
(100V / div)
ia
(0.5A / div)
Time (400ms / div)
27
Modified SPWM Waveforms 2
PWMa
(100V / div)
PWMb
(100V / div)
PWMc
(100V / div)
ia
(0.5A / div)
Time (4ms / div)
28
14
Modified SPWM Summary
29
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”
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