Synchroscopes for Multi-Engined Aircraft By V. C. Hunt O overcome the discomfort, nerve strain engine at will. The advantage of the former and fatigue to both passengers and crew presentation is that a complete indication of all Ton multi-engined aircraft resulting from the the engines relative to the master is visible con pole and this will produce a torque which will noise and vibrations of engines running at tinuously. cause the rotor to rotate towards the position of slightly varying speeds, the need of an accurate, The principles of operation of the synchroscope the maximum pole. Should the generator feeding easily read indication of such variations is readily are as follows: the rotor continue to be slow, the rotor will be apparent. A direct comparison of r.p.m. in forced continually to realign itself, thus causing In the electric tachometer generator, it may be dicators is ineffective because the permissible rotation in a certain direction. Should, however, considered that three coils of wire exist which form tolerance on each indicator might result in a total the generator in question be speeded up to run the three phases of an electric field spaced equi difference of the order of 50 r.p.m. even though faster than that feeding the stator, the reverse distant apart at 120 deg. Now as a magnet moved the speed of the engines under comparison is the in respect to a coil of wire induces a voltage in that rotation will take place; as the rotor pole will be same. The development of the 3-phase electric coil so the permanent magnet rotor revolving magnetized faster than the stator pole and an tachometer indicators overcame this difficulty within these three coils induces a voltage which opposite torque applied. by comparing the relative frequencies of two or reaches its peak when the greatest amount of FIG. 3 shows a typical installation for a 4- more tachometer generators (not the voltages), magnetic flux crosses the centre of the coil. In engined aircraft, of a synchroscope designed with before this has been translated into an indicated duced voltage moves around the field in a direc three separate assemblies combined in one in r.p.m., by means of a small synchronous motor tion corresponding to the movement of the rotor; strument. These can be accommodated in the operating a magnetic drag unit; the desired result voltage in each coil increases or decreases as a standard 2⅜ in. dia. case. is obtained even though the possibility of per pole of the magnet approaches or recedes from Each assembly consists of a rotor supported on missible errors in a yet serviceable indicator has the coil. FIG. 1 represents in graph form the in two bearings and housed inside the stator. The not been obviated. These two frequencies arc stantaneous position of the magnet when produc stator is fed directly from the generator, through used to operate what is, in reality, a beat- ing the maximum current in each phase of a the common connexion plug, whilst the rotor is frequency indicator with a pointer rotating in 3-phase motor for the maximum magnetic flux. fed through rings on the shaft in contact with proportion to the difference in speed. As the It will be apparent that there are three similar brushes connected in turn with their generator. difference in speed of the two engines results in a alternating voltage curves displaced in the same The whole by virtue of a minimum of wearing difference in the frequencies developed by the two angular position as the coils in the generator. surfaces needs very little maintenance and mal tachometer generators, so the pointer of the functioning can only be due to excessive friction The synchroscope consists of a 3-phase stator synchroscope will rotate either clockwise or anti in the bearings, faulty contacts or poor brush and a 3-phase rotor (see FIG. 2). By connecting the clockwise according to which engine is running stator to a tachometer generator a rotating field contacts. faster. When both engines are running at the is produced just as in a motor, the rotating volt An interesting development of this instru same speed the frequencies will be alike and the ages in the generator producing rotating currents ment for a twin-engined aircraft is a dual pointer pointer will remain stationary. Any difference in proportional to the voltage. With no imput to r.p.m. indicator giving indication of synchroniza voltage output of the two generators will not the windings of that rotor, it will follow the rotat tion by means of a rotating disk visible through a affect this final indication, but will have some ing field. Similarly by energizing the rotor with cutaway on the lower part of the dial, this disk bearing on the speed difference at which the no imput to the stator a motor action is obtained. taking the place of the pointer on the ordinary pointer begins to rotate where this rate of rota When both the stator and the rotor are energized synchroscope. tion is directly proportionate to the difference by a separate source, i.e. their respective genera in engine speed. However, as a large variation tors, the two respective fields cause the rotor to between engine speeds will not cause any pointer rotate at a speed equal to the difference in the rotation at all, but merely be shown as a flickering frequencies of the field; or, assuming that the of the pointer, this is of no material importance. frequencies of the fields are identical, as when the It is obvious that any number of engines can generators are running at the same speed, the be compared with the same 'master' engine by motor action is not evident. At the instant when using an individual synchroscope for each, or, the rotating field is producing a maximum north alternatively, by using one synchroscope wired pole at phase one the north pole will attract the to a selector switch to bring into comparison any south pole of the rotor until the south pole is in line with it. By the time phase one has passed through zero and has become a maximum south pole the rotor pole will have become a maximum north pole, since the periods are the same. Pro vided that the frequencies of the rotating fields remain constant to one another the poles of the stator will always be in step with the poles of the rotor, causing the rotor to remain in a stationary position. As the generator feeding the rotor slows up, the rotor pole will not reach its maximum magnetic strength until a little later than the stator. By this time the maximum opposite pole in the stator will be a little offset from the rotor BOOK REVIEW REPRESENTATION IN FRANCE The Editor can thoroughly recommend to any An Outline of the Control of Production firm in the Aircraft Industry requiring a re presentative in France an ex-officer who served Introduction to Production Control. D. Tiranti this close connexion with the contents it is with the French Naval Air Service during 1940- and W. F. Walker. [Chapman and Hall. hardly possible, and would indeed ill become us, to be critical of them. We believe that all pro- 1945. The gentleman concerned is both an ex 15s. 0d.] duction engineers could not fail to benefit from a perienced pilot and a fully trained engineer with As Mr. Tiranti states in his Acknowledgments', study of the book and we heartily commend it as, a wide knowledge of aeroplanes and aero much of the material contained in this book has so far as we know, the first to be published in engines gained over a considerable num originated from articles by him which appeared in ber of years. He both speaks and writes England on an aspection of production in which AIRCRAFT ENGINEERING during 1943, 1944 and English. America has taken the lead and where many 1945, and we are happy to think that we gave early universities offer courses for students. This book If any firm desiring further information will encouragement to the scientific study of a sub is calculated to help in the development of a communicate with the Editor, he will be glad to ject which is now generally acknowledged to be similar technique in this country. one of great industrial importance. In view of put them in touch. March 1947
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Mar 1, 1947
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