DESC:FIELD OF THE INVENTION The present invention is an indirect method of detecting mechanical overload situations and a remote method of diagnosing mechanical problems in single frequency induction motor actuators subject to unpredictable external loads. The present invention also encompasses wind turbine energy generating power plants (wind turbines) using this protection and diagnosis method for protecting the gearing of their yaw and blade pitch actuators against damage which may be caused by irregular aerodynamic forces generated by stormy or turbulent winds. The present invention thus defines a safe motor velocity range N2-N3 corresponding to reducer torque loads smaller than the rated torque load, a temporary velocity range N1-N4 which would cause overheating and excessive wear if it were allowed more than tem-porarily, and forbidden velocity zones permitting the diagnosis of blockage and brake slippage or failure, and a blockage detection threshold Nmin for detecting movement blockage, as well as a diagnosis aid. Discussion of prior art: Wind energy generating power plants, hereafter referred to as wind turbines, are intended to convert wind energy into another form of energy, in most cases electrical current, which may then be transported to end users, transformed or converted to other forms of energy. Wind energy generating plants (wind turbines) of the type shown in Figure 1 commonly include a rotor, often with blades resembling those of a propeller, which are caused to rotate under the influence of air flow resulting from the wind; this rotor rotation is trans-mitted via a shaft through an optional gearbox and converted into energy, preferably electrical energy, for instance by an electricity generator. For optimal operation in a sufficient range of wind speeds and wind directions, wind energy generating power plants require at least two types of actuators, namely yaw actuators and blade pitch actuators, the former being intended for orienting the nose cone of the rotor into the wind for maximum efficiency of the rotor, and the latter for controlling the pitch of each rotor blade, or feathering it, according to wind strength and/or rotor speed. As shown in Figure 2 (Prior Art), each of yaw actuators include a motor driving at least one drive pinion into rotation, often through a reducer, said drive pinion meshing into the teeth of a crown gear fixedly mounted on the top of tower. In stormy weather, in turbulent winds or by excessive wind strength, all major structural parts of the wind power plant are subjected to extremely large forces threatening the structural integrity of all essential wind turbine components, as well as possibly vibration and resonance problems, as the very large aerodynamic loads often are not exerted symmetrically on all blades. This can cause nacelle and tower vibration: The actuator systems are therefore very likely to sooner or later be subjected to intense and rapidly repetitive strain concentrated at the teeth of drive pinions which engaging the teeth of the main nacelle/tower crown gear and slew/yaw turntable bearing. This repetitive strain may cause metal fatigue and eventual failure in the teeth of the gear crown, of the actuator pinions and of the gearboxes, due to excessive applied torque. Since excessive torque resulting from storm effects typically will also be converted through the gearing into large motor torques, known solutions to this stormy weather problem have included detection and/or control of the motor torque, under the assump-tion that limiting the torque at motor level, or turning it off entirely so that it can freely rotate, will have the effect through the gearing of also limiting the torque on the drive pinion and the meshing teeth of the crown gear. Induction motors, also called AC asynchronous motors, which typically have several stator windings and a squirrel cage rotor as sole moving part, are normally used for this actuator application because of their extreme ruggedness, low maintenance and high power capabilities. Their torque/speed characteristic when connected to a standard single phase (with a starter capacitor), or three phase, fixed frequency supply such as that of the AC mains at 50Hz or 60Hz has been intensively studied in many textbooks, and in particular, the exemplary torque/speed characteristics of present Fig. 5 and 7 are theoretically discussed, along with the definitions of the motor and generator modes of any induction motor are discussed in textbook Woodson, Herbert H., and James R. Melcher; Electromechanical Dynamics., part 1: Discrete Systems, John Wiley and sons, 1968. Substantial disadvantages of connecting directly these motors directly on the AC mains have long been known and discussed in the above textbook. In particular the speed can only safely vary without instability in a narrow region around the fixed nominal speed Ns determined by the fixed frequency of the AC mains. Further, the torque vs. speed curve is as presented in Fig.7, with velocity regions R2 and R4 in which the torque vs speed slope is positive and thus yield an unstable operating point in critical high torque situations, such as during a wind storm: this may cause sudden motor stalling or violent self-reinforcing oscillations (for instance between points PB3 and PB2). Such wild, sudden behavior of the motor will subject the reducer gears to substantial strain in region R2; likewise, the motor will exhibit an uncontrolled speed runaway behavior in region R4 if wind gusts entrain the motor to a velocity of rotation higher than Nlg, the velocity corresponding to the lower extremum. High wind runaway behavior of single frequency induction motor actuators thus has been a serious safety problem of prior art actuators, because of its unrecoverable nature once allowed to start. Known but costly solutions to this runaway problem include larger brakes able to stop an already started runaway movement, larger dimensioning of the motors, and variable frequency drives which are able to move frequency Ns beyond Nd to recover stable control. The present invention proposes a simpler solution which includes giving advance warning and taking action while the operating point is still in the stable speed range, where the motor still provides stable