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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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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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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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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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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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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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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”