Ebook

Chapter 4

Six-Step Commutation

To better understand BLDC motor behavior when an external voltage is applied, we’ll use the configuration shown earlier where the rotor consists of a single pole pair while the stator is made up of three coils spaced at 120 degrees. The coils, which are here referred to as phases A, B, and C, can be energized by passing a current through them. The north pole of the rotor is shown in red, and the south pole in blue.

In the beginning, none of the coils is energized and the rotor is stationary. Applying voltage across two phases, here A and C, generates a combined magnetic field along the dashed line. As a result, the rotor starts to turn to align itself with the stator magnetic field as seen in the animation.

There are six possible ways of energizing coil pairs, shown below. By commutating two phases at a time, the resulting stator magnetic field is rotated, which causes the rotor to turn and end up in the positions shown as below. In the below animation, the rotor angle is measured with respect to the horizontal axis and there are six different rotor alignments, each 60 degrees apart from each other.

This means that if the correct phases are commutated every 60 degrees, the motor will continuously spin as can be seen in the following animation. The term for this type of control is six-step commutation or trapezoidal control.

More pole pairs can be added to such machines, requiring for the commutation to take place more often. To properly commutate the motor at the right times with the correct phases, the controller needs to know the exact position of the rotor at any time, which is usually measured by using Hall sensors.