Motorola M68HC08 User Manual

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Sensorless BLDC Motor Control

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M68HC08

Microcontrollers

MC68HC908MR32

Designer Reference Manual

DRM028/D Rev. 0, 03/2003

MOTOROLA.COM/SEMICONDUCTORS

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MC68HC908MR32

Designer Reference Manual — Rev 0

by: Libor Prokop Motorola Czech System Laboratories Roznov pod Radhostem, Czech Republic

DRM028 — Rev 0 Designer Reference Manual

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Revision history

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To provide the most up-to-date information, the revision of our documents on the World Wide Web will be the most current. Your printed copy may be an earlier revision. To verify you have the latest information available, refer to:

http://www.motorola.com/semiconductors

The following revision history table summarizes changes contained in this document. For your convenience, the page number designators have been linked to the appropriate location.

Revision history

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Date

February,

2003

Revision

Level

1 Initial release N/A

Description

Page

Number(s)

Designer Reference Manual DRM028 — Rev 0

4 MOTOROLA

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Designer Reference Manual — Sensorless BLDC Motor Control

List of Sections

Section 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Section 2. System Description. . . . . . . . . . . . . . . . . . . . . 15

Section 3. BLDC Motor Control . . . . . . . . . . . . . . . . . . . .23

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Section 4. Hardware Design. . . . . . . . . . . . . . . . . . . . . . .57

Section 5. Software Design . . . . . . . . . . . . . . . . . . . . . . . 75

Section 6. User Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

Appendix A. References. . . . . . . . . . . . . . . . . . . . . . . . .161

Appendix B. Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . .163

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List of Sections

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Designer Reference Manual DRM028 — Rev 0

6 MOTOROLA

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Designer Reference Manual — Sensorless BLDC Motor Control

Section 1. Introduction

1.1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

1.2 Application Functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

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1.3 Benefits of the Solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Section 2. System Description

2.1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

2.2 System Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

2.3 System Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Section 3. BLDC Motor Control

3.1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

3.2 Brushless DC Motor Control Theory. . . . . . . . . . . . . . . . . . . . .23

3.3 Used Control Technique. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.4 Application Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

Section 4. Hardware Design

4.1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57

4.2 System Configuration and Documentation . . . . . . . . . . . . . . . . 57

4.3 All HW Sets Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.4 High-Voltage Hardware Set Components. . . . . . . . . . . . . . . . .66

4.5 Low-Voltage Evaluation Motor Hardware Set Components . . .70

4.6 Low-Voltage Hardware Set Components . . . . . . . . . . . . . . . . .72

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Table of Contents

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Section 5. Software Design

5.1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

5.2 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.3 Data Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

5.4 Main Software Flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

5.5 State Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89

5.6 Implementation Notes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103

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Section 6. User Guide

6.1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109

6.2 Application Suitability Guide . . . . . . . . . . . . . . . . . . . . . . . . . .109

6.3 Warning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112

6.4 Application Hardware and Software Configuration . . . . . . . . .113

6.5 Tuning for Customer Motor. . . . . . . . . . . . . . . . . . . . . . . . . . .126

Appendix A. References

Appendix B. Glossary

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Designer Reference Manual — Sensorless BLDC Motor Control

List of Figures

Figure Title Page

2-1 System Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

3-1 BLDC Motor Cross Section. . . . . . . . . . . . . . . . . . . . . . . . . . . .24

3-2 3-Phase Voltage System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3-3 BLDC Motor Back EMF and Magnetic Flux . . . . . . . . . . . . . . .26

3-4 Classical System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

3-5 Power Stage — Motor Topology. . . . . . . . . . . . . . . . . . . . . . . .28

3-6 Phase Voltage Waveform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3-7 Mutual Inductance Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3-8 Detail of Mutual Inductance Effect . . . . . . . . . . . . . . . . . . . . . . 33

3-9 Mutual Capacitance Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3-10 Distributed Back-EMF by Unbalanced

Capacity Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3-11 Balanced Capacity Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . .36

3-12 Back-EMF Sensing Circuit Diagram . . . . . . . . . . . . . . . . . . . . . 37

3-13 The Zero Crossing Detection . . . . . . . . . . . . . . . . . . . . . . . . . .38

