Designing High-Performance and Power Efficient 3-Phase...

Designing High-Performance and Power Efficient

3-Phase Brushless DC Motor Control Systems

April 17, 2014 Revision 2.1

By John T. Lee, Carlos Ribeiro, and Miguel Mendoza; Micrel, Inc.

Synopsis

Today motor control systems are used by engineers use for both digital and analog technologies

to conquer past challenges including motor speed control, rotation direction, drift and motor

fatigue. The application of MCUs has enabled this generation of engineers the opportunity to

dynamically control motor actions so that they respond to environmental stresses and conditions.

This will help to provide a longer operational lifetime and reduce maintenance which means

lower cost. Currently, motor manufactures are gravitating towards 3-Phase BLDC motors

because they provide more torque for less power and have a longer operating time due to no

direct contact from the commutator and electrical terminals such as is found in the brushed

motors. Regrettably, the use of 3-Phase motor control adds additional complexity compared to

brushed DC or AC motors and the relationship between digital and analog components becomes

very important.

This whitepaper discusses a summary of the concerns that should be factored in when using

analog components and microcontroller in a 3-Phase BLDC motor application. It also covers the

suitable power management devices and power level shifters that enable the microcontrollers to

drive motors form power sources from 12V all the way up to 300V DC voltages.

Who Needs BLDC Motors Anyway?

Recently, designers have been favoring the use of a higher efficiency BLDC motor. This trend

holds true for numerous markets and a diverse array of applications. Presently, applications can

use or are already using BLDC motors as a replacement for dated AC motors or mechanical

pump technology. Important benefits of using BLDC motors include:

Higher efficiency (75% vs. 40% of an AC motor)

Less heat generated

Higher reliability (no brushes to wear out)

Safer to operate in a dangerous environment (no brush dust generated as is found with

brushed motors).




The use of BLDC motors in key sub-systems also reduces the overall system weight. As the

BLDC motor is commutated entirely electronically, it is much simpler to control the torque and

RPM of the motor and at much higher speeds. Governments the world over are dealing with very

real power deficits that are caused by insufficient electrical power grids. Moreover, there are

many regions of the world that must routinely deal with blackouts during peak demand seasons.

The result is that those countries are now either giving subsidies or preparing to hand out

subsidies for the more efficient use of BLDC motors.

Key Parameters AC Motors BLDC Motors

Size and Weight 100% 55%

Efficiency 40% - 50% 70% -75%

Speed Control Difficult Easy & Linear

Accuracy and Speed 3% - 5% 0.5%

Torque Control Poor Excellent Table 1. Advantages of Brushless DC Motors

Strategic Market Segments and Applications

Automotive. There are many instances in automotive markets where mechanical and hydraulic

pumps/movement controls are being replaced. Applications for this trend cover everything from

fuel pumps, power steering, seat control, automotive HVAC (Heating Venting and Air-

Conditioning), moon roof movement, windshield wiper motors, and more. It has been calculated

that each one of these functions can save up to one mile per gallon by switching to BLDC

motors. The reasoning behind this movement is obviously fuel savings and power efficiency.




Figure 1. Window Lift Functional Block Diagram

Home Appliances. There are quite a few appliances in the home appliance market that can

benefit from the use of high efficiency BLDC motors. These include pumps, fans, air

conditioners, blenders, hand power tools, and other kitchen appliances.

Figure 2. Blender Motor Control Functional Block Diagram




Industrial Systems. Most pumps, fans, air conditioners, mixers, and HVAC units require a

motor drive. The EU has issued an edict that requires all new industrial appliances to use the 3-

Phase “Inverter Drive” of the BLDC motors.

Figure 3. Air Conditioning Functional Block Diagram




White Goods. The electricity use in many clothes washers and dryers can be reduced with the

adoption of higher efficiency BLDC motors.

Figure 4. Washer Motor Functional Diagram

Segments Driving Force Applications

Automotive Fuel Savings

Higher Efficiency and Reliability

Electrical Pumps

Power Steering

Wipers

White Goods EU-Drive Towards Clean Energy

and Power Efficiency

Washers

Dryers

Refrigerators

Air Conditioning

Industrial EU-“Inverter Drive” Power Efficiency Pump

Fans

Heaters

Home Appliance Clean Energy and Power Efficiency

Government Subsidy

Blenders

Refrigerators

HVAC Table 2. Key Areas for Brushless DC Motor Drives




What Drives BLDC Motors?

There are several methods for driving a BLDC motor. Some fundamental system requirements

are listed below:

a. Power Transistors. These are usually MOSFETs or IGBTs that can tolerate high

voltages (matched to the requirements of the motor). Most home appliances use motors

that produce half to three quarter of a horsepower (1hp = 734W). Hence, typical current

capabilities of up to 10A are utilized. For a high voltage system (>350V typically),

IGBTS can be used.

b. MOSFET/IGBT Drivers. Generally, a set of MOSFET or IGBT drivers are utilized.

