17.5.1 Six-Step Commutation of Three-Phase BLDC Motor

One method for controlling a three-phase Brushless DC (BLDC) is six-step commutation. Six-step commutation is also called 120° commutation, or trapezoidal control, and uses six steps or ‘sectors’ over one electrical cycle to energize a BLDC motor. Each sector is equivalent to 60 electrical degrees, with the six sectors resulting in 360 electrical degrees or one electrical cycle.

Sequencing through these steps moves the motor through one electrical cycle, which in mechanical terms, corresponds to one pair of rotor magnet poles moving past stator windings. A given BLDC motor has a certain number of pole pairs, defined by Np as a positive integer. If the motor is rotated one mechanical revolution, this corresponds to Np electrical cycles. This yields one electrical cycle for a 2-pole motor, two electrical cycles for a 4-pole motor, three electrical cycles for a 6-pole motor and so on.

A six-step commutation drive is typically implemented using a three-phase bridge circuit, as shown in Figure 17-39. Each phase of the motor is connected to a half-bridge driver and controlled with a complementary PWM pair output. At any given time in the six-step commutation scheme, only two of the three motor windings are energized. Current in the motor winding flows from one phase to another, in either direction.

Various PWM switching techniques can be employed for six-step commutation. The following Table 17-42 summarizes the three schemes presented in this manual.