3-14 Commutation Control Stages . . . . . . . . . . . . . . . . . . . . . . . . . .39

3-15 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40

3-16 BLDC Commutation with Back-EMF

Zero Crossing Sensing Flowchart. . . . . . . . . . . . . . . . . . . . . . . 42

3-17 BLDC Commutation Time with Zero Crossing Sensing . . . . . .43

3-18 Vectors of Magnetic Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . .47

3-19 Back-EMF at Start Up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

3-20 Calculation of the Commutation Times During the Starting

(Back-EMF Acquisition) State. . . . . . . . . . . . . . . . . . . . . . . . . .49

4-1 High-Voltage Hardware System Configuration . . . . . . . . . . . . .59

4-2 Low-Voltage Evaluation Motor Hardware

System Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4-3 Low-Voltage Hardware System Configuration . . . . . . . . . . . . .63

4-4 MC68HC908MR32 Control Board . . . . . . . . . . . . . . . . . . . . . . 65

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List of Figures

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4-5 3-Phase AC High Voltage Power Stage . . . . . . . . . . . . . . . . . . 67

4-6 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73

5-1 Main Data Flow — Part1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78

5-2 Main Data Flow — Part 2: Alignment, Starting,

5-3 Closed Loop Control System . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5-4 Main Software Flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5-5 Main Software Flowchart — Main Software Loop. . . . . . . . . . .86

5-6 Software Flowchart — Interrupts . . . . . . . . . . . . . . . . . . . . . . . 89

5-7 Application State Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . 90

5-8 Stand-by State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5-9 Align State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .94

5-10 Back-EMF Acquisition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97

5-11 Running State. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

5-12 STOP State. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102

5-13 Fault State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103

6-1 High-Voltage Hardware System Configuration . . . . . . . . . . . .114

6-2 Low-Voltage Evaluation Motor Hardware

6-3 Low-Voltage Hardware System Configuration . . . . . . . . . . . .116

6-4 Controller Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117

6-5 Execute Make Command . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

6-6 PC Master Software Control Window . . . . . . . . . . . . . . . . . . . 124

6-7 Follow-up for Software Customizing to Customer Motor . . . . 128

6-8 Follow-up for Advanced Software Customizing . . . . . . . . . . .129

6-9 Follow-up for Software Customizing Trouble Shouting. . . . . .129

6-10 PC Master Software Parameters Tuning Control Window . . .130 6-11 PC Master Software Parameters Tuning Control Window . . .131 6-12 PC Master Software Current Parameters Tuning Window . . . 141

6-13 PC Master Software Start Parameters Tuning Window . . . . .149

6-14 PC Master Software Speed Parameters Tuning Window. . . .154

Running Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80

System Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

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List of Tables

Table Title Page

2-1 Software Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

2-2 High Voltage Hardware Set Specifications. . . . . . . . . . . . . . . . 20

2-3 Low Voltage Evaluation Hardware Set Specifications . . . . . . .21

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2-4 Low Voltage Hardware Set Specifications . . . . . . . . . . . . . . . .22

3-1 PC Master Software Communication Commands . . . . . . . . . . 51

3-2 PC Master Software API Variables. . . . . . . . . . . . . . . . . . . . . .52

4-1 Electrical Characteristics of Control Board . . . . . . . . . . . . . . . . 66

4-2 Electrical Characteristics of Power Stage. . . . . . . . . . . . . . . . .68

4-3 Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69

4-4 Electrical Characteristics of the EVM Motor Board. . . . . . . . . .71

4-5 Characteristics of the BLDC motor . . . . . . . . . . . . . . . . . . . . . . 71

4-6 Electrical Chatacteristics of the 3-Ph BLDC

Low Voltage Power Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . .74

5-1 Software Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76

6-1 Required Software Configuration

for Dedicated Hardware Platform . . . . . . . . . . . . . . . . . . . . . .118

6-2 Start-up Period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .146

6-3 PWM Frequency Setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . .157

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List of Tables

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Designer Reference Manual DRM028 — Rev 0

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Designer Reference Manual — Sensorless BLDC Motor Control

Section 1. Introduction

1.1 Contents

1.2 Application Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.3 Benefits of the Solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.2 Application Functionality

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This Reference Design describes the design of a low-cost sensorless 3-phase brushless dc (BLDC) motor control with back-EMF (electromotive force) zero-crossing sensing. It is based on Motorola’s MC68HC908MR32 microcontroller which is dedicated for motor control applications. The system is designed as a motor drive system for medium power three phase BLDC motors and is targeted for applications in automotive, industrial and appliance fields (e.g. compressors, air conditioning units, pumps or simple industrial drives). The reference design incorporates both hardware and software parts of the system including hardware schematics.

