There seem to be some misunderstandings about types of motors and their control so I think it may be worth providing a little bit of clarification. I'm not going to quote all the relevant comments on this, but I though this was worth repeating:
Originally Posted by
yoko1
1. The stab trim motor has four speeds - Main Electric fast/slow (0.4/0.2 deg/sec) and Automatic fast/slow (0.27/0.09 deg/sec) and two directions. If the previously posted schematics are to be taken at face value, then the commands for speed and direction come from outside the motor unit, but the actual processing of those commands into a specific speed and direction occur within a controller housed within the motor unit. While no documentation regarding this controller and how it operates has been publicly released, it is reasonable to assume that the accident investigating bodies would have access to it if they felt it was needed.
It seems pretty clear from the schematics and descriptions from Eaton that the contemporary motor unit works by turning the 400Hz 3 phase supply into DC and then the motor controller turns this back into a 3 phase supply at the frequencies required to achieve the various designed speeds. Control of speed control this way over quite a wide range is fairly easy with modern power electronics. This type of speed control is very similar to that in inverter drives for induction motors, and the motor stator will be pretty similar to that of a three phase induction motor, but the rotor is likely to be permanently magnetized rather than magnetized by induction. The motor may have a sensor that provides feedback to the controller as to current rotor position, but it is also possible for control circuits to deduce this from the current/voltage relationships in the three phases. However this is not the same as a servo motor.
A servo motor implies a system where a position (rather than speed) can be demanded and there is a feedback loop using a position sensor (traditionally an analogue potentiometer) to allow the controller to determine the current distance from the demanded position. Any type of motor could theoretically be used in such a system. Positional control can also be achieved using a stepper motor, which can be rotated in small steps of specific angle and will remain locked in that position unless the holding torque is exceeded. This can be open loop (relative motion from a starting point) or incorporated in a system with feedback.
50 years ago power electronics were much more primitive and it was therefore much more difficult to control motor speed over a wide range. However it has been said that pre NG there were two separate motors and I would guess they were both three phase AC motors which can be speed controlled using relays rather than power electronics, by switching the sequence of connections for the stator poles, but only in quite a limited range of integer ratios typically 2:1 or 3:1 which happen to be the same ratios seen above (0.4:0.2 and 0.27/0.09). Different gear ratios between the two motor rotors and the output shaft would explain the different Main Electric and Automatic speeds. Eventually along come robust power electronics and whatever speeds are required can be provided with just one motor and probably more reliably (less mechanical parts (bearings, gears and relays) to wear out).