Regenerative Braking of BLDC Motors - Microchip Technology Inc

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Regenerative Braking of BLDC Motors By Daniel Torres, Applications Engineer Patrick Heath, Marketing Manager High-Performance Microcontroller Division Microchip Technology Inc.

Transcript of Regenerative Braking of BLDC Motors - Microchip Technology Inc

Page 1: Regenerative Braking of BLDC Motors - Microchip Technology Inc

Regenerative Braking of BLDC Motors

By

Daniel Torres, Applications Engineer

Patrick Heath, Marketing Manager

High-Performance Microcontroller Division

Microchip Technology Inc.

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Different electrical braking scheme

Kinetic Energy

Kinetic Energy

Dynamic BrakingOR

Regenerative Braking

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Regenerative Brake in BLDC motor

Kinetic Energy

BLDC Hub Motor used

in e-bike application

While braking, energy

is stored in the battery

Regenerative braking stores energy back into the battery, while

increasing the life of friction pads on brake shoe. However, to bring

the bike to a complete stop, the mechanical brakes are required.

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Configuration of a 3-Phase Rectifier using Simulink

powergui

Continuous

V_DC _BUS

v+-

V_CA

v+-

V_BC

v+-

V_AB

v+-

Scope

RPM to rad /sec

1/9.55

RPM

1563

I_DC _BUS

i+-

I_C

i+-

I_B

i+-

I_A

i+-

HURST MOTOR

w

mA

B

CFilter

Diode5Diode 4Diode 3

Diode 2Diode 1Diode

The Hurst BLDC motor running at

1563 RPM, generates the EMF,

which is rectified by a 3 phase

diode configuration and filtered to

DC to charge the battery.

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3-Phase Rectifier Simulation Results

Filtered DC Bus Voltage

Voltage constant of this motor = 7.24 Vpeak/ Krpm

For a speed of 1563 RPM, the voltage = 11.3 Vpeak

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MOSFET Bridge as a 3-Phase Rectifier (using Simulink)

powergui

Continuous

V_DC _BUS

v+-

V_CA

v+-

V_BC

v+-

V_AB

v+-

Scope

RPM to rad /sec

1/9.55

RPM

1563

Mosfet5

g D

S

Mosfet4

g D

S

Mosfet3

g D

S

Mosfet2

g D

SMosfet1

g D

S

Mosfet

g DS

I_DC _BUS

i+-

I_C

i+-

I_B

i+-

I_A

i+-

HURST MOTOR

w

mA

B

C

Filter

Constant 5

0

Constant 4

0

Constant 3

0

Constant 2

0

Constant 1

0

Constant

0

Body Diode of

MOSFET acts

as rectifierAll MOSFETs are turned OFF

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3-Phase MOSFET Bridge Rectifier Simulation Results

Filtered DC Bus Voltage

The DC bus voltage results for the 3-phase MOSFET bridge are

similar to the rectified 3-phase diode circuit.

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4-Quadrant Motor Operation

For a BLDC motor to operate in 2nd quadrant, the value of the back EMF

generated by the BLDC motor should be greater than the battery voltage (DC bus

voltage). This ensures that the direction of the current reverses, while the motor

still runs in the forward direction.

V > E

VE

I

Torque

Speed

Forward

Motoring

E > V

VE

IForward

Braking

Reverse

Braking

|V| > |E|

VE

I

Reverse

Motoring|E| > |V|

VE

I

E

2 1

3 4

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Energy Flow

Generating (Braking)

(Current from Motor to Battery)

Motoring

(Current from Battery to Motor)

For current to flow into battery, the bus voltage should be higher

than the battery terminal voltage. Hence we have to boost the

voltage developed from motor higher than the battery.

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Limitation of a Direct Connection

Since this motor is rated for 24-Volts, the battery terminal

voltage would be 24-Volts. To generate 24-Volts from the

motor (or higher voltage), the motor should run at a speed of

3,400 RPM or higher. Hence we have to figure out ways to boost

the back EMF generated by the motor so that even at lower

speeds, the motor can work as brake.

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Simple Boost Converter(using Simulink)

Boost _Converterpowergui

Continuous

V_INPUT

v +-

V_DC _BUS

v+-

Series RLC Branch

Scope

Pulse

Generator

Mosfet2

g D

S

Load

I_DC _BUS

i+-

Filter

Diode

DC Voltage Source

The output voltage is

proportional to the duty

cycle of the MOSFET.

