330 



ALTERNATING CURRENTS 



large unit is required and where any sudden changes of load are 

 partly absorbed by the inertia of the direct-current armature. 

 A few such motors, situated at various points in a large system, 

 may make it possible to operate the generating station and many 

 of the transmission lines and sub-stations at high power-factor, 

 in spite of low power-factor in the consumers' loads. 



Even with these advantages of the synchronous motor, electri- 

 cal engineers often prefer to use induction motors for motor- 

 generator sets, because of their simplicity and greater reliability. 



FIG. 305. Sv 



driving direct-current generators. 



Figure 305 shows synchronous motors driving direct-current 

 generators in a sub-station. 



Electric Propulsion. Synchronous motors are also coming 

 into use for the electric propulsion of cargo and merchant ships. 

 Such ships, when under way, operate at a constant speed and the 

 constant-speed characteristic of the synchronous motor is not a 

 disadvantage, therefore. As such motors can be operated at 

 unity power-factor, the weight of motor, generator and connect- 

 ing leads is smaller than when induction motors are used. This 

 matter of weight is important in marine work. The air-gap of 

 synchronous motors is considerably greater than that of induc- 

 tion motors and the mechanical difficulties which a short air-gap 