One could choose either three “Half Bridge” drivers or 3-Phase drivers. These solutions

must be able to handle two times the motor voltage to manage the back-electromotive

force (EMF) generated from the motors. Moreover, these drivers should provide

protection of the power transistors via timing and switching controls that ensure the top

transistors are turned off before the bottom transistors turn on.

c. Feedback Elements/Control. Designers should have some sort of feedback element in

all servo control systems. Examples include optical sensors, hall effect sensors,

tachometers, and sensorless EMF sense—the least costly of all. Various feedback

methods are useful, depending upon the precision required, RPM, and torque needed.

Many consumer applications generally seek to use the sensorless technique of back-EMF

sense.

d. Analog to Digital Converter. In many situations, an A/D device is needed in order to

convert the analog signals to digital code, which are then sent to the system MCU.

e. MCU. All closed-loop control systems (BLDC motors are nearly always in this group)

require an MCU, which handles the servo loop control, calculations, corrections, PID

controls, and sensor management. These digital controllers are usually 16-bit, but less

sophisticated applications can use 8-bit controllers.

f. Analog Power/Regulators/References. In addition to the above-mentioned components,

many systems contain axillary power, regulation, voltage conversion, and other analog

devices such as supervisors, LDOs, DC-to-DC converters, and OP amps.




ZNEO Core MCUGain Calculations

Position and Velocity Calculations

PID Control System

Stabilty Compensation

Phase Detech

Back EMF Calculations

Sensor Management

ADC I/0s

MIC4604

Half Bridge Driver

Phase AProvide Protection and

Appropriate Voltage and

Current to Drive the

MOSFET

ON or OFF

MOSFETHigh Voltage

Switch

(ON / OFF)

MOSFET

MOSFET

MIC6270

EMF Feedback

Feedback Loop to

Detect Position,

Speed, and

Direction

MIC5235LDO

Provides Constant

Voltage Despite of

Input Voltage

Variations MIC4682

Buck RegulatorProvides Regulated

Output Voltage that

is Lower Than the

Input Voltage with

High Efficiency

24V DCAC

Bridge

Rectifier & PFCRectifies AC to DC

and Maintains the

Current in Phase with

the Voltage

MIC38HC44

Flyback Provides Regulated

Output DC Voltage

Isolated Supply

110V to

220V AC

12V DC

3V DC

MIC4604

Half Bridge Driver

Phase B

MIC4604

Half Bridge Driver

Phase C

Figure 5. Typical Functional Block Diagram of a 24V Brushless DC Motor Control

Micrel Advantages in Motor Drive

a. Power Drivers. Micrel maintains a broad line of MOSFET/IGBT drivers. Key

parameters include fast pulse delay, high peak current for gate charge/control, and up to

85V operation. Some examples include theMIC4604 family that can withstand up to 85V

of back-EMF motor voltage.

b. Voltage Reference and Supervisors. Micrel offers an extensive line of these devices

which are critical to the operations of MCU. Examples include: the MIC811, MIC2775,

and the MIC1232 voltage supervisors.

c. OP Amps/Comparators. Micrel has a line of low power op amps and comparators.

These devices are critical to the precision feedback control of servo systems. Examples

include the MIC6270, MIC841N, and the MIC833.

d. LDO. Micrel offers the broadest line of LDOs in the industry including the fastest

transient LDO, lowest input LDO, lowest dropout LDO, and highest current LDO.

Examples include the MIC49150, MIC29150, MIC5235, and the MIC5283.

e. DC-to-DC Switching Regulators. Micrel also offers an extensive line of DC-to-DC

convertors with the highest efficiencies. These are used in auxiliary power applications.

Examples include the MIC2605 Boost and MIC4682 Buck (Step-Down) switching

regulators.




Basic Theory of Operation for 3-Phase Brushless DC Motors

Brushless DC (BLDC) motors are synchronous motors that

have permanent magnets residing on the rotor and coil

windings. These produce electrical magnets on the stator of the

motor (see Figure 6). Electrical terminals are directly connected

to the stator windings; hence there are no brushes or mechanical

contacts to the rotor such as in brushed motors. BLDC motors

use DC power and a switching circuit to generate bi-directional

currents on the stator windings. The switching circuit entails the

use of both a high-side switch and low-side switch for each winding, which totals six switches

for one BLDC motor.

Modern motor designs use solid state switches such as MOSFETs or IGBTs (see Figure 7)

depending upon the speed and voltage requirements of the motor when compared to relays. Cost,

reliability, and size must all be factored in. The switching currents produce the appropriate

magnetic field polarity that attracts opposite polarities and repels equal polarities. This, in turn,

produces a magnetic force that rotates the rotor. Using permanent magnets on the rotor provides

the designer with a mechanical advantage: the reduction of both size and weight. BLDC motors

feature improved thermal characteristics compared to brush motors and induction motors,

making them ideal choice for the next generation of power savings for mechanical systems.

