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(1)

Regenerative Braking of BLDC

Motors

By

Daniel Torres, Applications Engineer Patrick Heath, Marketing Manager

High-Performance Microcontroller Division Microchip Technology Inc.

(2)

Different electrical braking

scheme

Kinetic Energy Kinetic Energy Dynamic Braking OR Regenerative Braking

(3)

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

(4)

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 m A B C Filter Diode 5 Diode 4 Diode 3 Diode 2 Diode 1 Diode

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.

(5)

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

(6)

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 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 m A 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 rectifier

(7)

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.

(8)

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 V E I Torque Speed Forward Motoring E > V V E I Forward Braking Reverse Braking |V| > |E| V E I Reverse Motoring |E| > |V| V E 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

(10)

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

(11)

Simple Boost Converter

(using Simulink)

Boost _Converter powergui Continuous V_INPUT v + -V_DC _BUS v + -Series RLC Branch Scope Pulse Generator Mosfet2 g D S Load I_DC _BUS Filter Diode DC Voltage Source

The output voltage is proportional to the duty cycle of the MOSFET.

(12)

Simple Boost Converter

Simulation Results

Boost voltage = 30 volts DC Input voltage = 12 volts DC

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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 RPM 3 0 RPM2 0 RPM 1 0 RPM 2000 Pulse Generator Mosfet5 g D S Mosfet4 g D S Mosfet3 g D S Mosfet2 g D S Mosfet1 g D S Mosfet g D S I_DC _BUS I_C i + -I_B i + -I_A i + -HURST MOTOR w m A B C Gain 1/9.55 Filter

By varying the duty cycle, the output voltage can be

(14)

Boost Converter 3-phase MOSFET

Bridge Simulation Results

DC Output Voltage ~26 volts @ 2000RPM and 50% duty cycle

(15)

Boost Converter 3-phase MOSFET

Bridge Simulation Results

DC Output Voltage ~38 volts @ 2000RPM and 70% duty cycle

(16)
(17)

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 (%) V o lt a g e ( 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

(18)

Test Versus Simulation Results

at 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 (%) C u rr e n t (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 (%) C u rr e n t (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).

(19)

Efficiency Simulation Results

Efficiency vs Duty (Simulation)

0 10 20 30 40 50 60 0 20 40 60 80 100 Duty (%) E ff ic ie n c y ( % ) 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

(20)

PID Control for Constant

Brake Force

Current command Error Current Feedback

PID Duty Cycle over voltage protectionDuty Cycle limit for MOSFET 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

W h e e l S p e e d

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?

Note: The Microchip name and logo are registered trademarks of Microchip Technology Inc. in the U.S.A. and other countries. All other trademarks mentioned herein are property of their respective companies.

References

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