Download (a) bipolar drive

Investigation on the Bipolar-Starting
and Unipolar-Running Method to Drive a
Brushless DC Motor at High Speed with
Large Starting Torque
PREM, Department of Mechanical Engineering
Hanyang University, Korea
Myung-Gyu Kim
Contents
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Motivation
Objective & Methodology
Driving methods of BLDC motor
Torque-speed-current relationship of BLDC motor
Torque nonlinearity of BLDC motor
Bipolar-starting and unipolar-running method
New Inverter Topology
System Implementation
Experiment
Conclusion
Motivation
• The trend of brushless DC (BLDC) motor
– High efficiency and good controllability over a wide range of
speed
 High speed applications of electromechanical systems.
• The characteristics of BLDC motor
– Small starting torque and long transient period in order to
run the motor at high speed
 One of the drawbacks of a BLDC motor in high speed
applications.
The research of high-speed BLDC motor focus on
the driving method that considers the starting torque.
Objective & Methodology
• Investigate (a) the method to drive a BLDC motor at high speed
with large starting torque.
(b) the new inverter topology.
(b) the effectiveness of the new inverter topology and
the bipolar-starting and unipolar-running method
experimentally.
• Investigation procedure
The new inverter topology
The winding pattern and
the driving method of
BLDC motor
The DSP-based
BLDC motor controller
The torque-speed-current
relationship of BLDC motor
The effectiveness of
the bipolar-starting and
unipolar-running method
Driving Methods of BLDC motor (1)
+ VS
A+
B+
+VS
A+
C+
B+
C+
A
A
B
B
A-
B-
C-
A-
C
B-
C-
-VS
(a)
(b)
Fig 1. Inverter circuits (a) bipolar drive (b) unipolar drive
Table 1. Commutation sequence of bipolar and unipolar drive
• Bipolar drive  Not using the neutral point.
• Unipolar drive  Using the neutral point.
C
Driving Methods of BLDC motor (2)
Fig 2. Torque curves (a) bipolar drive (b) unipolar drive
-The phase difference of 30 electrical degrees between
the commutation sequences of bipolar and unipolar drive.
Bipolar-starting and unipolar-running
method (1)
Torque-current relationship : T  KT I a
(1)
The equation of the voltage : V  I a Ra  K E
(2)
In SI unit
KT  KE  K
(3)
From eqn (1), (2), (3)
K2
K
Torque-speed relationship : T  
 V
Ra
Ra
(4)
KT :torque constant
K E :back emf constant
Ra :resistance
I a :current of energized phase
 :speed
V :voltage
-The slope of eqn (4) is independent of the terminal voltage and speed.
-The torque decreases linearly as the speed increase.
Bipolar-starting and unipolar-running
method (2)
• Difference between bipolar and unipolar drive
Design
variables
Bipolar drive
Torque constant
KT
Resistant
Ra
Torque-speed
Relationship
2
Unipolar drive
1
KT
2
1
Ra
2
2
K
K
K
K
T   T + T V T   T + T V
Ra
Ra
2 Ra
Ra
K
K
TS  T V
TS  T V
Starting torque
Ra
Ra
V
2V
0 
0 
No-load speed
KT
KT
Table 2. Major design variables of a BLDC motor
driven by bipolar and unipolar drive
Bipolar-starting and unipolar-running
method (3)
• Occur the torque nonlinearity of BLDC motor in practice
Magnetic effect of stator current
Magnetic saturation due to large input current
The reduction of torque constant due to large input current
- At same terminal voltage
Input current
Reduction of
torque constant
Starting torque
Bipolar drive < Unipolar drive
Bipolar drive < Unipolar drive
Bipolar drive > Unipolar drive
Bipolar-starting and unipolar-running method
- Starting torque of bipolar drive
- Maximum speed of unipolar drive
- Suitable for driving method of high-speed BLDC motor
New Inverter Topology (1)
• The basic inverter topology
+VS
A+
B+
C+
A
SW1
B
-VS
ADC link
B-
C-
C
SW2
Fig 3. Basic Inverter circuit for bipolar-starting and unipolar-running drive
- Proposed by PREM
- For switching from bipolar to unipolar drive,
switch 1 : open  ground, switch 2 : ground  -12V
- Problem  Need the additional input power
New Inverter Topology (2)
• The theoretical inverter topology
+VS
A+
B+
C+
N+
A
B
A-
B-
C-
C
N-
Fig 4. Theoretical Inverter circuit for bipolar-starting and unipolar-running drive
Table 3. State of the theoretical inverter circuit
- Mentioned by SGS-Thomson Microelectronics, Western Digital
New Inverter Topology (3)
• The problem of the theoretical inverter topology
Generated voltages due to the interaction of a rotating flux and a stationary coil
This back emf would drive current around the freewheel diode path.
