Some of the key advantages of using a BLDC motor include higher efficiency (75% vs. 40%), less heat generation, higher reliability (no brush wear), and safer operation (no brush dust or arcing). Using these motors in key subsystems can also reduce the weight of the overall system.

　　And because BLDC motors are fully electronically commutated, it's much easier to control torque and speed at higher speeds.

　　Observe carefully

　　BLDC motors are synchronous motors with permanent magnets integrated in the rotor. Like AC induction motors, they have coil windings in the stator. The windings create magnetic fields on the stator pole pieces, which can be rotated (see image on the left). The electrical terminals are connected directly to the stator windings, so the rotor has no brushes or mechanical contacts.

　　These motors use a DC power supply and switching circuits to generate bidirectional current in the stator windings. The switching circuit consists of high-side and low-side switches for each winding. A total of six switches are required for a BLDC motor. This switching action causes the magnetic field of the stator to rotate.

　　Current motor designs use solid state switches (such as MOSFETs or IGBTS) instead of relays due to cost, reliability and size issues, depending on the voltage and speed of the motor. The switching current creates the proper magnetic field polarity, which attracts the opposite polarity and repels the same polarity. Magnetic force rotates the rotor. Using permanent magnets on the rotor has the mechanical advantage of reducing size and weight. Compared to brushed and induction motors, BLDC motors have better thermal characteristics, making them ideal for next-generation energy-efficient mechanical systems.

　　BLDC motors typically use three phases (windings), each with a conduction interval of 120 degrees.

　　Since the current is bidirectional, each phase is divided into two steps at each conduction interval. This is called six-step commutation. One commutation sequence option is AB-AC-BC-BA-CA-CB. Each conduction stage is called a step, and at any time only two phases conduct current, and the third phase is floating. The unenergized windings can be used as feedback control, which is the basis of the sensorless control algorithm.

　　In order for the magnetic field in the stator to remain ahead of the rotor, the transition from one sector to another must occur at the precise rotor position for optimal torque. Maximum torque is achieved by switching circuits commutated every 60 degrees. All switch control algorithms are embedded in the microcontroller unit (MCU). The MCU can control the switching circuit with a MOSFET driver with appropriate characteristics, such as propagation delay, rise and fall times, and drive capabilities such as gate drive voltage and current synchronization required to switch the MOSFET/IGBT to the on or off state.

　　The rotor position is the key to determining the correct moment to commutate the motor windings. In applications where accuracy is required, Hall sensors or tachometers are used to calculate rotor position, speed and torque. In applications where cost is a factor, back electromotive force (EMF) can be used to calculate position, velocity and torque.

　　Back EMF is the voltage generated in the stator windings by the permanent magnets as the motor rotor turns. Back EMF has three important properties that can be used for control and feedback signals. First, its size is proportional to the speed of the motor, so engineers use MOSFET drivers that can run at least twice the voltage of the power supply. Second, as the speed increases, so does the signal slope. Finally, the signal is symmetrical around the crossover event.

　　Accurate detection of zero-crossing events is the key to implementing the back-EMF algorithm. Back EMF analog signals can be sent directly to the MCU through its mixed-signal circuitry. Typically, only one or more high voltage op amps are needed. These amplify and level shift (as needed) and send the control signal to the ADC commonly available in most modern microcontrollers.

　　When using sensorless control, the startup sequence is important because the MCU does not know the initial rotor position. The first step starts the motor by energizing both windings at once, while taking multiple measurements from the back EMF feedback loop until the precise position can be determined.

　　BLDC motors typically operate in closed-loop control systems that require an MCU. The MCU performs servo loop control, calculations, calibration, PID control, and sensor management, such as back EMF, Hall sensors, or tachometers.

　　These digital controllers are typically 8-bit or higher and require EEPROM to store firmware that executes the algorithms needed to set the desired motor speed and direction and maintain motor stability. MCUs often have ADCs that allow sensorless motor control architectures, saving cost and board space. The MCU provides the ability to optimize the algorithm for the application. Analog devices provide MCUs with energy-efficient power supplies, voltage regulation, voltage references, the ability to drive MOSFETs or IGBTs, and fault protection. Both technologies allow you to efficiently use a three-phase BLDC motor at a price comparable to induction and brushed motors.

　　Globally, many governments are facing power shortages as a direct result of inadequate grids. Many regions are now facing power outages during the peak demand season. These governments are now offering or planning subsidies for more efficient use of BLDC motors. BLDC implementation is one of many green initiatives that can save our resources without affecting our way of life.