braking action and is still able to effectively supplement the brakes, before the unstable speed range is entered. These operation point instability problems in critical high torque situations due to ex-traordinary external forces, causing either sudden stalling and/or self-reinforcing oscillations in the motor mode speed range or self-destructive runaway behavior in the generating mode speed range, can be problematic or even catastrophic in induction motor actuator applications such as wind turbine actuators or slewing units of tower cranes, or marine anchor windlasses, in which aerodynamic forces can rise very quickly to become the dominant influence on pinion torque, potentially causing an unrecoverable motor stall or destructive runaway situation at the worst possible moment. The above disadvantages, plus a few others such as the lack of speed controllability, and the non-gradual movement acceleration to a speed near the fixed nominal rotational speed (this can be particularly problematic in the presence of the large gyroscopic forces of a wind turbine) which does put more strain on the gears than a frequency controlled start, have hitherto seriously discouraged the use of AC asynchronous motors at fixed frequency in the role of yaw or blade pitch actuators, and have on the contrary strongly encouraged their use in combination with inverters and/or soft start circuits, which rem-edy all of the above-mentioned speed and stability problems of single-frequency operation and permit excellent torque observability and controllability even at very slow speeds, at the price of little added cost (the added price of the inverter and its sensors is partly compensated by a smaller size and therefore a smaller cost of the motor, not to mention a more advantageous, smaller weight), but significant added complexity, lower repairability, and higher sensitivity to lightning strikes and power surges. A more economical known method of using induction motors at fixed frequency while not relying on inverters has involved providing a torque limit signal whenever the absolute maximum torque ratings of the gears were about to be exceeded, or whenever the induction motor was about to leave its normal torque range. Such a torque threshold alarm normally gives sufficient advance warning time to feather the main rotor of the wind turbine or to apply the brakes, and disconnect the motors subjected to overly high torque loads before the situation degenerates and causes gear damage. One known direct torque sensing method involves trying to sense motor or gear torque directly through strain gauges directly mounted on motors or gear shafts. However such mounting places on rotating shafts are very exposed to damage and vibration, not easy to protect against stray signals, and generally make both the motor and the whole actuator a lot less rugged and reliable, and are difficult to communicate with because they cannot easily be connected by wire. Moreover this method would not be able to distinguish between motor mode torque and generator mode torque: this is a problem because gear reducers typically tend to withstand higher strains in motor mode and generator mode, so that another problem of the invention would be to be able to distinguish between generating mode measurements and motor mode measurements, so as to be able to fully use the torque ranges of the gear reducer in both modes without risking torque overload damage. Another known method involves single - phase sensing of the current or magnetic field intensity on one or more of the supply wires of the motor (either using a ferrite torus with a winding around one of the motor supply wires, by measuring the current through that winding, or using a small shunt resistance or a Hall sensor. This very simple and relatively economical method provides an almost straightforward reading of the torque, at least for motor speeds close enough to the nominal speed, by using the property of quasi-proportionality of torque and current intensity. However this single-phase current measurement has the following disadvantages: The relationship between current and torque varies with fluctuations of the mains volt-age: in case of overvoltage, this can cause a torque larger than the maximum absolute ratings to remain undetected, with fatal consequences for the gear reducer. By contrast, in case of undervoltage, the torque threshold involved in the decision of shutting down the induction motor actuator might become too low, and thus lead to premature turn off of the actuator motor, thus needlessly wasting actuator power, not using the wind power plant in the entire range of its wind strength and torque capabilities, and needlessly reducing the overall duration of the power producing time windows. Finally with single phase current sensing, the same current threshold has to be used for all three cases of emergency shutdown of the motor, namely: - blockage of the mechanical power converting mechanism - motor mode torque surcharge - generating mode torque surcharge This common threshold has at least two disadvantages, one for remote monitoring and diagnosis, and the other for the selection of individualized optimal motor shutdown torques: Since the same current threshold is used, it is not possible to distinguish between the above three causes of threshold excursion, and this is a serious disadvantage in particular if this data is remotely transmitted to a faraway remote monitoring center which has to decide which action to take on the basis of this insufficiently differentiated diagnosis information. Moreover, due to the different natures of the three cases of emergency shutdown of the motor, the ideal torque thresholds for all three should ideally be different, but it is not possible to make them different if only a current measurement is made. In order to solve at least the two aforementioned problems, and more easily differentiate between the three kinds of threshold excursions, it is possible to add a motor speed sensor such as a dynamo, but this does not solve all problems, in particular not the threshold dependency on the AC voltage amplitude, and does complicate the sensor arrangement, decreasing its reliability and increasing its cost. Problems solved by the invention The inventors have