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March, 1910 "TAE MARINE REVIEW 117 varies--after a certain point it will sim- ply stop altogether. We will now as- sume that there are 40 poles in our stator; we have already mentioned that there are four poles in our three-phase "generator, and if current from the latter be supplied to the motor, the result will be that it will rotate at one-tenth of the alternator' speed, 4, ¢, at 90. RP. M, Fig. 1 shows the arrangement. C._ is the stationary armature of a three-phase generator; B is the four-pole rotating magnet supplied with current by the small exciting dynamo (E) through the slip rings (D). The three-phase con- nections of the motor stator (L) are shown. K_ is the squirrel-cage rotor, which, as already explained, will drive the propeller at 90 R. P. M. when the generator is driven at 900. In the lower part of the diagram are shown the electric connections, which, however, will convey more to an electrical en- gineer than to the average reader. It will be noticed that on the shaft of B there is another machine (H). This may be regarded as a special form of induction coil in which the primary is rotating, the secondary remaining sta- tionary. It is really what is known as a transformer generator. The rotating "primary" has 12 poles, the three-phase winding of the secondary being taken to the slip rings (GG). -We will show now Gan > be ised. to give a hianer speed to the pro- pellér = Big. 22 shows the change that must be made in the con- nections; the gen- erator armature is now connected to the slip rings of the transform- -- er-generator, and the secondary of the latter is connected to the stator of the squirrel-cage motor; Now the "pri- mary' (H) of the transformer gener- ator is being driven at the same speed as the four-pole generator, and, as it has 12 poles, it follows that the current generated in the secondary would have three times the periodicity of the gen- erator current were it not for connec-. tions just made to the slip rings. The effect of supplying alternating current 'to H is, however, to produce a rotating . magnetic field turning. in the reverse direction to that in which the trans- former generator is driven, so that in effect for every three turns ahead there is one turn backwards. The result is that the alternating current generated in I (the stationary part of the trans- former generator) has only twice the periodicity of the alternator current in- stead of three times--hence, if I be connected, as shown, to the stator of the motor, the propeller will be driven at twice the speed at which it turned in' Big." 1-4. &) at 180- Re PM: Now we come to Fig. 3, which to all outward appearance is the same as Fig. 2. If, however, we interchange two leads between the alternator and the transformer generator, the effect is to how this machine . reverse the direction of rotation of the magnetic flux. We now have the flux turning in. the same direction as the primary of the transformer dynamo, so that, instead of subtracting the speed of rotation of the flux, it must be added, which gives us a resultant periodicity of 3+1=4 times the alternator period- icity. If, therefore, the secondary. (I be connected as before to the motor, four times the original speed will be ob- tained, 4.-¢.,.300 R. PM. 4 Thus, simply by altering electrical con- nections which can be arranged very simply on a controller, though very difficult to describe, it is: possible to ob- _tain three different speeds ahead with the engine running at constant speed, and all of them, moreover, speeds ad- mitting of the very highest propeller efficiency. There is the additional ad- vantage that the squirrel-cage motor be- ing of small size, and so simple that nothing can go wrong with it, can be tucked away in the stern and put as low down in the boat as can be desired, giving the advantage of a horizontal propeller shaft. The oil engine and the three-phase alternator, on the other hand, together with the transformer gen- -erator, can be put in whatever part of the vessel may be. most convenient, either right forward or, if convenient, on deck. m oe . A very small acquaintance with elec- tricity will be enough to show any reader that these changes in connection would cause a great deal of sparking were they carried out in the way de-. scribed, but a very simple arrangement entirely gets over the difficulty, the whole system being automatically brought to a no-voltage state before any change in the connections is made. This is done by means of the exciter (E) of the alternator; its field magnet winding (El) is so arranged that it can be short-circuited by the connection (F), which has the effect of reducing the field-magnet current to zero, and so pro- ducing a no-voltage state throughout oe os ER Se aa PER A ay on A ee a ae RR Pe ges See LL