Introduction

There are two standardized methods for driving Brushless DC (BLDC) motors or Permanent Magnet Synchronous Motors (PMSM): Trapezoidal Drive (or Block Commutation) and Sinusoidal Drive.

Trapezoidal Drive is simple to implement, even with 8-bit microcontrollers (MCUs), as it is a driving state machine with state switching dictated by the few feedback sources. In a Sensorless solution, the revolution indicator is the motor Back Electromotive Force (BEMF), which provides the position of the rotor every 60° based on the Zero-Cross Detection (ZCD) of each of the three motor's phases.

To measure the BEMF of a spinning motor, the coils must not be driven such that the signal is not distorted. This condition occurs when both the high-side, and this condition is encountered when both the high-side and the low-side MOSFETs of the half-bridge are driven to a logical low level.

BEMF measurement becomes possible in Trapezoidal Drive because two of the six steps of one complete electrical revolution fulfill the necessary condition one phase at a time, and the other phases’ interferences are filtered by hardware and software algorithms.

Sinusoidal Drive has less torque ripple, higher efficiency, and better torque at low speed when compared to Trapezoidal Drive, but with the cost of greater complexity. Moreover, in Sinusoidal Drive, all three phases are driven continuously. Thus, there is no window for the BEMF to be acquired directly.

The usual method used for BEMF acquisition in Sinusoidal Drive is Field Oriented Control (FOC), which requires detailed and fast individual coil current measurements, complex mathematical transformations, and dynamic signal reconstruction and prediction. For FOC, an MCU needs a clock frequency higher than 100 MHz, fast ADCs, and mathematical processing power found only in 16-bit and 32-bit cores.

The method presented in this technical brief creates the conditions necessary for a direct BEMF measurement while the drive is still sinusoidal and continuous for all three phases, thus eliminating the need for a complex acquisition and control algorithm.

The solution provides an easy way to obtain the ZCD from the motor for each 60° rotation, thus creating the possibility for drive synchronization to be simplified to a degree where even 8-bit MCUs can keep up and implement a simple Sensorless Sinusoidal Motor Control solution.