counter-intuitively realized that, although starting from the above described single phase current sensing and trying to improve it with a speed sensor would not solve all problems, very surprisingly just getting rid of the current sensor altogether and only keeping a speed sensor as sole sensing device provides a better solution. A most preferred embodiment of this better solution involves detecting a plurality of speed thresholds on the speed sensor output instead of detecting a single current threshold, because this method offers unparalleled opportunities to solve all problems mentioned above. This approach was also felt to be counterintuitive and apparently goes against technical prejudice because the range of speed values of an AC asynchronous motor in its usable range of operation is very narrow, thus demanding great precision and repeatability from any speed sensor, a priori increasing its cost; moreover, as the most important use of the sensor is to detect both generator mode and motor mode overloads in stormy weather, the speed fluctuations to be expected due to random and repetitive vibrations which will not fail to be superimposed on top of the useful speed signal in the presence of strong aerodynamic wind effects on a nacelle buffeted by stormy winds can a priori become quite a bit larger than the useful signal, casting doubt on the robustness of this method in presence of signal irregularities due to stormy weather. However the inventors have found that in actuators mounted on structures prone to contributing this kind of stormy weather pollution of the speed signal, the robustness of the system can be improved by filtering or averaging the speed signal from the speed sensor through an adequately chosen averager or low-pass filter having an averaging duration or time constant longer than two periods of the repetitive fluctuations of the speed signal. It is a principal object of the invention to give adequate advance warning of such potential stalling situations before they occur by signaling occurrences when the motor torque approaches stall values by specifying short-time torque ranges 102 and 103 (see fig.5) corresponding to high torque values barely sustainable by the motor 21, the associated reducer 22 and the pinion 23, and detecting (S6, S8 on the flow-chart of Fig.6) when they occur. AC asynchronous motors operated at fixed frequency are quite efficient and powerful but typically rotate relatively fast in a narrow range of speeds around their nominal speed. In order to bring their rotational speed to a more usable range, they are often used in cooperation with mechanical power converting mechanisms such as gearboxes, gear reducers or hydraulic pumps, which are able to divide the rotational speed by a fixed amount K and to multiply the motor torque to the very high values required for reliably moving large components of large or heavy apparatus such as drawbridges, wind turbine nacelles or rotor blades, cranes, radar antennas, etc., even in stormy wind conditions. However, the resulting large wind loads on such large components may at times exceed the rated loads of the mechanical power converting mechanisms, especially in cases where the motor is powered and the motor torque combines with the applied torque due to the wind load. According to another embodiment of the invention, a safe AC motor velocity range is defined, corresponding to gear torque loads smaller than the rated torque load, a temporary velocity range which would cause overheating gear tooth fatigue and excessive wear if it were allowed more than temporarily, and crucially a plurality of separately identifiable forbidden velocity zones identified by respective separate speed thresholds permitting the diagnosis of blockage, runaway and brake slippage or failure. Diverse further methodologies for AC asynchronous motor control with the help of inex-pensive microcontrollers are further taught in Texas Instruments Application Notes, in particular in Application Note bpra043 entitled “Digital signal processing solution for AC induction motor” which is hereby incorporated by reference. These disclosures not only teach the detailed physics of AC induction motors and very convincingly encourage the reader to use their lowest cost version, the squirrel cage mo-tor, with vector control rather than scalar control using either current sensing or position or velocity sensors to control the inverter, they also list a series of serious disadvantages discouraging the user from trying to use an AC asynchronous motor without any inverter at all, directly from the AC mains, without variable frequency supply. Chief among these arguments is the mechanical wear and tear on any gear reducer, resulting from the lack of a soft start solution. This argument, rather widespread in the prior art, may at first appear plausible, and would in fact be correct if the only design parameters for the reducer were the motor torque and load inertia, without external mechanical or aerodynamic effects on the load. This argument is however considered to be flawed and technically prejudiced in the case of actuators dimensioned for erratic worst case loads, such as is the case for wind tur-bines, in which the torque dimensioning of drive pinion 23 and gear reducer 22 is actually determined and dominated according to Equation 1 (eqn.1) not by the demultiplicated nominal motor torque KT0 and correspondingly higher inertial startup torque value Tj due to the rather more brutal start of a single frequency induction motor (which in the case of a calm weather start would be, assuming other torques to be minimal, substantially equal to KT0 ), but by the much higher aerodynamic torque transients Ta of worst case stormy weather. Indeed, the torque Tdp on the drive pinion 23 and on the gear teeth of the final gear stage of gear reducer 22 of wind turbine yaw actuators can be roughly modeled as follows: Tdp = KTm +Ta +Ti +Tg +Tf + Tj (eqn. 1) where K = reducing ratio of gear reducer 22 Tm = motor torque Ta = partially repetitive, partially random torque due to Irregular /imbalanced aerodynamic forces on different rotor blades (e.g. ground effect, turbulent winds) Ti = sum of repetitive torque effects due to static or dynamic rotor imbalance Tg = gyroscopic forces (to minimize this, nacelles are typically rotated very, very slowly!) Tf = friction forces (main bearing, meshing gear teeth, nacelle yaw brake, gear reducer losses) Tj = effect of inertia of all components due to rate of change of relative speed (rotational acceleration) between tower and nacelle, either due to the influence of a motor torque (motor suddenly turning on), or due to cyclical variations/tower repetitive torsion/vibration modes, etc. In order to reduce this pinion strain Tdp to an acceptable level even in the worst weather conditions when the brake is not used, it is commonly known to spread this loading between several yaw drive actuators 4 of the types shown on Fig. 2 and Fig. 3 and to tighten a nacelle yaw brake 17, which is used to prevent any rotation of the nacelle supporting frame 28 (see Fig.3) around the nacelle axis according to nacelle yaw main bearing 5 whenever the yaw actuators are not in use. Instead of an active nacelle yaw brake 17 a friction bearing could be used to prevent undesired yawing of the nacelle. In this context the friction provides a breaking mechanism. In other words, for a properly dimensioned wind turbine actuator, the nominal motor starting torque even without inverter control or soft start circuit, is likely to be significantly smaller than the design torque rating of the gear reducer and of pinion 23, because said rating has to be dimensioned not according to KTm = KT0 but according to the significantly higher value Ta. This means that the traditionally alleged need for inverters or variable frequency speed regulation of the motor in order to avoid the occurrence of violent startup torques is actually useless and counter-productive (it decreases the overall reliability and cost of the drive, and needlessly increases its response time), because the optimal pinions and gear reducers anyhow have to be sized bigger than the motor alone would demand because of aerodynamic constraints, so that the motor will anyhow not be able to cause much wear and tear to them in a normal calm weather startup situation, and might only exceptionally contribute to the design limits or absolute maximum ratings to be exceeded only when a startup is attempted while the wind is approaching storm levels. In addition, for certain markets where initial cost is all-important, and where the availa-bility of highly skilled technical personnel for repair of complex power electronics circuitry such as inverters is not guaranteed, the inventors reckon that the electronic complexity of the prior art circuits can be a problem in case any repairs will have to be handled by the untrained village electrician in the midst of the rainy season, working from a boat because roads are impassable and prevent timely and cost-effective delivery of spare parts, and reckon that contrary to some of the technical prejudices taught in the above mentioned documents, a very simple and rugged system and very economical systems for reliable torque limitation can be built for these markets, limiting wear and tear to acceptable limits. In addition, wind turbines being high structures, they are frequently hit by lightning, which tends to play havoc with sensitive microelectronics, microcomputers or the like, and therefore an extremely rugged solution which can work entirely without any kind of electronics may have a strong appeal in certain situations, not least as quick and easy-to-implement retrofit solution or radical repair solution for wind turbines. Such a retrofit solution would involve simply throwing away the existing inverter control, which possibly may have been damaged beyond repair by lightning transients or short circuits, just keeping/reusing the velocity or position sensor that provided it with feedback, and replacing it by an inexpensive kit according to the invention, involving rugged, low cost and easy to comprehend low technology solutions such as mere relays and adjustable potentiometers within the technical reach of any developing country village technician. DESCRIPTION OF THE INVENTION The invention as described below sets out to solve all the above mentioned problems in the unconventional manner stated below, and not just for wind turbines but also for any other kind of inverter-less fixed-frequency induction-motor driven actuator systems such as slewing units for tower cranes, marine anchor winches and the like, which may be exposed to irregular and unpredictable loads such as turbulent fluid conditions, wind, waves, etc., and/or also exposed to frequent lightning strikes. To solve the above objectives, the present inventionteaches an indirect torque sensing method for an actuator drive assembly using an induction motor for moving a first appa-ratus component relative to a second apparatus component as described in the present invention, as well as an actuator system, a torque-overload-protected drive assembly system for moving a first apparatus component relative to a second apparatus, a wind turbine, an azimuthal drive assembly for a wind turbine, a retrofit kit for wind turbines, a wind turbine management system using said method managing a wind-farm or wind-park, i.e. a system of several associated wind power plants, and a diagnosis aid of the invention using said sensing method. The torque-overload-protection infers only from an inferred velocity that a torque exerted on or by said motive element has exceeded a predefined threshold. Speed is proportional to the torque within an operating range of the system. An embodiment of the present invention discloses a motive arrangement for driving a motive element of an apparatus with respect to a second part, said motive arrangement including: -a driving element for driving engagement with the second part; -a driving unit comprising a housing to be fixedly connected to the motive element, a motive generation means for generating motion of the motive element, and a motion transferring means for transferring the motion of said motive generation means to said driving element; and -a velocity detector for inferring a velocity of said motive generation means -a torque monitoring unit for inferring only from said inferred velocity whether a torque exerted on or by said motive element has exceeded a threshold. Another embodiment of the