bldc_chr
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BLDC_basics.ppt 1
STMicroelectronics
ST7FMCxST7FMCx
BLDC Motor basics
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BLDC_basics.ppt 2
V22
BLDC DRIVE KNOWLEDGEBLDC DRIVE KNOWLEDGE
Why BLDC Motor What is a BLDC Motor
Why BEMF
Control Principle
Control Method
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BLDC_basics.ppt 3
V23
Why The BLDC MotorWhy The BLDC Motor Also called brushless Permanent Magnet DC (BLDC) or
synchronous DC motor
Advantages : High efficiency (up to 98%) Variable speed
Silent operation
Reliable/long life time (no brushes)
High Power/ Size ratio
High torque at start-up
Same stator as Induction 3 phase motor
Major applications (where above advantages are required): Compressor (air conditioner, refrigerator)
Appliances (refrigerator, vacuum cleaner*, food processor*) Industrial fan
Automotive (fuel* and water*pumps, cooling fan*, climate control)
Drawbacks: Overall system cost due to cost of electronic control
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BLDC_basics.ppt 4
V24
BLDC Synchronous MotorBLDC Synchronous Motor
Synchronous : Stator flux and rotor mechanical rotation speed aresame.
Stator description : 3 phase winding (1 or 2 or more pairs of poles)
Rotor description : Permanent magnet
For 1 pair of pole : 1 electrical cycle (6 steps) = 1 mechanical cycle
South Pole
1
23
4
5 6
NS 1
23
4
5 6
N
S
For p pair of poles 1 mechanical cycle = p electrical cycles
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BLDC_basics.ppt 5
V25
Why BEMF in a PMDC MotorWhy BEMF in a PMDC Motor
A
B C
ea
eb
ec
Neutral
(1) B-emf is the voltage induced in a winding by the movementof the magnet in front of this winding : e=d/dt. It is independant
of the energy supply to the motor (just by spinning by hand the
rotor for example it is possible to generate back-emf).
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BLDC_basics.ppt 6
V26
Control BasicsControl Basics
To run efficiently a BLDC motor, the current in each winding must be inphase with the b-emf (1) induced in this same winding by the rotation ofthe magnet (when this happens we autocommutation).
If current and b-emf are not in phase, then the BLDC will not run efficientlyor not run at all.
To keep current and b-emf in phase, the current must be applied with thecorrect timing and therefore the position or the rotor (which determines theb-emf) needs to be known.
t
current
Back-EMF
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BLDC_basics.ppt 7
V27
Rotor Position DetectionRotor Position Detection
ControlIing efficiently a BLDC requires to know the position ofthe rotor at all times.
At least 2 common ways are available to know the rotor position.
By using Hall effect sensors which allow to read physically andprecisely the rotor position (BLDC control with sensors).
By reading the back-emf and deducting the rotor position from
its reading (BLDC control without sensors). There are several
methods available to monitor the back-emf. Most of themrequire 6 step mode current supply to the motor.
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BLDC_basics.ppt 8
V28
PMDC PolarizationPMDC Polarization
PMDC
Vcc
L6386
HVGOUT
HIN
SD
SGND
LVG
PGND
LIN
L6386
HVGOUT
HIN
SD
SGND
LVG
PGND
LIN
L6386
HVGOUT
HIN
SD
SGND
LVG
PGND
LIN
300-400V
DC inputControl
Unit
Sensorless Wiring
Sensor Wiring
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BLDC_basics.ppt 9
V29
Drive Mode: 6 Step ModeDrive Mode: 6 Step Mode
2
T1 T1 T3 T3 T5 T5T4 T6 T6 T2 T2 T4
1 2 3 4 5 6
t
t
t
A
B
C
Current in winding A,B,C
UV
W
1
6
54
3
C
AT5
T6
T3 T1
T4 T2
B
+300V DC
-
+I
During one electrical cycle, there are 2 times when there is no current in the winding.
During one electrical cycle there is always one winding biased in one direction and an
other in the other direction.
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BLDC_basics.ppt 10
V210
One pair pole motorOne pair pole motor
1
N
S
A
B CT1-T4
2
N
S
T1-T6
A
B3
N S
A
B T3-T6
4
N
S
A
B CT3-T2
5
N
S
A
T5-T2B C 6
NS
A
B CT5-T4
Magnet of Rotor follow the statoric resultant flux
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BLDC_basics.ppt 11
V211
Sensorless BEMF MeasurementSensorless BEMF Measurement
BUS
POWER GND
N
A
B
C
ea
eb
ec
Voltmeter
The Goal is to measure the voltage of not polarized winding
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BLDC_basics.ppt 12
V212
State of the Art BLDC control method (A)State of the Art BLDC control method (A)
weaknesses: - Voltage dividers give reduced sensitivity & difficulty for low speed operation
- Filtering is optimised for narrow speed range
300V
~
POWER GND
N
N'
+ 5V
divider & filter
BUS
~
POWER GND
N
BUS
~
POWER GND
N
N'
divider & filter
virtual ground
Fully analog method with many drawbacks.
