Design discussion based on three-phase BLDC motor control system

0 Introduction

Today, engineers use digital and analog techniques in motor control systems to address challenges that were faced in the past, including motor speed control, rotation direction, drift, and motor fatigue. The use of microcontrollers (MCUs) provides contemporary engineers with the opportunity to dynamically control the motion of motors, allowing them to respond to environmental stresses and conditions. This helps extend operating life and reduce maintenance, thereby reducing costs.

Currently, motor manufacturers tend to manufacture three-phase BLDC motors. The reason is that BLDC motors do not directly contact the commutator and electrical terminals (brush motors do), which not only reduces power consumption and increases torque, but also extends operating time. Unfortunately, compared with brushed DC or AC motors, three-phase motor control devices are more complex. In addition, the relationship between digital and analog components becomes very important.

This article will briefly discuss the issues that should be considered when using analog components and microcontrollers in three-phase BLDC motor applications. It will also focus on power management devices and power level shifters suitable for driving microcontrollers from power supplies ranging from 12V to 300V DC.

1 Source of demand for BLDC motors

Recently, designers prefer to use efficient BLDC motors. This trend applies to many markets and various applications. Currently, many applications can or have used BLDC motors to replace outdated AC motors or mechanical pump technologies. The important advantages of using BLDC motors include:

●More efficient (up to 75%, AC motor is only 40%)

● Less calories

●High durability (no wear as it is a brushless type)

●It is safer to operate in hazardous environments (no dust is generated, while brushed motors do).

Using BLDC motors in major subsystems can also reduce the weight of the entire system. Since BLDC motors are fully electronically commutated, it is easier to control the torque and RPM of the motor at high speed.Governments around the world are dealing with a lack of effective power due to insufficient power grids. In addition, many parts of the world must deal with power outages during peak demand periods. As a result, these countries are providing subsidies or preparing to issue subsidies to enable more efficient use of BLDC motors.

Table 1 Advantages of brushless DC motors

2 Strategic market segments and applications

2.1 Automobile

The automotive market contains many examples where mechanical and hydraulic pumps/movement controls are replaced. Specific applications include fuel pumps, power steering, seat controls, automotive HVAC (heating, ventilation and air conditioning) top window movement and windshield wiper motors. It is calculated that converting to BLDC motors can save about one mile per gallon of gasoline for each of these functions. This is due to the significant fuel savings and power efficiency.

Figure 1 Schematic diagram of the window glass lifter

2.2 Home appliances

Some appliances in the home appliance market can benefit from the use of efficient BLDC motors. These include pumps, fans, air conditioners, blenders, hand tools, and other kitchen appliances.

Figure 2 Block diagram of agitator motor control principle

2.3 Industrial Systems

Most pumps, fans, air conditioners, mixers and HVAC require motor drives. The European Union has issued a directive requiring all new industrial appliances to use three-phase "variable frequency drives" with BLDC motors.

Figure 3 Air conditioning principle block diagram

2.4 Large household appliances

Using high-efficiency BLDC motors can reduce the electricity usage of many washers and dryers.

Figure 4 Schematic diagram of washing machine motor

Table 2 Key areas of brushless DC motor drive

3 BLDC Motor Drive

There are several methods that can be used to drive a BLDC motor; some basic system requirements are listed below:

3.1 High Power Transistor

These are usually field effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs) that can withstand high voltages (as required by motors). Most home appliances use motors with 1/2 to 3/4 horsepower (1 horsepower = 734 watts). Therefore, typical current capabilities can reach 10A. For high voltage systems (usually >350V), IGBTs can be used.

3.2MOSFET/IGBT Driver

Typically, a set of MOSFET/IGBT drivers is used. Either a "half-bridge" driver or a three-phase driver is available. These solutions must be able to operate at twice the motor voltage to handle the back electromotive force (EMF) generated by the motor. In addition, these devices need to provide power transistor protection through set-up time and switching control to ensure that the top transistor is turned off before the bottom transistor is turned on.

3.3 Feedback elements/control

Designers should include some "feedback elements" in all servo control systems. Examples include optical sensors, Hall Effect sensors, tachometers, and the simplest "EMF sensing". Various feedback methods are useful, depending on the accuracy required and the required RPM and torque. Many consumer appliances typically use a sensorless technique called back-EMF sensing.

3.4 Analog-to-Digital Converter

In many cases, an analog-to-digital device is required to convert analog signals into digital signals and send the digital signals to the system MCU.

3.5MCU Microcontroller

All closed-loop control systems (BLDC motors almost always belong to this group) require an MCU to perform servo loop control, calculations, corrections, PID control, and sensor management. These digital controllers are usually 16-bit, but less complex applications can use 8-bit controllers.

3.6 Analog Power/Regulator/Benchmark

In addition to the components listed above, many systems also include auxiliary power supplies, voltage conversion and other analog devices such as supervisors, LDOs, DC/DCs and operational amplifiers.

Figure 5 Typical block diagram of 24V brushless DC motor control

4 Advantages of Micrel Motor Drivers

4.1 Power Driver

Micrel has a wide range of MOSFET/IGBT drivers suitable for all industry applications. Key parameters include: fast pulse delay, high peak current for gate charge/control and operating voltage up to 85V. For example, Micrel MIC4604 series can withstand back EMF motor voltage up to 85V.

4.2 Voltage Reference and Manager

Micrel offers a range of devices that are critical to operating an MCU. Examples include: MIC811, MIC2775 and MIC1232 voltage supervisor circuits.

