Sensorless BLDC Motor Control

Sensorless BLDC motor control—sometimes called sensorless trapezoidal control of BLDC motors—uses back EMF (BEMF) for determining the location of the motor's rotor (the motor's rotating part) with respect to the motor's stator (the stationary part).

A voltage applied across a motor's winding forces the motor's rotor to turn. The movement of the rotor through the motor’s magnetic field, however, is analogous to the behavior of a generator, and consequently the motor not only receives an applied voltage but also generates its own voltage. This voltage is referred to as back electromotive force, or back EMF, and it is proportional to the motor's rotational speed. Back EMF can be used to determine a motor's rotor speed and position—no sensors are required. Controlling a motor by means of back EMF is not a simple task; most sensorless BLDC motors are controlled using a microcontroller, a digital signal processor, or a dedicated driver IC. The figure below shows a typical sensorless BLDC motor driver.

Figure 4. Typical sensorless BLDC motor drive.

The DRV10983 from Texas Instruments is a three-phase sensorless motor driver with integrated power MOSFETs capable of providing a continuous drive current of up to 2 A. It is highly integrated and requires few external components.

Figure 5. TI's DRV10983 Sensorless BLDC motor control driver. Image courtesy of Texas Instruments (page 1).

Not all sensorless BLDC motor controllers have integrated MOSFETs. Consider, for example, the A4964 from Allegro. This part requires the use of external N-channel power MOSFETs; it can operate in conjunction with a microcontroller or as an independent single-chip motor controller.

Figure 6. Allegro's A4964 sensorless BLDC controller can operate either with a microcontroller or as an independent motor controller. Image taken from the A4964 datasheet.

As mentioned earlier, the term trapezoidal is sometimes used when describing sensorless BLDC motor controllers. And when viewing the figure below, it's easy to see why: the voltage waveforms for each of the three motor phases have a trapezoidal shape.