1.3 Benefits of the Solution

The design of very low cost variable speed BLDC motor control drives has become a prime focus point for the appliance designers and semiconductor suppliers.

Today more and more variable speed drives are put in appliance or automotive products to increase the whole system efficiency and the product performance. Using of the control systems based on semiconductor components and MCUs is mandatory to satisfy requirements for high efficiency, performance and cost of the system.

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Once using the semiconductor components, it is opened to replace classical universal and DC-motors with maintenance-free electrically commutated BLDC motors. This brings many advantages of BLDC motors when the system costs could be maintained equivalent.

The advantages of BLDC motor versus universal and DC-motors are:

• high efficiency

• reliability (no brushes)

• low noise

• easy to drive features

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To control the BLDC motor, the rotor position must be known at certain angles in order to align the applied voltage with the back-EMF, which is induced in the stator winding due to the movement of the permanent magnets on the rotor.

Although some BLDC drives uses sensors for position sensing, there is a trend to use sensorless control. The position is then evaluated from voltage or current going to the motor. One of the sensorless technique is sensorless BLDC control with back-EMF (electromotive force) zero-crossing sensing.

The advantages of this control are:

• Save cost of the position sensors & wiring

• Can be used where there is impossibility or expansive to make additional connections between position sensors and the control unit

• Low cost system (medium demand for control MCU power)

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Section 2. System Description

2.1 Contents

2.2 System Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

2.3 System Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

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2.2 System Concept

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The application block diagram is shown in Figure 2-1. The sensorless rotor position technique detects the zero crossing points of back-EMF induced in the motor windings. The phase back-EMF zero crossing points are sensed while one of the three phase windings is not powered. The information obtained is processed in order to commutate the energized phase pair and control the phase voltage, using pulse width modulation.

The back-EMF zero crossing detection enables position recognition. The resistor network is used to step down sensed voltages to a 0–3.3 V level. Zero crossing detection is synchronized with the middle of center aligned PWM signals by the software, in order to filter high voltage spikes produced by switching the IGBTs (MOSFETs). The software selects by MUX command the phase comparator output that corresponds to the current commutation step. The multiplexer (MUX) circuit selects this signal, which is then transferred to the MCU input.

The voltage drop resistor is used to measure the dc-bus current which is chopped by the pulse-width modulator (PWM). The signal obtained is rectified and amplified (0–3.3 V with 1.65 V offset). The internal MCU analog-to-digital (A/D) converter and zero crossing detection are synchronized with the PWM signal. This synchronization avoids spikes when the IGBTs (or MOSFETs) are switched and simplifies the electric circuit.

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System Description

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MUX Command

Commutation Control

Three-Phase

Inverter

PWM

Generator

Dead Time

with

PWM

Duty Cycle

HC08MR32

3-ph

BLDC

Motor

DC Bus Current &

DC Bus Voltage

Sensing

Power line

DC-Bus Voltage/ Current Temperature

ADC

PC Master

START

STOP

SPEED

SCI

Required Speed

Required Alignment Current

3 BEMF Voltage

Zero Crossing

Comparators

MUX

Digital Inputs

Zero Crossing

Zero Crossing Period, Position Recognition

1/T

Actual Current

3 phase BLDC

Power Stage

BEMF Zero Crossing signal

Digital Outputs

Zero Crossing Time moment

Commutation Period

Actual Speed

Speed PI

Regulator

Current PI Regulator (for Alignment)

Figure 2-1System Concept

During the rotor alignment state, the dc-bus current is controlled by the current PI regulator. In the other states (motor running), the phase voltage (PWM duty cycle) is controlled by the speed PI regulator.

