114 ELECTRIC PROPULSION OF SHIPS. 



torque can be obtained without excessive current, and the running efficiency can be improved 

 somewhat. Such motors are in use on propeller drives at the present day. 



Turbine Electric (Synchronous) . — Fig. 3, Plate 14, shows a diagrammatic scheme of 

 connections for a synchronous motor drive. Power is supplied from the turbine-driven gen- 

 erator to the synchronous motor through one of the reversers. The generator and motor 

 fields are excited from a three-wire D. C. exciter set. The synchronous motor system dif- 

 fers from the induction motor system only in so far as the synchronous motor itself affects 

 the details of control and generator capacity, the main features of turbine-electric drive be- 

 ing otherwise the same. 



Although the characteristics of the ordinary synchronous motor are not suitable for 

 ship propulsion, the synchronous motor can be modified to give characteristics which meet 

 the requirements. The modification consists in providing the rotor with a substantial in- 

 duction winding of such design and arrangement as will not seriously detract from certain 

 purely synchronous motor characteristics which are desirable. 



The maneuvering operations can be accomplished in more than one way. If appreciable 

 torque is required to reverse, the method which appears to be most favorable is to 

 utilize the synchronous characteristics and the induction characteristics at different stages of 

 the reversing cycle. The motor is stopped as a synchronous generator loading into the gen- 

 erator windings which form a dead load (generator field not being excited), then is brought 

 up to nearly synchronous speed in the reverse direction as an induction motor, and finally is 

 pulled into step with the generator as a synchronous motor. With this method, the sequence 

 of operation for reversing would be approximately as follows : 



1. Reduce turbine to idling speed (25 per cent), 



2. Open generator and motor fields, 



3. Reverse motor connections, 



4. Apply motor field excitation bringing motor to rest, 



5. De-energize motor field, 



6. Energize generator field to double value, bringing motor to nearly synchronous speed 

 as induction motor, 



7. Apply normal excitation to motor field, pulling motor into synchronism with 

 generator, 



8. Adjust speed to desired value. 



The method described above is preferable where appreciable torque is required during 

 reversing. Usually, however, sufficient torque will be developed by the simpler method of 

 reversing as an induction motor, in which case the cycle of operation will be as follows : 



1. Reduce turbine to idling speed (25 per cent). 



2. Open generator and motor fields. 



3. Reverse motor connections. 



4. Energize generator field to double value, bringing motor to rest and reversing it to 

 nearly synchronous speed, as an induction motor. 



5. Apply normal excitation to motor field, pulling motor into synchronism with 

 generator. 



6. Reduce generator field to normal. 



7. Adjust speed to desired value. 



This method, therefore, simplifies the sequence to the extent of omitting one step and 

 eliminating the generator action of the synchronous motor. 



Although 8 and 7 steps respectively have been indicated in the sequence, as a matter 



