Brushless DC motor drive solution - the definition of Brushless
The brushless DC (BLDC) motor can be imagined to be opposite to the brush type DC motor, in which the permanent magnetism is on the rotor, and the winding is on the stator. Therefore, the motor without brush and commutator, brush DC motor and eliminates the sparks related weaknesses.
The motor is referred to as a DC motor, because the coil is driven by a DC power supply, and the DC power supply is applied to different stator coils in the form of a predetermined order. This process is referred to as commutation. However, BLDC is not appropriate, because the motor is actually a AC motor. In the circuit loop, the current in each coil is positive and negative. The stator is a salient pole structure, which is designed to generate a trapezoidal back EMF waveform, which can be used in the waveform of the applied voltage waveform. But in practice it's hard to do, and the back EMF waveform is usually more like a sine, rather than a trapezoidal. As a result, many control techniques used by PMSM motors (e.g. field oriented control) are also applicable to BLDC motors.
Another misunderstanding of the BLDC motor is about how it drives. The stator coil is driven by an open loop step application to determine the position of the rotor, and the rotor position decides which stator coil to be driven in the BLDC motor. The stator flux vector position must be synchronized with the position of the rotor flux vector (instead of the opposite) to make the motor operate smoothly. To achieve this goal, it is required 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 harness will increase the system cost, and reduce the reliability of the system.
In order to reduce these problems, many technologies have been developed to eliminate these sensors, and then to achieve sensorless operation. Most of the techniques rely on the reverse electromotive force waveform of the stator winding to extract the position information when the motor rotates. However, rely on back EMF sensing technology in motor rotate slowly or stationary and useless, because this time counter EMF waveform is weak or nonexistent. Therefore, we continuously develop new technologies to obtain the rotor position information from other signals at low speed or zero speed.
BLDC motor in terms of efficiency rating is an absolute advantage, generally can reach about 95%. At present, the research of new amorphous alloy materials is pushing this figure to a new high. There have been reports that the efficiency of the 100W range is 96%. BLDC motor is still competing for the world's fastest motor, said some of the motor speed can be up to hundreds of thousands of RPM (one of which has been reported in the application of RPM 400K).
The most commonly used BLDC motor topology using 3 phase stator structure. Thus, the standard 6 transistor inverter is the most commonly used power level, as shown in fig.. According to the operation requirements (including sensors and sensor, commutation and sinusoidal PWM and SVM), many methods of the driving transistor to achieve the desired goals are too numerous to mention. The flexibility of the PWM generator, which is typically located in the microprocessor, is highly demanded.