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The A/D converter is also used to sense the dc-bus voltage and the drive temperature. The dc-bus voltage is stepped down to a 3.3-V signal level by a resistor network.

The six IGBTs (copack with built-in fly back diode), or MOSFETs, and gate drivers create a compact power stage. The drivers provide the level shifting that is required to drive the high side switch. The PWM technique is used to control motor phase voltage.

2.3 System Specification

System Description

System Specification

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The concept of the application is that of a speed-closed loop drive using back-EMF zero crossing technique for position detection. It serves as an example of a sensorless BLDC motor control system using Motorola’s MC68HC908MR32 MCU. It also illustrates the usage of dedicated motor control on-chip peripherals.

The system for BLDC motor control consists of hardware and software. The application uses universal modular motion control development hardware boards, which are provided by Motorola for customer development support. For a description of these hardware boards refer to Appendix A. References 3.,4.,5.,6.,7., and the World Wide Web at:

http://www.motorola.com

There are three board and motor hardware sets for the application:

1. High-Voltage Hardware Set — For variable line voltage 115–230 Vac and medium power (phase current < 2.93 A)

2. Low-Voltage Evaluation Motor Hardware Set — For automotive voltage (12 V) and very low power (phase current < 4 A)

3. Low-Voltage Hardware Set — For automotive voltage (12 V or possibly 42 V) and medium power (phase current < 50 A)

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System Description

2.3.1 Software Specification

The application software is practically the same for all three hardware platforms. The only modification needed is to include one of three constants that customize the hardware and motor parameter settings.

The software (written in C language) specifications are listed in

Table 2-1. A useful feature of the software is serial communication with

PC master software protocol via RS232. The PC master software is PC computer software which allows reading and setting of all the system variables, and can also run html script pages to control the application from the PC. Another feature of the BLDC control software, is on-line

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Table 2-1. Software Specifications

3-phase trapezoidal BLDC motor control star or delta!

connected

Control Algorithm

Target Processor MC68HC908MR32

Language C-language with some arithmetical functions in assembler

Compiler Metrowerks ANSI-C/cC++ Compiler for HC08

Application

Control

MCU Oscillator

Frequency

MCU Bus

Frequency

Minimal BLDC

Motor

Commutation

Period

(Without PC

Master

Software

Communication)

Sensorless, with back-EMF zero crossing commutation

control

Speed closed loop control

Motoring mode

Manual interface (start/stop switch, speed potentiometer

control, LED indication)

PC master software (remote) interface (via RS232 using PC

computer)

4 MHz (with default software setting)

8 MHz (with default software setting)

333 µs (with default software setting and COEF_HLFCMT =

0.450)

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Table 2-1. Software Specifications

Minimal BLDC

Motor

Commutation

Period (with PC

Master Software

Control)

Targeted

Hardware

Software

Configuration

and Parameters

Setting

2.3.2 Hardware and Drive Specifications

The other system specifications are determined by hardware boards and motor characteristics. The boards and their connections are shown in

Section 4. Hardware Design. and Section 6. User Guide, 6.4.1 Hardware Configuration. The hardware set specifications are

discussed in the following subsections.

System Description

System Specification

520 µs (with default software setting and COEF_HLFCMT =

0.450)

Software is prepared to run on three optional board and

motor hardware sets:

• High-voltage hardware set at variable line voltage 115–230 Vac

(software customizing file const_cust_hv.h)

• Low-voltage evaluation motor hardware set (software customizing file const_cust_evmm.h)

• Low-voltage hardware set (software customizing file const_cust_lv.h)

Configuration to one of the three required hardware sets is

provided by inclusion of dedicated software customizing files. The software pack contains the files const_cust_hv.h, const_cust_lv.h, and const_cust_evm.h with predefined parameter settings for running on one of the optional board and motor hardware sets. The required hardware must be selected in code_fun.c file by one of these files #include.

Where software is configuration for different customer

motors, the software configuration for any motor is provided in the dedicated customizing file, according to the hardware board used.

