ELECTRIC PROPULSION OF SHIPS. 117 



as operation, control, economy, simplicity, flexibility, reserve, etc. The series arrangement, 

 in eliminating parallel operation of generators, obviates the necessity for close regulation, and 

 hence simple engine governors and simple field rheostats are entirely satisfactory. In other 

 words, satisfactory operation is independent of variation in voltage between the different 

 generators. 



Besides the operating advantages, the series arrangement inherently provides for full 

 power from each of the remaining generators in case of casualty to one or more of the gen- 

 erating sets without providing additional capacity, and consequently additional weight in 

 the motor. To obtain full power from each of the remaining sets with parallel operation 

 would necessitate increasing the motor field in order to lower its speed to such a value as 

 would require the total capacity of the remaining imits, thus necessitating a larger and 

 heavier motor. 



Fig. 5, Plate 15, shows a diagrammatic scheme of connections for a single-screw Die- 

 sel electric drive. In this particular case there are six generating sets and one double-unit 

 direct-connected motor. The six generators and the two motor units are connected in 

 series. The machines are distributed electrically as follows : 3 generators, 1 motor unit, 

 3 generators and 1 motor unit, to reduce the voltage strain, or the maximum voltage to 

 ground at any two points to one-half the total voltage of the system. The voltage of each 

 generator being 250, we have in effect a 1,500-volt system with only 750-volt insulation re- 

 quirements. The advantages of this arrangement are obvious, especially in the case of large 

 capacity drives and where there are several generators involved. The diagram shows an 

 arrangement for using as many as may be desired of the main generating sets for supply- 

 ing power to the ship's auxiliaries when in port. Although the generators (and motors) 

 operate as pure shunt machines when driving the ship, series windings on the generators are 

 automatically placed in circuit when the generators are coimected to the auxiliary bus, and 

 therefore the generators operate as compound machines when supplying the auxiliary load, 

 and when used for this purpose the generators are operated in parallel. Arranging the 

 generators for supplying power to the ship's auxiliary bus determines the voltage of the 

 individual machines (250 volts). 



The motors and generators being pure shunt wound machines, the motor speed is ad- 

 justed to any value within the requirements by the voltage control system. In this system 

 the generator and motor fields are separately excited (preferably from the same excitation 

 source). The motor fields are excited at constant potential, and in one direction, whereas 

 the generator field excitation is varied to obtain the motor speed desired. With this ar- 

 rangement the speed of the motors is directly proportional to the generator voltage, and 

 therefore any motor speed from zero to the maximum in either direction is obtained by 

 merely manipulating the generator field rheostat (a common rheostat is used for all genera- 

 tors). Since the rheostat handles only the generator field current, which is 1 to lj4 per 

 cent of the generator rating, the simplicity and economy of the control are obvious. With 

 the type of rheostat used, the excitation of the generators may be varied from full exci- 

 tation in one direction to full excitation in the opposite direction, without opening the field 

 circuit, and therefore the ship can be brought from full speed ahead to full speed astern 

 without opening a single circuit. 



Reserve power in the event of casualty to prime movers is greater with this type of 

 drive than with any other. On the basis of the power varying as the cube of the speed, a 

 three-engine unit Diesel electric ship can make 88 per cent speed with two generators and 

 70 per cent speed with one generator. 