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Simple Boost Converter Simulation Results

Boost voltage = 30 volts DC

Input voltage = 12 volts DC

Boost current = 2.5 Amps

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Boost Converter Based on a 3-phase MOSFET Bridge

BRAKE_MODEL_3

powergui

Continuous

V_DC _BUS

v +-

V_CA

v+-

V_BC

v+-

V_AB

v+-

Scope 2

Scope 1

Scope

RPM3

0

RPM2

0

RPM1

0

RPM

2000

Pulse

Generator

Mosfet5

g D

S

Mosfet4g D

SMosfet3

g D

S

Mosfet2

g D

S

Mosfet1

g D

S

Mosfet

g D

S

I_DC _BUS

i+-

I_C

i+-

I_B

i+-

I_A

i+-

HURST MOTOR

w

mA

B

C

Gain

1/9.55

Filter

By varying the duty

cycle, the output

voltage can be

boosted to different

magnitude.

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Boost Converter 3-phase MOSFET Bridge Simulation Results

DC Output Voltage ~26

volts @ 2000RPM and

50% duty cycle

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Boost Converter 3-phase MOSFET Bridge Simulation Results

DC Output Voltage ~38

volts @ 2000RPM and

70% duty cycle

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Picture of Test Setup

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Test Result of Boosted Voltage while running Motor at 2500 RPM

No Load voltage vs Duty

(Tested on Hurst Motor)

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80

Duty (%)

Voltage (V)

2000 RPM

2500 RPM

3000 RPM

This slide shows the output voltage of the motor after boost, for different speed and

duty cycles. The magnitude of the voltage will increase proportional to the shaft

speed. Another point evident from the plots is that the output voltage gets boosted

proportionally to the duty cycle.

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Test Versus Simulation Resultsat 2000 RPM

Current vs Duty (Simulation)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

Duty (%)

Current (A)

Current (A)

Current vs Duty (Tested on Hurst Motor)

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 20 40 60 80 100

Duty (%)

Current (A)

Series1

• For low value duty cycles, the

boosted voltage is low. Hence no

current flows into the battery.

• At around 30% duty cycle, the

voltage begins to boost and the

current flows into the battery.

This is the point where

regenerative braking starts.

• The peak current from simulation

is around 0.5Amps @ 70% duty

cycle. This translates to 24V * 0.5

Amps = 12 Watts. Since brake

force is proportional to the current,

this is the point of maximum brake

force.

• Beyond that point, the current

starts to fall, mainly because of the

motor construction (resistance and

inductance drops).

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Efficiency Simulation Results

Efficiency vs Duty (Simulation)

0

10

20

30

40

50

60

0 20 40 60 80 100

Duty (%)

Efficiency (%)

Efficiency (%)

This slide shows the efficiency curve of the brake setup, which gives a maximum efficiency of 55% at 50% duty cycle.

Since this is a simulated result, the actual figure of efficiency might be lower.

From the plot, it can be seen that the maximum efficiency point and maximum brake force points do not coincide:

• Max brake force @ 70% duty cycle

• Max efficiency @ 50% duty cycle

Hence, the braking algorithm can be designed to operate at either maximum efficiency or at maximum brake force

points.

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PID Control for Constant Brake Force

Current

command

Error

Current Feedback

PIDDuty Cycle Duty Cycle limit for

over voltage protectionMOSFET

BLDC Motor

Analog voltage

proportional to the

required brake force

1 1.5 2 2.5 3 3.5 4 4.5 5

0 0 0 0 0 0 0 0 0 0

5 0 0 0 0 0 0 0 0 0

10 0 0 0 0 0 0 0 0 0

20 0 0 0 0 0 0 0 0 0

40 0 0 0 0 0 0 0 0 0

60 0 0 0 0 0 0 0 0 0

80 0.1 0.1 0.12 0.12 0.15 0.15 0.15 0.18 0.18

120 0.2 0.2 0.22 0.22 0.25 0.25 0.25 0.28 0.28

160 0.3 0.3 0.32 0.32 0.35 0.35 0.35 0.38 0.38

200 0.4 0.4 0.42 0.42 0.45 0.45 0.45 0.48 0.48

240 0.5 0.5 0.52 0.52 0.65 0.55 0.55 0.58 0.58

280 0.6 0.6 0.62 0.62 0.75 0.85 0.85 0.88 0.88

320 0.7 0.7 0.72 0.72 0.85 1.05 1.25 1.48 1.68

380 0.8 0.8 0.82 0.82 0.95 1.25 1.55 1.78 2.08

440 0.9 0.9 0.92 0.92 1.05 1.55 2.15 2.28 2.88

500 1 1 1.02 1.02 1.15 1.95 2.65 2.78 2.98

Required Brake force

Wheel Speed

The PID loop will try to maintain a constant brake

force at different motor speeds. Hence the user will

get a linear response of brake force.

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Thank You

Questions?

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