Taking a brief look at ... motor phasing

Motor phasing refers to the number of phases that are externally supported by the motor and which must be driven by the control system. A motor phase can be thought of as a complete electromagnetic circuit which must be driven by the controller (amplifier) to create torque. Some motors like a DC servo have only a single phase (at least externally) while other motors like the step motor have 2, 3, or 5 phases.

If the motor presents more than a single phase to the outside world it is referred to as a multi-phase device. If this is the case the controller must concern itself with the sequential excitation of these phases to maintain motor rotation. The technique of controlling the sequence of the phase excitation is known as commutation and will be discussed in the next section.

Each phase of a multi-phase motor requires an amplification circuit and therefore the number of phases a motor has is a key contributor to the cost of the overall system. For example, a 2- phase step motor, as the name implies, requires two separate amplification circuits while a 3-phase brushless DC motor will require three separate amplification circuits. If a motor amplifier is purchased as a complete unit then all of this circuitry is contained within the amplifier. Because of the overhead cost of the amplifier’s case, connections, etc., the relative difference in cost between, for example, a DC servo motor amplifier and a brushless DC motor amplifier is minimized. However, if you are constructing your own amplifier using ICs such as MOSFETs, drivers, etc., the choice of motor type will directly affect the number of components required and therefore the board space and board cost.

The following table summarizes the most common configurations of motor types, number of phases, and number of motor lead wires:

Most common configurations of motor types