Since its successful development in the 1960s, brushless direct current (BLDC) motors have been proven to be more efficient and have a longer lifespan than previous brushed direct current (DC) motors. Along with the shift towards synchronous alternating current (AC) motors in high-power industrial applications, many other applications have also begun to use BLDC motors.
Nowadays, BLDC motors have penetrated into every aspect of consumers' daily lives. They can be found in battery powered tools such as drills and blowers, household appliances such as washing machines and printers, as well as electric bicycles and cars. In industrial environments, BLDC motors have been used for motion control and material handling applications. BLDC motors also provide power for unmanned ground vehicles (UGVs), drones, and similar unmanned aerial vehicles (UAVs), as well as surgical robots and assistive exoskeletons.
Brushed DC motors rely on metal or carbon commutator brushes to deliver electrical energy to the motor windings, while BLDC motors are non-contact. Due to the absence of friction and wear, it is more efficient, requires less maintenance, and has a longer lifespan for the motor. The performance of BLDC is also better, with faster speed, greater torque, and higher power to weight ratio. With the help of advanced control systems, BLDC motors can almost instantly change speed or torque and provide precise positioning to ensure safety.
The outstanding performance demonstrated by advanced BLDC motor drivers makes these motors and their control systems highly attractive to engineers designing modern robot and drone applications, which typically require features such as miniaturization, high speed, high precision, high safety, and low maintenance requirements.
Basic principle of BLDC motor
The BLDC motor has such a simple three part structure that it is simply unbelievable. The stationary stator is equipped with two to eight sets of copper windings, distributed on a circumference surrounded by or parallel to the rotor equipped with permanent magnets (Figure 1). The motor controller is connected to the stator to obtain position data and supply power to the winding.
Controller for three-phase BLDC motor
Figure 1: The three-phase BLDC motor controller changes the direction of the stator magnetic field by switching the energized state and current polarity of the stator windings (U, V, W phases). The rotor (blue part) with built-in permanent magnets rotates accordingly, thus maintaining the same direction as the stator magnetic field. (Image source: Qorvo)
Applying electricity to a set of windings in the stator will generate a magnetic field, and the permanent magnet of the rotor will respond to this magnetic field. The attraction between opposite magnetic poles causes the rotor to rotate. Before aligning the rotor with the stator magnetic field, the controller will switch the energized winding, change the direction of the magnetic field, and keep the rotor rotating continuously.
In fact, the current pulse sent by the controller to the stator will change from conduction to disconnection, and switch polarity at a certain frequency to represent the current using a certain waveform. The switching scheme shown in Figure 1 is represented by trapezoidal waves. Other types of motors, including permanent magnet synchronous motors (PMSM), have sine waves. This type of motor is structurally similar to a BLDC motor, but drives the magnetic field to rotate through varying currents, and the rotor remains synchronized and locked with the magnetic field. Adjusting the amplitude and phase of these waves can change the motor's speed and available torque.
The controller can also receive continuous feedback information from position sensors such as Hall effect sensors or photoelectric encoders. In sensorless BLDC motors, the measured value of reverse electromotive force (BEMF) - the current generated by the magnetic field generated by the energized winding in the unenergized winding - can be used to determine the position of the rotor.
Development of Motor Drivers
Given that the monitoring, power supply, and control of BLDC motors require complex structures, it is not surprising that old-fashioned BLDC motor controllers using solid-state electronic devices in industrial environments require independent cabinet space and bulky power and data cables to connect the motors. The increasingly sophisticated integrated circuits (ICs) are driving the continuous miniaturization of motor controllers, until they can be integrated onto printed circuit boards (PCBs). Despite achieving miniaturization, the functionality of today's motor controllers continues to expand.
For example, Qorvo's ACT72350 three-phase BLDC motor driver (Figure 2). This driver integrates a configurable analog front-end (AFE), a power management module adapted to various power configurations, and a dedicated motor driver (ASPD) into a 9 mm x 9 mm square flat no lead (QFN) surface mount device.
Qorvo ACT72350 Integrated Three Phase BLDC Motor Driver
Figure 2: The ACT72350 integrated three-phase BLDC motor driver integrates AFE circuitry and configurable power management functionality in a compact surface mount package. (Image source: Qorvo)
The configurable AFE of ACT72350 is equipped with three differential programmable gain amplifiers, four single ended programmable gain amplifiers, two 10 bit analog-to-digital converters, and ten comparators, making it a bridge connecting sensors and control circuits. This AFE can also receive pulse width modulation (PWM) control signals from an external microcontroller (MCU) through a serial peripheral interface (SPI).
The configurable power management module enables ACT72350 to accept DC input voltages ranging from 25 V to 160 V, including up to 20 seconds of battery capacity (nominal voltage of 72 V or 84 V when fully charged). The high-voltage switching power supply of this module can provide stable 12V or 15V output voltage, and can also provide stable 5V, 200mA power supply for ACT72350 modules and MCUs.
The ASPD of ACT72350 can use half bridge, H-bridge, or three-phase architecture to drive the motor (Figure 3). Three high-voltage side gate drivers with a voltage of 160 V and three low-voltage side gate drivers with a voltage of 20 V, each driver has a gate driving capability of 2 A (pulling current)/2 A (pouring current), which can achieve fast switching performance to improve motor speed.