Machine Topologies

SR machines can offer a wide variety of aspect ratios and salient pole topologies. This means that each application is likely to be better suited to a specific SR topology. Therefore it is difficult to give an overview of which topology offers what advantage or disadvantage without resorting to sweeping statements. However, here goes.

	Single-Phase Motor These are the simplest SR motors with fewest connections between machine and electronics. The disadvantages lie in very high torque ripple and inability to start at all angular positions. Maybe attractive for very high speed applications, but starting problems may preclude their use.
	Two-Phase Motor Problems of starting compared with single phase machines can be overcome by stepping the air-gap, or providing asymmetry in the rotor poles. This machine may be of interest where the cost of winding connections is important, but again high torque ripple may be detrimental.
	Three-Phase Motor Offers simplest solution to starting and torque ripple without resorting to high numbers of phases. Hence has been the most popular topology in its 6/4 form. Alternative 3-phase machines with doubled-up pole numbers can offer a better solution for lower speed applications. But again watch-out for torque ripple especially in the voltage control single-pulse operating mode.
	Four-Phase Motor Maybe popular for reducing torque ripple further, but the large number of power devices and connections will probably limit four phase to a limited application field. Five- and six-phase motors can offer better torque ripple reduction compared with four-phase and three-phase.