Trapezoidal commutation is the most basic method of spinning a 3-phase Brushless-DC motor. This is accomplished by energizing the windings in a 6-step pattern every 60 electrical degrees so that one phase is sourcing motor current, another phase is sinking motor current, and the last phase remains unconnected (Hi-Z). This produces a 120° trapezoidal-shaped current waveform for each phase as shown in Figure 5-1.

Figure 5-1 Trapezoidal Commutation Phase Voltage and Current Waveforms

To determine the real-time position of the motor and commutate the motor to the next state, Hall sensors are commonly placed between the 3 motor phases to measure the motor’s magnetic field and output digital signals to the MCU as shown in Figure 5-2. The three hall sensor signals are accepted as logic inputs and a 6-step table is used to decode the next PWM control state. This is commonly referred to as Hall-sensored trapezoidal commutation, which is a low-cost, simple solution to implement that can generate high amounts of torque and speed and minimal MOSFET switching losses. However, it is low resolution and results in torque ripple and audible noise due to a non-ideal current drive.

Figure 5-2 Hall Sensor A/B/C Waveforms for Six-Step Commutation