A quick search of the patents issued, as well as a review of recent articles, shows a wide range of what authors call “servo stepper motors,” “closed-loop step motors,” “hybrid servo motors” and the like. Although the names may sound similar, the performance can vary quite substantially. Most of the “closed-loop stepper motors” are not actually servo motors!
Transverse magnetic flux motors -- also known as step motors -- become Hybrid Servos when you operate them closed loop. This is the same transformation that happens between running a 3-phase synchronous motor from line voltage as opposed to running it closed loop as a brushless servo. In the case of hybrid motors, there are many degrees of what is advertised as "closed loop," and thus significant differences in the performance improvements seen.
The transverse flux permanent magnet motor -- also known as a hybrid step motor or hybrid servo motor -- has a wide range of performances, depending upon how you drive these motors, and whether you operate them in open loop or one of the many variants of closed loop methods you use. In this third installment we will cover some of the many ways to drive these motors, as well as how these choices affect the performance of these motors.