Popular Science丨Six Elements of Brushless DC Motor Control

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Popular Science丨Six 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.

This blog post contains an excerpt from the Motor Control Fundamentals for Dummies eBook, which describes the key elements of motor control for brushless DC (BLDC) motor management and how to improve motor efficiency while reducing cost, saving space, and improving performance. You can download the entire book in the eBook section of the Qorvo Design Center.

Motor Control for Dummies https://www.qorvo.com/design-hub/ebooks/motor-control-for-dummies

E-book https://www.qorvo.com/design-hub/ebooks/motor-control-for-dummies

Here are some highlights from this e-book:

Different motor types

Several motor control topologies are 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 do not require motor brushes and are therefore widely used in many applications today. These BLDC topologies use commutation logic to move the rotor, which improves the efficiency and reliability of the motor.

Commutation in a brushed motor is achieved by the brush/commutator interface. This interface creates friction and arcing that degrades the performance of the brushes over time. This friction generates heat and shortens the life of the motor.

BLDC motors have many advantages over brushed motors. They are more energy efficient, smaller, lighter, quieter, more reliable, and more durable. In addition, they offer precise speed control, making them better suited for variable speed applications.

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

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 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 with a digital output that is equivalent to the polarity of the detected magnetic field. A separate Hall sensor is required for each phase of the motor. Therefore, 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 machines requires knowledge of the rotor position so that the motor can be commutated effectively.

Motor Control Software Algorithms

Today, software algorithms, such as computer programs (a set of instructions designed to perform a specific task), 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 6 states. As shown in the figure below, a three-bit code can be used to represent the opcode number between 1 and 6. The sensor is used to provide a 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 an action based on the legal opcode (1 to 6). As shown in the figure below, 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, pushing the rotor in the direction of movement. This process is repeated as the motor rotates.

The above content is excerpted from Qorvo Motor Control For Dummies. You are welcome to download our free e-book to get more insights on power management.