present invention discloses the motive arrangement, wherein said motive generation means is an induction motor. Another embodiment of the present invention discloses the motive arrangement, wherein said torque monitoring unit operates on the basis of rotational speed. Another embodiment of the present invention discloses the motive arrangement, wherein said motive generation means has at least three windings supplied with fixed frequency alternating current. Another embodiment of the present invention discloses the motive arrangement, wherein said torque monitoring unit monitors at least four different velocity thresholds. Another embodiment of the present invention discloses the motive arrangement, wherein said torque monitoring unit causes disconnection of said motive generation means in case predetermined thresholds are exceeded or repeatedly exceeded or not reached within a certain time span. Another embodiment of the present invention discloses the motive arrangement, wherein said torque monitoring unit transmits information about which thresholds were exceeded as diagnosis data to a remote monitoring unit or monitoring center, and/or records said information as time-stamped data in a local memory device. Another embodiment of the present invention discloses a wind energy power plant in-cluding the motive arrangement as described in the present invention. Another embodiment of the present invention discloses an actuator drive assembly sys-tem for moving a first apparatus component relative to a second apparatus component, including a drive assembly which includes: - an AC asynchronous motor or induction motor directly powered at a predetermined, constant and fixed frequency by AC Mains (no inverter, fixed frequency); - a mechanical power converting mechanism for converting an input mechanical power supplied by the motor into an output mechanical power suitable for moving said first component relative to said second component; - a speed sensor for providing a speed signal sensing the rotational speed of said motor and characterized in that it includes a torque monitoring unit including at least one speed comparator adapted to compare two or more predefined speed thresholds with said speed signal so as to permit inference of the sustained motor-mode torque or driven generating mode torque from the measured speed, whereby the speed comparator output directly controls actions intended to reduce the motor torque below acceptable limits or to return the speed of the motor to a predetermined intended range. Another embodiment of the present invention discloses an actuator drive assembly sys-tem such that said predetermined, constant and fixed frequency is single phase or three phase AC Mains current of fixed frequency, used without any inverter or soft start circuit. Another embodiment of the present invention discloses an indirect torque sensing method for a drive assembly for moving a first apparatus component relative to a second apparatus component, said drive assembly including: -An AC asynchronous motor or induction motor; -a mechanical power converting mechanism for converting an input mechanical power, torque, speed or movement supplied by the motor into an output mechanical power, torque, speed or movement suitable for moving said first component relative to said second component; -a velocity sensor for sensing the velocity of a component of said drive assembly, for in-stance that of a motor driveshaft and outputting a velocity signal commensurate to a velocity value (N) of the motor such that said motor is supplied exclusively with fixed frequency AC current of constant frequency f0 (50Hz or 60Hz); -the torque control method being characterized in that the motor velocity value N derivable from the velocity signal is periodically compared to two or more predetermined threshold velocity values (N1, N2, …) in order to determine and signal, solely on the basis of this comparison of the motor velocity value N with predetermined threshold velocity values, whether or not a maximum torque rating of said mechanical power converting mechanism is being exceeded or not. Another embodiment of the present invention discloses the indirect torque sensing method, wherein at least two different thresholds (N2,N3)of said at least one or more predetermined threshold velocity values (N1,N2, …) define a safe torque range (T2,T3) for long duration operation of said mechanical power converting mechanism according to the speed/torque characteristic (Fig.5) of said motor, and wherein any variation of the motor velocity value N outside of said safe torque range as revealed during normal operation by said comparison with said two different thresholds (N2,N3) triggers an overload signal message. Another embodiment of the present invention discloses the indirect torque sensing method, wherein at least two different thresholds of said at least one or more predetermined threshold velocity values (N1,N2, …) define according to the speed/torque characteristic (Fig.5) of said motor at least one temporary overload torque range of said mechanical power converting mechanism (T1 to T2 or T3 to T4) and at least one maximum temporary overload duration (Dmh or Dgh) of said mechanical power converting mechanism for short duration operation, said maximum overload duration being defined as the duration over which an applied torque in the temporary overload torque range would cause overheating or lasting damage to said mechanical power converting mechanism, wherein any variation of the motor velocity value N inside of said temporary overload torque range as revealed by said comparison with said two different thresholds (N2,N3) triggers a new comparison after a time period corresponding to said maximum temporary overload duration, whereby a time limit expiration message is sent and the motor is de-energized unless the motor velocity value N has returned inside of said safe torque range as revealed by said comparison with said two different thresholds (N2,N3). Another embodiment of the present invention discloses the indirect torque sensing method, wherein at least four different thresholds (N1,N2,N3,N4) of said at least one or more predetermined threshold velocity values (N1,N2, …) define according to the speed/torque characteristic