Trapezoidal Control

The trapezoidal (six-step) commutation makes two switching power devices on each motor phase in a pre-determined sequence. This method is popular due to the simplicity of its control algorithm. It uses a six step sequence using three Hall-effect sensors to get rotor position information. It is very effective at controlling motor speed, but suffers from torque ripple (vibration) during commutation, especially at low speeds.

The figures below show the layout of a BLDC motor and its hall effect sensors as the stator rotates.

Open

1

Open

2

As can be seen, the Hall effect sensors (1, 2 and 3) get triggered at different times and different combinations as the stator rotates. Therefore, each phase (A, B, C) has to be triggered appropriately. Note: Only two phases can be triggered at once.

The activation for each hall effect sensor can be graphed as shown below -

The high and low states of each sensor can be tabulated in binary form as shown below. Depending on the sensor state, the corresponding phase can be triggered to carry current, also tabulated.

Based on this table, the timing diagram for activating each of the phases can be created as below -