PWM frequency 15.626 kHz with default software setting,

possibly changeable in const.h file

2.3.2.1 High-Voltage Hardware Set Specification

This hardware set is dedicated for medium power (phase current <

2.93 A) and main voltage. The specifications for a high-voltage hardware

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System Description

and motor set are listed in Table 2-2. The hardware operates on both 230 Vac and 115 Vac mains.

Table 2-2. High Voltage Hardware Set Specifications

Input voltage: 230 Vac or 115 Vac

Hardware Boards

Characteristics

Motor -Brake Set Manufactured EM Brno, Czech Republic

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Maximum dc-bus voltage: 407 V

Maximal output current: 2.93A

EM Brno SM40V

Motor type:

Pole-Number: 6

3 phase, star connected

BLDC motor,

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Motor Characteristics

Brake Characteristics

Maximum dc-bus voltage: 380 V

Drive Characteristics

Speed range: 2500 rpm (at 310 V)

Maximum electrical

power:

Phase voltage: 3*220 V

Phase current: 0.55 A

Brake Type:

Nominal Voltage: 3 x 27 V

Nominal Current: 2.6 A

Pole-Number: 6

Nominal Speed: 1500 rpm

Speed range:

Optoisolation: Required

Protection:

150 W

SG40N 3-Phase BLDC Motor

< 2500 rpm (determined by motor

used)

Over-current,

over-voltage, and under-voltage fault protection

Load Characteristic Type: Varying

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System Description

System Specification

2.3.2.2 Low-Voltage Evaluation Hardware Set Specification

This hardware set is dedicated for 12 V voltage and very low power (phase current < 4 A). The specifications for a low-voltage evaluation hardware and motor set are listed in Table 2-3. It is targeted first of all to software evaluation with small motors.

Table 2-3. Low Voltage Evaluation Hardware Set Specifications

Input voltage: 12 Vdc

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Hardware Boards

Characteristics

Maximum dc-bus voltage: 16.0 V

Maximal output current: 4.0 A

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Motor Characteristics

Drive Characteristics

Load Characteristic Type: Varying

2.3.2.3 Low-Voltage Hardware Set Specification

Motor type:

Speed range: < 5000 rpm (at 60 V)

Maximal line voltage: 60 V

Phase current: 2 A

Output torque: 0.140 Nm (at 2 A)

Speed range: < 1400 rpm

Input voltage: 12 Vdc

Maximum dc-bus voltage: 15.8 V

Protection:

4 poles, three phase, star

connected, BLDC motor

Over-current,

over-voltage, and under-voltage fault protection

This hardware set is dedicated for medium power (phase current < 50 A) and automotive voltage. The specifications for a low-voltage hardware and motor set are listed in Table 2-4. The hardware power stage board is dedicated for 12 V, but can be simply configured to a 42 V supply (described in documentation for the ECLOVACBLDC board). The supplied motor is targeted for 12 V.

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System Description

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Table 2-4. Low Voltage Hardware Set Specifications

Input voltage: 12 Vdc or 42 V

Hardware Boards

Characteristics

Motor -Brake Set Manufactured EM Brno, Czech Republic

Maximum dc-bus voltage: 16.0 V or 55.0 V

Maximal output current: 50.0 A

EM Brno SM40N

Motor type:

Pole-Number: 6

3 phase, star connected

BLDC motor,

nc...

cale Semiconductor,

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Motor Characteristics

Brake Characteristics

Drive Characteristics

Maximum dc-bus voltage: 15.8 V

Speed range: 3000 rpm (at 12 V)

Maximum electrical

power:

Phase voltage: 3*6.5 V

Phase current: 17 A

Brake Type:

Nominal Voltage: 3 x 27 V

Nominal Current: 2.6 A

Pole-Number: 6

Nominal Speed: 1500 rpm

Speed range: < 2500 rpm

Input voltage: 12 Vdc

Protection:

150 W

SG40N 3-Phase BLDC Motor

Over-current,

over-voltage, and under-voltage fault protection

Load Characteristic Type: Varying

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Designer Reference Manual — Sensorless BLDC Motor Control

Section 3. BLDC Motor Control

3.1 Contents

3.2 Brushless DC Motor Control Theory. . . . . . . . . . . . . . . . . . . . .23

3.3 Used Control Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

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3.4 Application Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