of said motor at least two distinct temporary overload torque ranges (T1 to T2 and T3 to T4) and corresponding maximum overload durations (Dmh and Dgh) for short duration operation in those temporary overload torque ranges, said maximum overload durations being defined as the durations over which an applied torque in the corresponding temporary overload torque range would cause overheating or lasting damage to said mechanical power converting mechanism wherein any variation of the motor velocity value N inside of said temporary overload torque range as revealed by said comparison with said two different thresholds (N2,N3) triggers a temporary overload signal message and a new comparison after a time period corresponding to said maximum temporary overload duration, whereby a time limit expiration message is sent and the motor is de-energized unless the motor velocity value N has returned inside of said safe torque range as revealed by said comparison with said two different thresholds (N2,N3). Another embodiment of the present invention discloses the indirect torque sensing method, wherein at least one (T3, T4) of said two distinct temporary overload torque ranges of said mechanical power converting mechanism corresponds according to the speed/torque characteristic of said motor to generating mode velocities in a velocity range (N3,N4) greater than the nominal motor velocity Ns at said frequency f0, said generating mode velocities corresponding to cases where external influences, such as wind gust effects on a yawing wind turbine turning away from the wind direction, on said first or second apparatus components are mechanically driving the motor over its nominal speed through the mechanical power converting mechanism, forcing it in a velocity range where it no longer acts as a motor but tends to resist the effect of this external influence by entering its generating mode. Another embodiment of the present invention discloses the indirect torque sensing method, wherein the said safe torque range (T2,T3) of said mechanical power converting mechanism corresponds according to the speed/torque characteristic of said motor to velocities in a velocity range (N2,N3) having an upper velocity threshold or boundary (N3) beyond the nominal motor velocity Ns at said frequency f0, said upper velocity threshold or boundary corresponding to cases where external influences on said first or second apparatus components are mechanically driving the motor over its nominal speed through the mechanical power converting mechanism, forcing it in a velocity range where it no longer acts as a motor but tends to resist the effect of this external influence by entering its generating mode. Another embodiment of the present invention discloses the indirect torque sensing method, wherein the velocity signal is measured a predetermined time interval Ds after motor start, whereby the motor speed N at that instant is compared to a predetermined minimum speed threshold Nmin in order to identify an error, problem or damage of the drive assembly, such as a blockage, whereby a blockage message is sent and the motor is de-energized unless the motor velocity value N exceeds said predetermined minimum speed threshold Nmin as revealed by said comparison. Another embodiment of the present invention discloses the indirect torque sensing method, wherein the drive assembly is fitted with a brake, such that the brake can be applied or engaged whenever the motor is de-energized, and such that the velocity signal is monitored and compared to a very low velocity threshold Nb whenever the brake is applied or engaged in order to signal an abnormal brake slippage or defect if the threshold is exceeded, whereby a brake slippage message is sent unless the motor velocity value N remains less than said predetermined minimum speed threshold Nb as revealed by said comparison. Another embodiment of the present invention discloses a torque-overload-protected drive assembly system for moving a first apparatus component relative to a second apparatus component, said drive assembly system comprising a drive assembly, which in turn includes: -An AC asynchronous motor or induction motor; -a mechanical power converting mechanism for converting an input mechanical power, torque, speed or movement supplied by the motor into an output mechanical power, torque, speed or movement suitable for moving said first component relative to said second component, -a velocity sensor for sensing the velocity of a component of said drive assembly, for in-stance that of a motor driveshaft and outputting a velocity signal commensurate to a velocity value (N) of the motor characterized in that said drive assembly system also includes: -A torque monitoring unit including a velocity comparator for comparing the measured speed signal from said velocity sensor to one or more preset velocity values, outputting a motor power shut-off signal signaling excessive torque solely on the basis of said velocity comparisons. Another embodiment of the present invention discloses an azimuthal drive assembly for a wind turbine, including one or more torque-overload-protected drive assembly systems, each such system according to any of the preceding devices and/or employing a method as disclosed in the invention, said first apparatus component being a wind turbine nacelle, a main yaw bearing supporting said nacelle on a tower and allowing it to rotate around a vertical axis, said tower constituting said second apparatus component, an azimuthal brake and its brake controller, wherein the brake is adapted to be automatically applied in case of torque overload identified by velocity comparison in the torque monitoring unit. The motor is shut off at the same time that the break engages. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 represents a schematic perspective view, partly in cutaway form, of a wind tur-bine nacelle on its tower, also showing exemplary blade pitch control units of the inven-tion and nacelle azimuth angle control drive assemblies of the invention. Figure 2 represents a more detailed perspective view of an embodiment of a drive assembly of the invention suitable for the nacelle represented in the previous figure. Figure 3 represents a detailed section view of a drive assembly in its nacelle mounting through a vertical plane containing the nacelle rotation axis in accordance with another embodiment of the present invention. Figure 4a represents a block diagram of a nacelle yaw drive assembly and control system embodiment of the invention and of its interactions with a nacelle brake. Figure 4b represents a simple and rugged implementation of an embodiment of the invention relying only on standard electromechanical elements such as relays rather than any kind of electronics, for use in remote, poorly accessible regions, marine environments or difficult environments plagued by lightning strikes or power surges. Figure 5 represents a typical velocity/torque characteristic of an AC asynchronous motor of a drive assembly in its mounting in accordance with another embodiment of the pre-sent invention, with specification of the torque range limitations of its gear reducer and deduction of the corresponding velocity thresholds N1-N4 and Nmin of the invention. Figure 6 represents an apparatus controller flow-chart suitable for implementing an embodiment of the present invention. Figure 7 represents a typical torque versus speed characteristic of an induction motor drive operated at a fixed frequency and driving loads LA, LB (which are a combination of friction loads and external forces) for discussing regions R2 (acceleration to stable) and R4 (unstable operation). DETAILED DESCRIPTION OF THE INVENTION In the following figures, the same or similar types of elements or corresponding parts are provided with the same reference numbers in order to prevent the item from needing to be reintroduced. Fig.1 shows a perspective representation of a wind energy generating plant 1, also referred to as a wind turbine in the following, for converting wind energy into mechanical energy via a rotor resembling a propeller, its rotation under wind influence generating electricity by means of an electricity generator 16, which may be an alternator or dynamo, usually mounted in a nacelle 2 placed on top of a tower 3. The rotor can be turned into the wind 10 by actuating one or more yaw actuators 4, each having drive pinions 23 cooperating with tower crown gear 8 for rotating the nacelle 2 around the vertical axis of one or more nacelle yaw bearing(s) 5 in any of the clockwise or counterclockwise directions 29. The engineering details of the above mentioned elements, as well as the structure of a wind energy generating power plant and methods for its operation and management are generally well known by wind power engineers, in particular from the textbook entitled: “Windkraft Systemauslegung, Netzintegration und Regelung”(Wind Power System De-sign, Grid Integration and Control), by Siegfried Heier, 4th Edition, published by B.G. Teubner, Stuttgart, Leipzig, Wiesbaden, Germany, February 2005. In the interest of conciseness, the above textbook is incorporated by reference in the present description to avoid repeating in the present description various engineering, design and dimensioning details and interactions of the various elements of wind power systems which are already commonly known by every wind power plant engineer, substantially as described in this textbook. The preferred embodiments of the present invention as described below are particularly directed to the design, use, system integration, system monitoring, system protection and system diagnosis involving a specific kind of actuator 4, namely an actuator driven by an AC asynchronous electric motor 21, also known as induction motor, operated exclusively at a fixed predetermined AC frequency, in particular without the use of an inverter, a variable frequency drive (VFD) or a soft start circuit. AC asynchronous electric motors are extremely rugged, economical and dependable and their ruggedness and dependability can be further increased by using them without any power electronic circuits when speed variability or very precise torque control is not an issue, as the above discussion of dimensioning constraints of azimuth drives equation 1 has shown: the added complexity of such circuits adds costs, surge protection and reliability issues, which may become decisive in applications subjected to frequent power surges or where simplicity and dependability is paramount for safety reasons. However, the actuator solutions and teachings described below are not only suitable exclusively for the technical field of wind turbines, but are applicable in any fixed AC frequency induction motor driven actuator application involving risks of torque overloads, torque instability problems, torque runaway problems and the like, which are in particular due to unfortunate interaction of large and unpredictable forces on the actuator loads with the torque /speed characteristic of the motor, to offset most of the disadvantages, torque instability problems and dangerous idiosyncrasies of induction motors used in these applications. Specifically, all embodiments of the torque limiting and diagnosis method, system and apparatus of the invention first and mainly involve directly or indirectly measuring the speed of motor 21 with a speed sensor 25 and then appropriately use the typical and known general features of speed/torque characteristic 100 of AC asynchronous motors as already discussed at length in relation with Figures 5 and 10. The breakdown torque in generator mode is much higher (e.g. 2.3 times) than in motor mode. This fact is often not displayed in speed/torque diagrams as known from educational textbooks but this fact is very important for the invention. The breakdown torque in motor mode and generator mode corresponds to different amounts of current. For instance the actual speed of the motor shaft 47 can be measured with a speed sensor 25 having a sensor shaft 46 directly or indirectly coupled to the motor shaft, in order to precisely determine a current torque estimate for any measured speeds in range R3. As shown on Figures 5 and 7, the speed/torque characteristic of AC asynchronous motors is substantially linear in the narrow speed range R0 of optimal efficiency around the nominal speed of the motor, yielding a linear correspondence between speed and torque, and strictly monotonic (strictly decreasing) in the mathematical sense in the main stable