Patent GB-167560-A (GB)
Title: Improvements in and relating to systems of electric ship propulsion
IPC: B63H23/24
Filed: 1920-05-07
Abstract: 167,560. British Thomson - Houston Co., Ltd., (General Electric Co.). May 7, 1920. Dynamos and motors combined for transmitting power.-An electric ship-propelling or like system, comprising turbo-alternators arranged to supply current to one or more motors directly driving the propeller shaft, is characterized by the provision of controlling-means which are operated when the slip or lag in the motor reaches a pre-determined value, and which automatically effect all the circuit operations involved in starting, stopping, or manoeuvring, on an installation such as that described in Specification 164,873, wherein, the propeller-driving motor is arranged to operate as a synchronous generator for braking purposes, as an induction motor for acceleration and reversing, and as a synchronous motor for normal operations. The controlling-means comprise differential mechanisms, the elements or which combine the synchronous speed and the actual motor speed so that a resultant rotation or movement proportional to the motor slip or lag is obtained which, in turn, is combined with a constant-speed device, so as to control the torque characteristic of the motor as the extent of the lag or slip attains a predetermined angle or number of cycles per second. Such control means may also operate to increase the stability of normal synchronous working. The invention may also be applied to an installation in which the propelling-motors are of the pure induction type. Moreover the step of synchronous braking may be eliminated, when this is unnecessary or undesirable, in systems including propeller-driving motors of the synchronous type. As shown in Fig. 2 the field windings of the generator 3 and motor 4 are connected either for normal or double excitation through slip-rings 13, 14 or 19, 20 with a three-wire system 48, 49, 50. With the switch 51 closed, normal excitation is supplied to the motor 4, whereas the switch 54 applies double excitation from the mains 48, 50. Similarly the switches 52 or 53 apply normal or double excitation to the generator. The switch 55 is provided to prevent short-circuiting the three-wire system during transition. The various elements are shown in the position they occupy during operation in the astern direction. During normal operation the generator and motor run in synchronism, normal excitation being applied to both field windings. To reverse the ship it is necessary first to slow the propeller down by super-exciting the motor field winding and de-energizing the generator field, thus causing the motor to operate as a synchronous generator; the current so produced is'dissipated in the field windings. To accelerate the propeller subsequently in the reverse direction, the generator field windings are super-excited and the motor is unexcited, the motor therefore operating as an induction motor until the slip has fallen to a value at which it is possible, to apply normal excitation to both generator and motor and run them in synchronism. A direction relay A operated by a disc 60 positively driven from the propeller shaft controls contacts A<1> - - A<4> and serves to eliminate unnecessary synchronous braking action say when picking up from " drifting " ahead to " full speed " ahead. Relay B controls contacts B<1> - - B<4> and is operated by a disc 71 which is driven through differential gear 74, 76 at a speed which is half the algebraic sum of the speed of the disc 60 and that of a constantspeed motor 77. Relay C controls contacts C<1> - - C<4> and is operated by a dise 73 which is driven through differential gearing 80, 82 at half the algebraic sum of the speed of the disc 71 and that of a synchronous motor 83 driven at a speed of 100 revolutions per minute counter-clockwise from the motor stator winding through a transformer 88. Under the conditions shown in Fig. 2 the speeds of the discs 60, 71, 73 are respectively 200, 105 and 2¢ revolutions per minute clockwise, the constant-speed motor 77 running at 10 revolutions, also clockwise. The controller finger g is energizing through contacts. A<4>, C<4> a line 89, which, in turn, through contacts 54a, 53a closes the switches 51, 52 for normal synchronous running astern. On chang. ing over to full-speed ahead, the controller 57 is first moved to the " off " position, whereupon all . switches and line contacts are opened. The controller segment 58 then energizes fingers a, c to reverse the phase rotation between the generator and motor by closing the switches 7, 8, 10. Simultaneously the motor 77 is reversed through the finger e to run at 10 revolutions per minute counter-clockwise. The disc 71 then drops to 95 revolutions and the disc 73 is accordingly reversed, moving the relay C to its upper position. The segment 58 finally energizes a finger i and establishes a circuit through contacts B<4>, 51<a> to close the switch 54 and thereby overexcite the