The current would build up in an uncontrolled fashion.
The current would contribute to losses and would produce negative torque.
B-
A+
C-
Back-emf
A
①
0°
②
60°
B+
A-
C+
B
C
③
④
120°
180°
240°
Electrical angle, deg.
300°
Fig 5. Ideal back-emf waveform
360°
New Inverter Topology (4)
• The new inverter topology
+VS
AS+
BS+
A+
B+
C+
CS+
Additional
sub-TR
N+
A
Inverter-TR
Freewheeling
diode
A-
B
C
BAS-
CBS-
N-
CS-
Fig 6. New Inverter circuit for bipolar-starting and unipolar-running drive
- Proposed by PREM
- Energized current path is similar with that of the theoretical inverter topology
Use the additional sub-TR to control the current driven
around the freewheeling diode path by back-EMF
New Inverter Topology (5)
• The state of new inverter circuit and freewheeling current
Table 4. State of the new inverter circuit
+VS
A+
AS+
B+
BS+
C+
CS+
N+
A
B
A-
BAS-
CBS-
C
CS-
N-
Fig 7. Freewheeling current of unipolar drive
Commutation mode A+  B-
· A+ diode : not used.
· A- : Freewheeling diode.
· Closed loop 형성.
System Implementation (1)
Inverter Circuit
+VS
A+
AS+
B+
BS+
C+
CS+
BLDC motor
N+
A
B
A-
BAS-
CBS-
C
CS-
N-
Speed feedback /
Switching signal
Digital I/O
DSP /
Drive Circuit
PC
Fig 8. Developed DSP-based BLDC motor controller
-Drive circuit  Control the inverter-TR and additional sub-TR
-DSP  All operating for driving the motor.
-Motor Controller  Run the motor with bipolar or unipolar driving method
and switch from one method to another at any speed
System Implementation (2)
Part I :
Switch signal
for inverter
Freewheeling
Diode
Part II :
Switch for
neutral point
Inverter
Transistor
Additional
sub Transistor
Part III :
Switch for
sub-TR
(a)
(b)
Fig 9. (a) New Inverter circuit (b) Driver circuit
System Implementation (3)
+VS
Freewheeling
diode
sub-TR (P-channel)
Gate
signal
Neutral point
Phase terminal voltage
(off signal)
-Variation of phase terminal voltage
: -20 ~ 30V
2k
Photocoupler
Protect
reverse voltage on signal
DSP Input
signal
-Off signal : use the phase terminal voltage
directly.
-On signal : 12V, 0V
0V
(a)
Neutral point
Freewheeling
diode
-Use the photocoupler
for the behavior of additional sub-TR
Phase terminal voltage
(off signal)
Gate
signal
sub-TR
(P-channel)
-Rising time
I) Inverter Transistor : ns
II) Photocoupler : µs
Protect
reverse voltage
DSP Input
signal
on signal
+VS
(b)
Fig 10. Switch for the additional sub-TR
(a) upper part (b) lower part
Experiment (1)
Motor Analyzer
Emulator
PC
Motor
Switching
DSP
Current probe
Torque meter
Oscilloscope
Fig 11. Experimental setup to measure toque-speed-current characteristics
-BLDC motor spec.