A virtual ground is created. The signal from it is divided and then filtered before entering
comparator.
The signal from the winding we want to measure b-emf from is also divided and filtered
before entering comparator.
This method implies to dimension components according to nominal point of operation
of motor.
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BLDC_basics.ppt 13
V213
State of the Art BLDC control method (B)State of the Art BLDC control method (B)
BUS
~
POWER GND
N
HV
~
POWER GND
N
+ 5V
divider & filter
divider & filterHV/2
During On state of PWM the neutral voltage value is equal to HV/2.
We create a voltage reference equal to HV/2 on the second input of Voltage comparator
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BLDC_basics.ppt 14
V214
State of the Art BLDC control methodState of the Art BLDC control method
Filtered voltageof rebuilt reference node N'
Filtered Phase voltage
Zero crossing event
filt1.hgl
Phase current
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BLDC_basics.ppt 15
V215
ST Sensorless BLDC control methodST Sensorless BLDC control method
GND
A
BC
T1 PWM "ON"
T4 always ON
bemf
300V
V/2
back-EMF whole signal sensing
high sensitivity + large speed range + very low speed operation + starting
with full torque
no filtering delay
high signal to noise ratio
GND
A
B C
T1 PWM "OFF"
T4 always ON
bemf
300V
GND
+ 5V
voltage clamping
ST7MC
ABC
T5
T6
T3
T4
+300V DC
+I
T1
T2
What happened before this chart : winding C has been demagnetised.
The window of b-emf reading is now opened.
Every time the T1 transistor is off, current flows in the free-wheeling diode and
voltage at point N is at ground.
B-emf zero crossing can be monitored at T1 sampling frequency on the
comparator output.
The C voltage is clamped at +5V/-0.6V (but what we are interested in is around
0V) by the on-chip clamping diode.
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BLDC_basics.ppt 16
V216
ST Sensorless BLDC control methodST Sensorless BLDC control method
End of demagnetisation End of demagnetisation
Back-EMF Back-EMF
Zero Crossing
Floating phase Floating phase
Conducting phase
2 bemf detection : One for crossing coming from negative area, one for crossing comingfrom positive area.
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BLDC_basics.ppt 17
V217
Sensorless PMDC, in current
Mode
Sensorless PMDC, in current
Mode
PMDC
MCIA
MCIB
MCIC
MCO5
MCO4
MCO3
PB
MCO2
MCO1
MCO0
MCCFI
MCCREF
ST7MC
Vcc
L6386
HV
GOUT
HIN
SD
SGND
LVG
PGND
LIN
L6386
HV
GOUT
HIN
SD
SGND
LVG
PGND
LIN
L6386
HV
GOUT
HIN
SD
SGND
LVG
PGND
LIN
300-400V
DC input
6 steps output to PMDC
B-emf input to ST7MC
5Volts High side outputs15Volts High side outputs
Current limit input
Current sense
15V Low side outputs
5Volts Low side outputs
DC bus is sustained by electronic capacitor after the rectifier bridge.
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BLDC_basics.ppt 18
V218
Control Principles of PMDCMaximum efficiency with delay manager
Control Principles of PMDCMaximum efficiency with delay manager
t
zero crossing detection times
back-EMF
t
switching timeof step n+1
current
C: commutation
Z: zero CrossingD: end of demagnetization
D
Z
Z
image of ST7MC Motor control timer(MTIM)
C
C
Delay
For a given speed, the best efficiency of the PMDC motor is when the current in awinding is in phase with the b-emf generated by rotation of the magnet in front of this
same winding.
The regulation process is going to be about keeping these 2 signals in phase.
Zero crossing and end of Demagnetisation are physical events.
The commutation is a calculated event (by the Motor control cell coprocessor),
according to the delay the motor control cell wants to put.
This delay is either computed from the value of the timer at the zero crossing (n) , or
from the value of the timer at the zero crossing before (n-1). This is required for sometypes of motors (dissymmetrical motors) to avoid B-emf to be detected at different
times. The choice between n and n-1 is done by SW.
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BLDC_basics.ppt 19
V219
ST7MC Regulation loops(simplified diagram)
ST7MC Regulation loops(simplified diagram)
P.I.D
or Fuzzy regulation
Feedback Speed
Step
Time
Motorb. emf
I
Target
Motor Speed
Motor efficiency regulation loop
Speed regulation loop
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V220
See Tutorial on Motors on
www.st.com/stonline/products/support/motor/tutorial/tutorial.htm
See Tutorial on Motors on
www.st.com/stonline/products/support/motor/tutorial/tutorial.htm
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