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BLDC_basics.ppt 1

STMicroelectronics

ST7FMCxST7FMCx

BLDC Motor basics

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BLDC DRIVE KNOWLEDGEBLDC DRIVE KNOWLEDGE

Why BLDC Motor What is a BLDC Motor

Why BEMF

Control Principle

Control Method

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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 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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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