4.3 Operational Amplifier/Comparator

Micrel has a range of low power operational amplifiers and comparators. These devices are essential to ensure accurate servo system feedback control. Examples include: MIC6270, MIC841N and MIC833.

4.4LDO

Micrel offers the industry's broadest range of LDOs, including fast transient LDOs, low input LDOs, minimum dropout LDOs, and high current LDOs. Examples include: MIC49150, MIC29150, MIC5235, and MIC5283.

4.5 DC/DC Switching Regulator

Micrel also offers a wide range of high efficiency DC/DC converters. These can be used for auxiliary power supplies and include the MIC2605 step-up and MIC4682 step-down (buck) switching regulators.

5 Basic operating principles of three-phase brushless DC motors

Brushless DC (BLDC) motors are synchronous motors with permanent magnets in the rotor and coil windings. They generate electromagnetic fields on the motor stator (see Figure 5). The electrical terminals are connected directly to the stator windings; therefore, there are no brushes or mechanical devices connected to the rotor (as in brushed motors). BLDC motors use a DC power supply and a switching circuit to generate bidirectional current in the stator windings. The switching circuit must use a high-side switch and a low-side switch in each winding, so a BLDC motor uses a total of 6 switches.

Modern motor designs use solid-state switches, such as MOSFETs or IGBTs, depending on the speed and voltage of the motor compared to relays. In addition, cost, reliability, and size must also be considered (see Figure 6). The switching current creates the appropriate magnetic field polarity, which attracts opposite polarities and repels like polarities. This generates a magnetic force that causes the rotor to rotate. Using permanent magnets for the rotor provides mechanical benefits to the designer; it also reduces size and weight. Compared with brushed motors and induction motors, BLDC motors have better thermal characteristics, making them an ideal choice to set off a new wave of energy-saving mechanical systems.

Figure 6 BLDC motor cross section

BLDC typically uses three phases (windings), each with a 120-degree conduction interval (see Figure 7).

Figure 7 Six-step commutation

Due to the bidirectional current flow, each phase has two steps per conduction interval. This is a tinned six-step commutation. For example, the commutation sequence could be AB-AC-BC-BA-CA-CB. Each conduction phase marks a step, and only two windings are conducting current at any time, with the third winding floating. The unexcited winding can be used as feedback control, forming the basis of the sensorless control algorithm feature.

In order to keep the magnetic field inside the stator ahead of the rotor and generate the best torque, the transition from one sector to another must be completed at a precise rotor position. Maximum torque is achieved by switching the circuit every 60 degrees. All switching control algorithms are contained in the MCU. The microcontroller can control the switching circuit through the MOSFET driver. The MOSFET driver contains the appropriate response time (such as maintenance delay and rise and fall time) and drive capability (including the gate drive voltage and current synchronization required to switch the MOSFET/IGBT "on" or "off" state).

The rotor position is important to determine the correct torque required to commutate the motor windings. In applications where higher accuracy is required, Hall sensors or tachometers can be used to calculate the rotor's position, speed, and torque. In applications where cost is a primary consideration, back electromotive force (EMF) can be used to calculate position, speed, and torque.

Back EMF is the voltage generated by the permanent magnets in the stator windings. This occurs when the motor rotor rotates. There are three main back EMF characteristics that can be used for control and feedback signals. First, the back EMF level is appropriate for the motor speed. Therefore, designers use MOSFET drivers that operate at a voltage at least twice the standard voltage. Second, the slope of the back EMF signal increases with speed. Third and last, the back EMF signal is symmetrical during the "crossover event" shown in Figure 8. Accurate detection of crossover events is key to executing the back EMF algorithm. The back EMF analog signal can be converted to the MCU per mixed signal circuit using high-voltage operational amplifiers and analog-to-digital converters (widely used in most modern microcontrollers). At least one ADC is required for each.

Figure 8 Crossover events

When using sensorless control, the enable sequence is critical, as the MCU is initially unsure of the initial position of the rotor. The motor is started first, both windings are energized, and several measurements are taken from the back-EMF feedback loop until the precise position is determined.

BLDC motors are typically operated using a closed-loop control system with a MUC. The MCU performs servo loop control, calculations, corrections, PID control, and sensor management (such as back EMF, Hall sensors, or tachometers) (see Figure 9). These digital controllers are typically 8-bit or higher and require EEPROM to store firmware to obtain the algorithms required to set the desired motor speed, direction, and maintain motor stability. Typically, the MCU provides an ADC that allows a sensorless motor control architecture. This architecture saves valuable cost and board space. The MCU is highly configurable and flexible to meet the needs of optimizing application algorithms. Analog ICs provide efficient power supplies, voltage regulation, voltage references, the ability to drive MOSFETs or IGBTs, and fault protection for the MUC. Both technologies can operate three-phase BLDC motors efficiently and at a price comparable to induction and brushed motors.

Figure 9 Closed-loop control

6 Conclusion

By using BLDC motors in mission-critical subsystems, weight can be reduced. This means more fuel savings in vehicle applications. Since BLDC motors are fully electronically commutated, it is easier to control the torque and RPM of the motor at high speeds. Many countries around the world are facing a shortage of effective power due to insufficient power grids. To be sure, a few countries are providing or are preparing to provide subsidies for more efficient use of BLDC motors. BLDC deployment is one of the trends that promote green environmental protection and save precious global resources without adversely affecting our lifestyles.