BLDC generally use three phases (windings) with each phase having a conducting interval of

120 degrees (see Figure 8).

Figure 6. Cross-section of BLDC

motor

300 V SUPPLY

300 V SUPPLY

Q 1

Q 2

Q 3

Q 4

Q 6 Q 5

Figure 7. Switching Circuit Using IGBTs




Figure 3. 6 Step Commutation

A

B C

AC AB CB CA BA BC AC

A

C

B

Because the current is bi-directional, each phase has two steps per conducting interval. This is

called six-step commutation. For example, the commutation phase sequence can be AB-AC-BC-

BA-CA-CB. Each conducting stage is one step and only two windings conduct current at any

time, leaving the third winding floating. The un-energized winding can be used as a feedback

control that forms the basis for the characteristics needed for sensorless control algorithms.

To keep the magnetic field in the stator moving ahead of the rotor and to create optimal torque,

the transition from one sector to another must happen at exact rotor positions. Maximum torque

is attained via the switching circuit commutating every 60 degrees. All the switching control

algorithms are implanted in the MCU. The microcontroller can control the switching circuit

through the MOSFET drivers, which contain the appropriate response times, such as prop delays,

rise and fall times, and drive capability including the gate drive voltage and current sync required

to turn the MOSFET/IGBT to the “ON” or “OFF” state.

The rotor position is crucial in determining the correct moment to commutate the motor winding.

In applications where precision is required, hall sensors or a tachometer are used to calculate the

position, speed, and torque of the rotor. In applications where cost is the most important

consideration, the sensorless technique—which is the calculation of the back electromotive force

(EMF)—can be used to calculate position, speed, and torque.

Figure 8. Six Step Commutation




Back-EMF is defined as the

voltage that is created in the stator

winding by the permanent magnet.

This occurs when the rotor of the

motor is turning. There are three

vital back-EMF characteristics that

can be utilized for control and

feedback signals. The first is that

the magnitude of the back-EMF be

proportional to the speed of the

motor. For this reason, designers

use MOSFET drivers that can operate on at least two times the voltage supply. The second factor

is that as speed increases, the slope of the back-EMF signal becomes greater. The third is that the

back-EMF signal is symmetrical around the Crossing Event, as illustrated in Figure 9. Detecting

the Crossing Event precisely is the key to implementing the back-EMF algorithm. The back-

EMF analog signal can be translated to the MCU per a mixed signal circuit using high voltage op

amps and analog-to-digital converters which are widely used in most modern microcontrollers.

There needs to be at least one ADC per winding, for a total of three ADCs.

When using sensorless control, the start sequence is critical because the MCU doesn’t innately

know the rotor’s initial rotor position. The first step initiates the motor energizing two windings

at a time while taking several measurements from the back-EMF feedback loops and it does this

until a precise position can be determined.

Figure 10. Closed Control Loop

Figure 9. Crossing Event




BLDC motors generally operate using a closed loop control system that requires an MCU. The

MCU performs the servo loop control, calculations, corrections, PID controls, and sensor

management using back-EMF, a hall sensor, or tachometer (see Figure 10). These digital

controllers are generally 8-bits or higher and require EEPROM to store the firmware that creates

the algorithms necessary to set the desired motor speeds, direction, and to maintain motor

stability. The MCU often offers ADCs that allow for sensorless motor control architecture. This

architecture saves on valuable cost and board space. The MCU features construability and the

flexibility necessary to optimize the algorithms for the application. The analog ICs can give the

MCU efficient power supply, voltage regulation, voltage references, the ability to drive

MOSFETs or IGBTs, and fault protection. Both technologies can operate 3-Phase BLDC motors

efficiently and at a comparable price point versus induction motors and brushed motors.

Conclusion

The need to migrate towards higher efficiency BLDC motors in many markets and applications

is becoming increasingly commonplace. This is due to certain key benefits:

Higher efficiency (75% vs. 40% of an AC motor)

Lower heat generation

Higher reliability (no electrical contacts)

Safer to operate in a dangerous environment (no brush dust generated as in brushed

motors).

By using the BLDC motor in key sub-systems, the overall weight can also be reduced. This

means that the application can offer better fuel economy in vehicles. As the BLDC motor is

entirely commutated electronically, it is much easier to control the torque and RPM of the motor

and at much higher speeds. Around the world, many countries are facing insufficient power due

to electrical power grid deficiencies. To be certain, a small number of countries are now either

giving subsidies or getting ready to provide subsidies for the more efficient use of BLDC motors.

The BLDC deployment is but one of the many trends addressing the green initiative to save the

world’s precious resources without adversely impacting our way of life.

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