Servo System

When I was at school, unfortunately long time ago, the definition of a servo system was related to a system to what is now call a close loop system with at least one loop of control that has the feedback mechanism connected to the load. No matter the technologies involved. For instance a mechanical speed regulator on an diesel engine that is running an alternator (a generator) was (and still is) a servo system. Before the NeFeB magnet was invented (better say economically available for low/normal industrial applications), the highest performing servo systems was typically hydraulic (rockets was driven in that way). Before the NeFeB magnet, there was AlNiCo magnets or Dc motors but was not that easy get magnetic flux at 1+ Tesla, so they were not real competitors for high performance applications (that as I said at that time was hydraulic) and nobody was naming them servo system (even if they was servo systems).

The first applications based on NeFeB magnets in the motors (linear or rotational) were able to arrive very close to hydraulic, in some applications, so those guys start to use the word "servo" for their system because probably that was giving some help to their marketing. Theoretically a system that is based VSD should have the right to be named servo system just when it has a feedback system that is connected to the load. I've read a lot of comments were the word servo is associate to high performance. I have to say that on a lot of industrial drives, the micros that are handling the closing of the current and speed or position loops, are nicer in drives that are typically sold/specialized for PM motors. That anyway is just a marketing strategy because a drive that is controlling a PM or an Induction motor, from a power and control electronic stand point can be extremely similar.

About performance, I would remember to everybody, that a system with an SR motor that is running at very high speed, can reach higher performance (power density and ratio between system inertia/power) of a NeFeB system (retain the magnets at ultra high speed is not that simple, rotors stat to get heavier...) Control and SR motor require even more calculations power of what is needed for a PM motor because their high torque ripple... My conclusion is that the word servo, is just a marketing slogan....