Taking a brief look at ... motor commutation

Motor commutation refers to the technique used to properly sequence the various motor phases described above. By definition, therefore, a single phase device such as a DC brush motor does not require external commutation. Interestingly this does not mean the motor is not commutated, it just means the motor does not require external commutation. A DC Servo motor is commutated, however the commutation is performed inside the motor using brushes or contacts to continuously sequence the motor phases and keep the motor rotating.

For multi-phase devices, such as step motors or brushess DC motors, commutation must be performed externally by the control electronics. In the case of a brushless DC motor, commutation is typically performed using devices known as Hall-sensors. These signals allow the controller to excite each of the 3 motor phase coils at the right time based on the shaft angle of the motor. Hall sensors are used by the controller to create three motor coil currents located 120 electrical degrees apart.

Another popular brushless DC motor commutation scheme is sinusoidal commutation which typically uses the motor’s position encoder to generate continuously varying sinusoidal signals. Using this scheme these signals are also phased 120 electrical degrees apart. This technique results in smoother motion with no discontinuities in the resultant motor torque output.

In the case of a step motor most engineers do not think of this device as requiring “commutation” but this is definitely what is occurring. In the world of step motors the commutation techniques that the amplifiers employ are given special names such as full step, half step, or microstep drive. These different techniques refer to the number of different power levels that are applied to each motor coil during an electrical cycle.

A full step drive uses a full positive or full negative technique, a half step drive can separate the torque level into 3 distinct levels (full positive, 0 output, full negative) and a microstep drive can generate a more or less sinusoidal signal resulting in the maximum smoothness and highest degree of control. Whatever drive method is used, typical step motors have 2 phases and require that the phase signals be located 90 electrical degrees apart.

The following table summarizes the most common commutation techniques for each motor type along with the relative advantage and disadvantage of each method:

Common Configurations