3.2 Brushless DC Motor Control Theory

3.2.1 BLDC Motor Targeted by This Application

The brushless dc motor (BLDC motor) is also referred to as an electronically commutated motor. There are no brushes on the rotor, and commutation is performed electronically at certain rotor positions. The stator magnetic circuit is usually made from magnetic steel sheets. Stator phase windings are inserted in the slots (distributed winding) as shown in Figure 3-1, or it can be wound as one coil on the magnetic pole. Magnetization of the permanent magnets and their displacement on the rotor are chosen in such a way that the back-EMF (the voltage induced into the stator winding due to rotor movement) shape is trapezoidal. This allows a rectangular shaped 3-phase voltage system (see Figure 3-2) to be used to create a rotational field with low torque ripples.

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STATOR

STATOR WINDING (IN SLOTS)

SHAFT

ROTOR

AIR GAP

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PERMANENT MAGNETS

Figure 3-1. BLDC Motor Cross Section

The motor can have more than just one pole-pair per phase. This defines the ratio between the electrical revolution and the mechanical revolution. The BLDC motor shown has three pole-pairs per phase, which represent three electrical revolutions per one mechanical revolution.

Voltage

VOLTAGE

Phase A

PHASE A

Phase B

PHASE B

Phase C

PHASE C

30° 90° 150° 210° 270° 330°

120°

ELECTRICAL

electrical

ANGLE

angle

Figure 3-2. 3-Phase Voltage System

The easy to create rectangular shape of applied voltage ensures the simplicity of control and drive. But, the rotor position must be known at certain angles in order to align the applied voltage with the back-EMF (voltage induced due to movement of the PM). The alignment between

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back-EMF and commutation events is very important. In this condition, the motor behaves as a dc motor and runs at the best working point. Thus, simplicity of control and good performance make this motor a natural choice for low-cost and high-efficiency applications.

Figure 3-3 shows a number of waveforms:

• Magnetic flux linkage

• Phase back-EMF voltage

• Phase-to-phase back-EMF voltage

Magnetic flux linkage can be measured. However, in this case it was

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calculated by integrating the phase back-EMF voltage (which was measured on the non-fed motor terminals of the BLDC motor). As can be seen, the shape of the back-EMF is approximately trapezoidal and the amplitude is a function of the actual speed. During speed reversal, the amplitude changes its sign and the phase sequence changes.

BLDC Motor Control

Brushless DC Motor Control Theory

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BLDC Motor Control

Phase Magnetic

Flux Linkage

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Ps i_ A

Ps i_ B

Ps i_ C

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PH. A

Phase Back EMF

Atop Btop Ctop

Cbot Abot Bbot

Phase-Phase

Back EMF

Figure 3-3. BLDC Motor Back EMF and Magnetic Flux

The filled areas in the tops of the phase back-EMF voltage waveforms

PH. A

A–B

PH. B

PH. C

PH. B

PH. C

“NATURAL” COMMUTATION POINT

“Natural” commutation point

ACTING POWER SWITCH IN THE POWER STAGE

B–C

C–A

SPEED REVERSAL

Speed reversal

Ui_A

Ui_B

Ui_C

Ui_A B

Ui_B C

Ui_CA

indicate the intervals where the particular phase power stage commutations occur. The power switches are cyclically commutated through the six steps; therefore, this technique is sometimes called six step commutation control. The crossing points of the phase back-EMF voltages represent the natural commutation points. In normal operation the commutation is performed here. Some control techniques advance the commutation by a defined angle in order to control the drive above the pulse-width modulator (PWM) voltage control.

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BLDC Motor Control

Brushless DC Motor Control Theory

3.2.2 3-Phase BLDC Power Stage

The voltage for 3-phase BLDC motor is provided by a 3-phase power stage controlled by digital signals. Its topology is the one as for the AC induction motor (refer to Figure 3-5). The power stage is usually controlled by a dedicated microcontroller with on-chip PWM module.

