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L99ASC03 Fully-Integrated Motor Driver IC for Three-Phase Brushless Motors Slide 6

The question is when should commutation happen? This is where the six-step drive mode comes into light. A three phase brushless DC motor is typically driven in six-step drive mode. Shown on the right side is the three phase inverter connection with the motor. Shown on the left side is the ideal current waveform of the motor. At any given time, only two out of three phases are conducting current. For example, when phase A and phase B conduct current, phase C is floating. This conducting interval lasts sixty electrical degrees, which is called one step. So totally, there are six steps in one cycle. A transition from one step to another is called commutation. From step one to step two, current commutates from phase B to phase C. This phenomenon can be used to read the back-EMF from a phase, which is floating and not conducting current.

PTM Published on: 2015-04-06