speed range R3 of speed of operation between the motor mode and generating mode extrema 108 and 109. Therefore within the main stable speed range R3, which is an intrinsic characteristic of any AC asynchronous motor for a predetermined frequency and voltage of operation, and which corresponds to the only normal range of operation of any AC asynchronous motor, any speed in that range uniquely corresponds to a specific torque, with speeds above the nominal or reference speed Ns of the motor necessarily corresponding uniquely to gener-ating mode torques, whereby the motor is behaving as a generator opposing and braking the movement of the load due to external forces, and speeds below the nominal or refer-ence speed Ns uniquely corresponding to a torque in the whole range of motor mode torques. Due to this simple, unambiguous, monotonic decreasing and permanent relationship between speed and torque within the main stable speed range of a given induction motor, the method of the invention is able to precisely estimate not only the motor torque quantitatively (since the permanent relationship evidenced by the monotonic decreasing part of characteristic 100 of Figs.5 and Fig10 can easily be tabulated), but also all the torques on the various components of the mechanical power converting mechanism 20 by measuring the rotational speed, so as to determine from a preset table the magnitude and direction of the corresponding torque, and it further is able to determine whether the motor is acting in its motor mode or in its externally driven generating mode. It is essential to be able to distinguish between these two modes, because the specifica-tions of reducer 22 heavily depend on whether the main motive torque is normally ap-plied on the motor side (on motor shaft 47) or exceptionally on the drive pinion side (via output pinion shaft 48) back to the motor: in this second case, the internal losses of re-ducer 22 typically are significantly larger, and the tolerable torque applied this way can be significantly less than the corresponding maximum equivalent torque when applied by the motor. As a result, a major advantage of this strictly monotonic relationship in range R3 between torque value T and measured speed value N according to a crucial aspect of the invention is the possibility of selecting and setting arbitrarily as many distinct limit torque thresholds Ti as desired, which are all uniquely defined by a distinct corresponding speed threshold Ni and most advantageously and crucially, the possibility of setting absolute maximum limit torque thresholds Tj in the generating mode (wherein the motor is driven faster by outside forces) which may be set substantially differently (and normally substantially lower) in absolute magnitude or intensity than the corresponding absolute maximum limit torque thresholds Ti foreseen in the motor mode, thus permitting a much safer and much fuller utilization of the torque capabilities of the gear reducer in particular in the motor mode, leading to the crucial advantage of being able to associate substantially higher powered motors providing higher motor torque to a given gear reducer, or conversely, significantly smaller, cheaper and lighter gear reducers than hitherto possible for a given motor power or torque, in the certainty that unforeseen torque excursions in both motor modes will be optimally detected in both modes and will permit precautionary motor turn-off in either mode at the optimal limit value for that mode, thereby suddenly decreasing the exerted torque to a safer value regardless of the motor mode or conditions, thus optimally protecting the gear reducer in all modes better than it was hitherto possible, and thereby allowing lighter, significantly more economical gear reducer designs with much tighter specifications with significantly reduced risk of catastrophic gear failure or gear teeth fatigue. The inability to distinguish between motor mode and generating mode, and of setting different limits for either mode actually was a major weakness of the prior art method of torque estimation by single phase current sensing: although for any gear reducer the absolute maximum rated torque value in generating mode driven by outside forces usually is substantially less -in absolute value- than the absolute maximum rated torque value in motor mode, the current sensing threshold - very disadvantageously - had to be set to one and the same value for both modes, and this resulted in the design necessity of selecting the current limit corresponding to the lower absolute torque value. This led in the current sensing prior art to a substantial under-utilization of the torque range capabilities of the reducer in motor mode due to a safety shut off in motor mode which had to occur for lower torques than absolutely necessary. This lower torque limit strictly necessary in turn led to the necessity of feathering the rotor and applying the yaw brakes of the wind power plant in much milder wind conditions than strictly necessary, leading to frustratingly unnecessary and substantial loss of power income for the operator, who saw its wind turbines shutting down one after the other in perfectly ideal wind! Importantly, this method of measuring and possibly recording motor speed (rpm) for uniquely and exactly estimating the corresponding motor torque is crucial for diagnosis and warranty claim purposes in case of reducer gear failure, because the entire history of torques on the drive pinion DP and on the last stage of the gear reducer 22 can be deter-mined or at least estimated with hitherto unparalleled accuracy, by recording the entire history of motor speeds into an inexpensive high capacity memory card (a few gigabytes, of the type commonly used for digital cameras) recording the motor speed and therefore the applied torque for every second of operation for the entire life service of the motor: these roughly amount to the motor torque multiplied by the reducing ratio K (K turns of the motor 21 yields 1 turn of pinion 23, in case external forces, friction and inertia effects can be neglected). Similarly, the maximum driven torque of the reducer, usually specified by a maximum torque TDPmax on output drive pinion shaft 48, can be estimated at motor level to a fraction x (0