3.2.3 Why Sensorless Control?

As explained in the previous section, rotor position must be known in order to drive a brushless dc motor. If any sensors are used to detect rotor position, sensed information must be transferred to a control unit

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(see Figure 3-4). Therefore, additional connections to the motor are necessary. This may not be acceptable for some applications (see 1.3

Benefits of the Solution).

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AC LINE VOLTAGE

SPEED

SETTING

–

CONTROL SIGNALS

3.2.4 Power Stage — Motor System Model

In order to explain and simulate the idea of back-EMF sensing techniques a simplified mathematical model based on the basic circuit topology has been created. See Figure 3-5.

POWER STAGE

CONTROL UNIT

MOTOR DRIVE

POSITION FEEDBACK

Figure 3-4. Classical System

POSITION SENSORS

LOAD

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backEMF a

Ra Rc

Figure 3-5. Power Stage — Motor Topology

The second goal of the model is to find how the motor characteristics

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depend on the switching angle. The switching angle is the angular difference between a real switching event and an ideal one (at the point where the phase-to-phase back-EMF crosses zero).

B u

backEMF b

backEMF c

Frees

The motor-drive model consists of a normal 3-phase power stage plus a brushless dc motor. Power for the system is provided by a voltage source (U

). Six semiconductor switches (S

A/B/C t/b

), controlled elsewhere, allow the rectangular voltage waveforms (see Figure 3-2) to be applied.

The semiconductor switches and diodes are simulated as ideal devices. The natural voltage level of the whole model is put at one half of the dc-bus voltage. This simplifies the mathematical expressions.

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3.2.4.1 Stator Winding Equations

The BLDC motor is usually very symmetrical. All phase resistances, phase and mutual inductances, flux-linkages can be thought of as equal to, or as a function of the position θ with a 120° displacement.

The electrical BLDC motor model then consists of a set of the following stator voltage equations (EQ 3-1.).

BLDC Motor Control

Brushless DC Motor Control Theory

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The task of this section is to explain the background of the back-EMF sensing and to demonstrate how the zero crossing events can be detected. Parasitic effects that negatively influence the back-EMF detection are discussed and their nature analyzed.

3.2.4.2 Indirect Back EMF Sensing

Let us assume a usual situation, where the BLDC motor is driven in six-step commutation mode using PWM technique, where both top and bottom switches in the diagonal are controlled using the same signal (so called “hard switching PWM” technique). The motor phases A and B are powered, and phase C is free, having no current. So the phase C can be used to sense the back-EMF voltage. This is described by the following conditions:

---± u 2

d–id==,

SAbSBt, PWM←

iSb–i==i

backEMF aubackEMF bubackEMF c

++ 0=

−

---u

0=i

d0=,

(EQ 3-1.)

(EQ 3-2.)

The branch voltage u

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---u

xa=

can be calculated using the above conditions,

∑

backEMF x

LacLbc–()

---- uVC–+–= td

(EQ 3-3.)

BLDC Motor Control

After evaluation the expression of the branch voltage uVc is as follows:

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---u

backEMF c

–()

---L

acLbc

(EQ 3-4.)

----–= td

The same expressions can also be found for phase A and B:

---u

backEMF a

---u

backEMF b

–()

---L

baLca

–()

---L

cbLab

----–= td

(EQ 3-5.)

(EQ 3-6.)

The first member in the equation (EQ 3-6.) demonstrates the possibility to indirectly sense the back-EMF between the free (not powered) phase terminal and the zero point, defined at half of the dc-bus voltage (see Figure 3-5.). Simple comparison of these two levels can provide the required zero crossing detection.

As shown in Figure 3-5, the branch voltage of phase B can be sensed between the power stage output B and the zero voltage level. Thus, back-EMF voltage is obtained and the zero crossing can be recognized.

When Lcb = Lab, this general expressions can also be found:

---u

backEMFx

where x A B C,,==

(EQ 3-7.)

There are two necessary conditions which must be met:

• Top and bottom switches (in diagonal) have to be driven with the same PWM signal

• No current goes through the non-fed phase that is used to sense the back-EMF

Figure 3-6 shows branch and motor phase winding voltages during a

0–360° electrical interval. Shaded rectangles designate the validity of the equation (EQ 3-7.). In other words, the back-EMF voltage can be sensed during designated intervals.

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