• used in hard disk drive.
• Y-winding, 8 poles, 12slots
• The rated operating speed of 5400rpm
-Torque meter
• Load torque : 0.5mN·m
40
0.8
30
30
0.6
20
20
0.4
10
10
0.2
0
-10
-20
-40
0
1
2
3
Time (sec)
4
-10
-0.2
-40
0
-3
Fig 12. Terminal and neutral voltage
of bipolar drive
-Use the new inverter circuit for bipolar drive
-Traditional waveform of bipolar drive
-Maximum speed : 6900rpm
-0.4
current of phase A
terminal voltage of phase A
-30
5
x 10
0
-20
terminal voltege of phase A
voltage of neutral point
-30
0
1
2
3
Time (sec)
-0.6
-0.8
5
4
x 10
-3
Fig 13. Terminal voltage and current
of bipolar drive
Current (A)
40
Voltage (V)
Voltage (V)
Experiment (2)
Experiment (3)
30
Back-emf
2
1.5
Current by back-emf
20
20
1
10
10
0.5
Voltage (V)
Voltage (V)
30
40
0
-10
-20
terminal voltage of phase A
voltage of neutral point
-30
-40
0
1
2
3
Time (sec)
0
0
-10
-0.5
-20
-1
current of phase A
terminal voltage of phase A
-30
4
5
x 10
-40
0
1
-3
Fig 14. Terminal and neutral voltage
of unipolar drive
2
3
Time (sec)
-1.5
-2
5
4
x 10
-3
Fig 15. Terminal voltage and current
of unipolar drive
-Use the theoretical inverter circuit for unipolar drive.
-The current would produce negative torque.
-Maximum speed : 7000rpm
Current (A)
40
Experiment (4)
Back-emf
0.8
Current ripple
30
20
20
10
10
Voltage (V)
Voltage (V)
30
40
0
-10
0.6
No current
0.4
B
0.2
0
0
-10
-0.2
A
-20
-30
-40
0
-20
terminal voltage of phase A
voltage of neutral point
1
2
3
Time (sec)
-30
4
5
x 10
-40
0
1
-3
Fig 16. Terminal and neutral voltage
of unipolar drive
-0.4
current of phase A
terminal voltage of phase A
2
3
Time (sec)
-0.6
-0.8
5
4
x 10
-3
Fig 17. Terminal voltage and current
of unipolar drive
-Use the new inverter circuit for unipolar drive.
-No current which would be produce negative torque.
-Maximum speed : 11500rpm
Current (A)
40
40
0.8
30
30
0.6
20
20
0.4
10
10
0.2
0
-10
-20
-40
4
4.4
4.8
5.2
Time (sec)
0
-10
-0.2
-20
terminal voltage of phase A
gate signal of sub-TR As+
-30
0
-30
5.6
6
x 10
-4
Fig 18. Terminal voltage and gate signal
of unipolar drive
-40
4
-0.4
current of phase A
terminal voltage of phase A
4.4
4.8
5.2
Time (sec)
-0.6
-0.8
6
5.6
x 10
-4
Fig 19. Terminal voltage and current
of unipolar drive
-Current ripple A.
-Difference of rising time between inverter transistor and photocoupler.
-Need for the freewheeling current.
Current (A)
40
Voltage (V)
Voltage (V)
Experiment (5)
40
0.8
30
30
0.6
20
20
0.4
10
10
0.2
0
-10
-20
-40
6
6.4
6.8
7.2
Time (sec)
0
-10
-0.2
-20
terminal voltage of phase A
gate signal of sub-TR As+
-30
0
-30
7.6
8
x 10
-4
Fig 20. Terminal voltage and gate signal
of unipolar drive
-40
6
-0.4
current of phase A
terminal voltage of phase A
6.4
6.8
7.2
Time (sec)
-0.6
-0.8
8
7.6
x 10
-4
Fig 21. Terminal voltage and current
of unipolar drive
-Current ripple B
-Difference of rising time between inverter transistor and photocoupler
-Also Appeared by the basic inverter circuit.