motor field winding so that synchronous braking may be set up as the propeller is. dragged through the water by the inertia of the moving ship. The speed of the motor 83 now depends upon the frequency of the braking currents so set up. When the speed of the gear,. owing tc the slowing-down of the propeller shaft, falls below 10 revolutions per minute (i.e. when. the shaft has only 5 per cent of its normal speed), the torque required to hold and reverse the propeller is within the capacity of the motor oper-- ating as an induction motor. The disc 71, at the same time, reverses and closes first the contacts B<1>, B<3> and, after a short interval, opens the contacts B<2>, B<4> in order to set up the necessary circuits for motor induction action. Accordingly the closing of contact B<3> complete a circuit from. the controller finger h through contacts A<2>, 52<a> to close the switch 53 to over-excite the generator; whilst the opening of the contact B<4> interrupts the circuit from the finger i through contact 51<a> to open the switch 54 and so de-energize the motor field winding. Simultaneously the motor 83 is reversed and jumps into synchronism with the generator. The disc 73 accordingly reverses a second time, without however disturbing any existing circuits. As the propeller shaft reverses, the relay A is moved to its upper position, similarly without affecting existing circuits, the. contact A<2> being shunted by contacts C<2>, 51<b> to maintain the switch 53 closed. The propeller speeds up until the slip value is only three cyclesper second; whereupon the disc 73 reverses for the third and last time. The consequent closing of the contact C<1> completes a circuit from the finger j through contacts A<1>, C<1> to line 89, and, the switch 54 being open, through contact 54<a> to close the switch 51 for normal excitation of the motor. After an interval to allow current to. build up in the motor field, the switch C<2> opens to de-energize the switch 53. This bridges the contact 53<a> and closes the switch 52, so that both motor and generator are normally excited for synchronous working in the " full speed ahead " direction. The installation shown in Fig. 5 includes means for increasing the stability of normal synchronous working. For this purpose resistances 108, 109 are inserted in the leads to the motor and generator field windings and are arranged to be automatically short-circuited if, during synchronous working, any sudden increase of load (arising, for example, when the rudder is turned hard over) threatens to throw the generator anc. motor out of step. The operating- mechanism for this comprises a device 110 having a conducting ring 111 provided with insulating segments 112 and co-operating brushes 113, 114. The ring 111 is mounted on the planet carrier of a differential mechanism comprising a gear 115 driven from the shaft 75 of a small reversible motor 94 supplied from an auxiliary synchronous generator 95 on the propeller shaft. The other element 116 of the differential is driven from the shaft of a motor 102<1> which corresponds to the motor 83, Fig. 2. During normal synchronous working, the ring 111 is maintained stationary; but as the rotating element 5 of the main motor lags with respect to the rotating element 2 of the generator (with variation in lead), the rotating element of the motor 94 will reflect these variations, whereas the rotating element of the motor 102' (which in this case is driven through a transformer 103 from the generator stator 3) will remain unchanged. The ring 111 therefore rotates to an extent that depends upon the lag and is consequently a measure of the stability of operation of the motor. When the angular lag has reached a predetermined limit, the brushes 113, 114 move to complete a circuit from the controller finger c through a coil 117 and contacts 51<d>, 52<d> to close contacts 117<a>, 117<b> to short-circuit the resistances 109, 108. This increases the field strength and restores stability. Arms 118 carried by the ring 111 are arranged to operate stability-indicating devices 119, 120; and they may also serve to close circuits to pilot motors, such as 127, which operate automatically to adjust the resistances 108, 109 as demanded by variations in the main motor load. Both these methods of regulating the stabilizing-resistances are only operative when the main switches are set for synchronous working (i.e. not during manoeuvring conditions). The invention is also described in detail as applied to an installation in which the pure type of induction motor is employed and in combination with turbine speed-regulating mechanism of the kind described in Specification 3026/13, [Class 110 (iii), Turbines &c.], controlled either manually or automatically by means of a pilot motor. It may also be applied to a battle-cruiser installation in which there are two or four turboalternators and two or four propellers, each propeller being driven by a single motor or by two motors mounted on the same shaft. Each propeller is