This document discusses regenerative braking of BLDC motors. It describes how kinetic energy from a motor during braking can be stored in a battery through regenerative braking. It then provides simulations and test results of using a 3-phase MOSFET bridge rectifier with a boost converter to regeneratively brake a BLDC hub motor on an e-bike. The maximum efficiency was 55% at 50% duty cycle, but maximum braking force occurred at 70% duty cycle. A PID controller is proposed to maintain a constant braking force at varying motor speeds.

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

2. 2
Different electrical braking
scheme
Kinetic Energy
Kinetic Energy
Dynamic Braking
OR
Regenerative Braking

3. 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
increasing the life of friction pads on brake shoe. However, to bring
the bike to a complete stop, the mechanical brakes are required.

4. 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
Diode5Diode4Diode3
Diode2Diode1Diode
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. 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. 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
All MOSFETs are turned OFF

7. 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. 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

9. 9
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.

10. 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
speeds, the motor can work as brake.

11. 11
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.

12. 12
Simple Boost Converter
Simulation Results
Boost voltage = 30 volts DC
Input voltage = 12 volts DC
Boost current = 2.5 Amps

13. 13
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
+-
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
boosted to different
magnitude.

14. 14
Boost Converter 3-phase MOSFET
Bridge Simulation Results
DC Output Voltage ~26
volts @ 2000RPM and
50% duty cycle

15. 15
Boost Converter 3-phase MOSFET
Bridge Simulation Results
DC Output Voltage ~38
volts @ 2000RPM and
70% duty cycle

16. 16
Picture of Test Setup

17. 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 (%)
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.

18. 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 (%)
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).

19. 19
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.

20. 20
PID Control for Constant
Brake Force
Current
command
Error
Current Feedback
PID
Duty Cycle Duty Cycle limit for
over voltage protection
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
WheelSpeed
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.

21. 21
Thank You
Questions?
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