Brushless DC (BLDC) Motor Control Engineering: Three-Phase Inverters, Back-EMF & Field-Oriented Control (FOC)

Brushless DC (BLDC) and Permanent Magnet Synchronous Motors (PMSM) have superseded brushed motors in drones, electric vehicles, industrial robots, and precision CNC actuators due to superior torque-to-weight ratio, higher speed capabilities, and zero mechanical brush wear.

1. Motor Architecture & Six-Step Trapezoidal Commutation

A BLDC motor features permanent magnets on the rotating rotor and 3-phase electromagnetic windings (U, V, W) on the stationary stator. Electronic commutation is performed by a 3-phase inverter composed of 6 power MOSFETs (3 half-bridges).

In standard six-step commutation, at any given 60-degree electrical sector, two phases carry current (one driven high, one grounded) while the third phase floats, providing 6 distinct stator flux vectors that drag the rotor forward.

2. Sensorless Zero-Crossing Back-EMF Detection

In sensorless drives, three 120° Hall effect sensors are omitted. The un-energized third floating phase generates a trapezoidal back-EMF voltage. By filtering and monitoring when this voltage crosses the virtual motor neutral point (V_dc / 2), the microcontroller detects rotor zero-crossing and advances commutation by exactly 30 electrical degrees.

3. Field-Oriented Control (FOC) & Space Vector PWM

While six-step commutation creates noticeable torque ripple, Field-Oriented Control (FOC) vector control treats the 3-phase AC stator currents as a decoupled 2-axis DC system (d-axis for field flux, q-axis for torque):

• Clarke Transform: Converts 3-phase stationary coordinates (Ia, Ib, Ic) into 2-phase stationary orthogonal coordinates (I_alpha, I_beta).

• Park Transform: Rotates (I_alpha, I_beta) using rotor angle theta into rotating coordinates (Id, Iq) aligned with rotor flux.