142 Book of Engineering 



constant voltage from a direct driven exciter, 

 or, in certain circumstances, from a battery. 

 The second field is a series field, acting in 

 opposition to the shunt field. 



Taking as an example a zoo-h.p. equip- 

 ment, it will be found that the reverse series 

 field automatically limits the currents which 

 can be applied to the traction motors, even 

 when starting from rest. Thus there is no 

 possibility, by faulty manipulation on the 

 part of the driver, of obtaining too great 

 an acceleration or of spinning the wheels. 

 It should also be noted that no starting 

 resistance is required in the motor circuit. 

 Further, the shape of the output curve of 

 the generator is such that over a wide 

 range of speed the engine is being utilised 

 at or near its maximum output. At the 

 same time, however, the engine cannot be 

 overloaded. 



The control of the power given by the 

 generator to the traction motors can be 

 varied either by control of the engine speed, 

 or by control of the field of the generator. 

 A combination of these two methods is 



