Brushless DC motor drive solution - brushless definition
Brushless DC (BLDC) motors can be imagined to be diametrically opposed to brush DC motors, which are permanently magnetically located on the rotor and wound around the stator. Therefore, the motor does not brush and commutator, eliminating the disadvantages associated with the brush type DC motor spark generated.
The motor is called a direct current motor, because the coil is driven by a DC power supply, and the DC power supply is applied to different stator coils in a predetermined sequence. This process is called commutation. However, BLDC is not appropriate, because the motor is actually an AC motor. In the circuit cycle, the current in each coil is positive and negative. Generally, the stator is a salient pole structure, which is designed to produce a trapezoidal back EMF waveform, as far as possible in line with the applied commutation voltage waveform. But in fact, it is difficult to do, the back EMF waveform is usually more like sine, rather than trapezoidal. Therefore, many control techniques used in PMSM motors, such as field oriented control, are also suitable for BLDC motors.
Another misconception about the BLDC motor is how it drives. The stator coil is different from the stator coil which is driven in the open loop step application. In the BLDC motor, the rotor position determines which stator coil to drive. The stator flux vector position must be synchronized with the rotor flux vector position (rather than the opposite) to make the motor operate smoothly. To achieve this goal, it is necessary to understand the rotor position to determine the stator coil to be driven. A variety of technologies can be used to achieve this goal, but the most commonly used technique is to use the Holzer effect sensor to monitor the rotor position. Unfortunately, these sensors and their associated connectors and wiring harnesses increase system cost and reduce reliability.
In order to reduce these problems, a variety of technologies have been developed to eliminate these sensors, and thus to achieve sensorless operation. Most of the techniques rely on extracting the position information from the back EMF waveform of the stator winding when the motor rotates. However, the technology based on back EMF sensing is useless when the motor rotates slowly or still, because the back EMF waveform is weak or nonexistent. Therefore, we continue to develop new technologies to obtain rotor position information from other signals at low speed or zero speed.
BLDC motor in terms of efficiency rating, the absolute advantage, generally can reach about 95%. At present, the research of new amorphous alloy materials is pushing this figure to a new high. It has been reported that the 100W range is 96%. BLDC motor is still competing for the world's fastest motor, said some of the motor speed can reach hundreds of thousands of RPM (one of the applications have been reported 400K RPM).
The most commonly used BLDC motor topology using 3 phase stator structure. Therefore, the standard 6 transistor reverser is the most commonly used power level, as shown in the figure. According to the requirements of the operation (including sensors and sensors, commutation and sine, PWM and SVM, etc.), there are many ways to drive transistors to achieve the desired goals, numerous. This is a general requirement for the flexibility of the PWM generator in the microprocessor!
