Commutation is used in motion systems to direct current through the windings of a three-phase motor. The two different forms of commutation are:
Trapezoidal Commutation (six state) directs current through two phases of the motor, based on Hall sensor feedback, which senses the position of the rotor relative to the stator. Because there are only six states of current per electrical cycle, the torque angle curve is not always at the ideal 90 degrees, resulting in torque ripple.
|Phase A||Phase B||Phase C|
Motor Phases – Trapezoidal Commutation (six state)
Three-Phase Trapezoidal Commutation
Sinusoidal Commutation directs current in the form of a sinewave through all three phases of the motor, which is sinusoidally synchronized by the drive. Each sinewave must be 120 electrical degrees from each other. Sinusoidal Commutation is achieved by utilizing an encoder feedback loop (algorithm) to maintain a 90 degree torque angle curve. Because the current is Sinusoidal, and there are no discontinuities in the current flow, there is theoretically zero torque ripple – although algorithmic commutation imperfections, winding uniformity, magnet placement variation, magnet width and strength variation, and non-zero ADC drive offset voltages, can attribute to some torque ripple. In many cases, Trapezoidal Commutation with hall sensors is used at motor startup, and then switches to Sinusoidal Commutation, using the encoder after the first Hall state change. This process avoids errors or inaccuracies from the drive’s commutation algorithm and takes advantage of Sinusoidal Commutation from the encoder(s) once in motion.
Three-Phase Sinusoidal Commutation
For information on converting units between Sinusoidal and Trapezoidal Commutation, read the ‘Motor Unit Conversions – What They Mean and How They are Used’ Tech Paper.