Current (A)
40
Voltage (V)
Voltage (V)
Experiment (6)
Experiment (7)
-Nonlinear torque-speed relation.
18
16
Bipolar drive
Unipolar drive
Torque (mNm)
14
-Bipolar drive generates a large
starting torque.
12
-Unipolar drive runs the motor
higher than the speed of a bipolar drive.
10
8
-Switch the driving method at 1500rpm.
6
4
2
0
0
-Starting torque : 12.98  14.98mN·m
 15%
Maximum speed : 6900  11500rpm
67%
2000 4000 6000 8000 10000 12000 14000
Speed (rpm)
Fig 22. Torque-speed curve of bipolar and unipolar drive
Experiment (8)
2000
Speed (rpm)
1500
-Bipolar drive speed up a little more
rapidly than unipolar drive
1000
500
0
Bipolar drive
Unipolar drive
0
0.1
0.2
0.3
0.4
0.5
Time (sec)
Fig 23. Speed variation of a BLDC motor with 1 disk
Experiment (9)
12000
-Switch time : 4000rpm.
Speed (rpm)
10000
-Max. speed of the bipolar driving and
bipolar-starting and unipolar-running
driving methods : 11500rpm.
8000
6000
4000
2000
0
0
0.1
Bipolar drive
Unipolar drive
Bipolar-starting and Unipolar-running drive
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Time (sec)
Fig 24. Speed variation of a BLDC motor with no disk
-The same start motion between
bipolar drive and bipolar-starting and
unipolar running.
Experiment (10)
3
2
1
0
-1
-2
-3
0
3
2
1
0
-1
-2
-3
0
3
2
1
0
-1
-2
-3
0
-No load condition
-Bipolar drive
0.2
0.4
(a)
0.6
0.8
1 • start up the motor with 1.3A
• 6900rpm with 0.15A
-Unipolar drive
0.2
0.4
(b)
0.6
0.8
• start up the motor with 2.1A
1 • 11500rpm with 0.3A
-Bipolar-starting and unipolar- running drive
0.2
0.4
(c)
0.6
0.8
• switched at 4000prm
• start up the motor with 1.3A
1 • 11500rpm with 0.3A
Fig 25. Variation of phase current of a BLDC motor
(a) bipolar drive (b) unipolar drive (c) bipolar-starting and unipolar running drive
2
2
1.5
1.5
1
1
0.5
0.5
Current (A)
Current (A)
Experiment (11)
0
-0.5
-1
30°
0
-0.5
-1
30°
-1.5
-1.5
-2
0.165
0.17
0.175
TIme (sec)
0.18
0.185
Fig 12. Variation of phase A current
from bipolar to unipolar drive
-2
0.165
0.17
0.175
Time (sec)
0.18
0.185
Fig 13. Variation of phase C current
from bipolar to unipolar drive
-The phase difference of 30 electrical degrees between the commutation sequences
of bipolar and unipolar drive.
-Switched from bipolar drive and unipolar drive smoothly.
Conclusion
• The bipolar-starting and unipolar running method of BLDC
motor.
– It runs the motor to high speed with large starting torque.
– It reduce the rising time of the motor.
– It can protects the inverter circuit by reducing large input
current during start-up
– The effectiveness of the this method is verified by
experimentally.
• This method can be effectively applied to drive a BLDC
motor under large load conditions to high speed.
Future work
• Performance degradation occurred by the current ripple
of the new inverter circuit.
– Speed, Torque, Efficiency, etc.
– Comparison between the basic inverter circuit and
new inverter circuit.
• Optimal switching time from a efficiency point of view.
• Another driving method to improve the starting torque
of BLDC motor.
– Tripolar driving method, 12-step driving method, etc.