6 Elements of Brushless DC Motor Control

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6 Elements of Brushless DC Motor Control [Copy link]

Rising consumer demands for power, reliability, functionality, and performance are driving the rapid development of electronic devices, including lawn mowers, refrigerators, vacuum cleaners, cars, and more. Manufacturers expect to deliver on all fronts. Motor control plays a major role in delivering on these promises, and understanding the fundamentals is the first step to achieving this goal.

Different motor types

There are several motor control topologies available today: brushed, brushless DC (BLDC), stepper, and induction. BLDC and permanent magnet synchronous motor (PMSM) are the two most closely related types of brushless motors.

Brushless motors are widely used in many of today’s applications because they do not require motor brushes. These BLDC topologies use commutation logic to move the rotor, which improves the efficiency and reliability of the motor. Let’s take a closer look.

Learn about BLDC and PMSM types of motors

BLDC and PMSM motors operate on the same principle as synchronous motors. The rotor continues to follow the stator at each commutation, so the motor continues to run. However, the stator windings of these two DC motors have different geometries, which results in different back electromotive force (BEMF) responses. The BLDC BEFM is trapezoidal. The BEMF of the PMSM motor is sinusoidal, so the coil windings are wound in a sinusoidal manner. To maximize performance, these motors are usually commutated with a sine wave.

BLDC and PMSM motors (Figure 1) generate an electromotive force through their windings when they are in operation. In any motor, the EMF generated due to motion is called back electromotive force (BEMF) because the EMF induced in the motor opposes that of the generator.

Figure 1: BLDC and PMSM motors typically use sinusoidal commutation.

Field Oriented Control Description

To achieve the sinusoidal waveform for controlling a PMSM motor, a field-oriented control (FOC) algorithm is required. FOC is often used to maximize the efficiency of a PMSM three-phase motor. Sinusoidal controllers for PMSM are more complex and more expensive than trapezoidal controllers for BLDC. However, the increased cost also brings some advantages, such as reduced noise and harmonics in the current waveform. The main advantage of BLDC is that it is easier to control. In the end, it is best to choose a motor based on the application requirements.

Sensored and sensorless BLDC and PMSM motors

BLDC and PMSM motors are available with or without sensors. Sensored motors (Figure 2) are suitable for applications that require the motor to be started under load. These motors use Hall sensors, which are embedded in the stator poles. Essentially, the sensor is a switch whose digital output is equivalent to the polarity of the detected magnetic field. A separate Hall sensor is required for each phase of the motor. A three-phase motor requires three Hall sensors. Sensorless motors require algorithms that use the motor as a sensor. They rely on BEMF information. By sampling the BEMF, the position of the rotor can be inferred, eliminating the need for hardware-based sensors. Regardless of the motor topology, controlling these motors requires knowledge of the rotor position so that the motor can be commutated effectively.

Figure 2: Schematic diagram of BLDC and PMSM motors.

Motor Control Software Algorithms

Today, software algorithms (a set of instructions designed to perform a specific task) such as computer programs are used to control BLDC and PMSM motors. These software algorithms monitor motor operation to improve motor efficiency and reduce operating costs. Some of the key functions in the algorithm include motor initialization, Hall sensor position detection, and switching signal checks to increase or decrease the current reference.

How the controller processes motor sensor information

The three-phase BLDC motor has six states. As shown in Figure 3, the three-bit code can represent the opcode number between 1 and 6. The sensor is used to provide three-bit data output through 6 of the 8 opcodes (1 to 6). This information is useful because the controller can determine when an illegal opcode is issued and perform operations based on the legal opcodes (1 to 6). The algorithm takes the Hall sensor opcode and decodes it. When the Hall sensor opcode value changes, the controller changes the power delivery scheme to achieve commutation. The microcontroller uses the opcode to extract the power delivery information from the lookup table. After the three-phase inverter is powered with a new sector command, the magnetic field moves to the new position, while pushing the rotor in the direction of movement. This process is repeated as the motor rotates.

Figure 3: A three-digit code can be used to represent opcode numbers from 1 to 6.