CHARACTERISTIC CURVES. 



407 



When the combined current output of A and B is, say, 50 

 amperes, then the two operating points P and P have equal 

 ordinates and the distance PP represents 50 amperes. 



34. Characteristic of the compound generator. The compound 

 dynamo has been heretofore treated, both as a generator and as a 

 motor, as a modification of the shunt dynamo and there is no 

 need in the present instance to discuss the compound generator 

 minutely. Fig. 41 shows a typical external characteristic of an 



Volts 



/ Amperes 



Ampere-turns 

 on field 



(Voltage constant) 



Fig. 41. 



Amperes output 

 Fig. 42. 



over-compounded generator of which the terminal voltage rises 

 with increase of current up to and beyond full rated load. 

 The rapid fall of terminal voltage in the dotted portion of the 

 curve is due to the great demagnetizing action of the current in 

 the armature, to internal drop R a 7 o + R e f e , and to the decrease of 

 that part of the field excitation which is due to the shunt field 

 winding. 



Armature characteristic. Calculation of the number of series 

 field turns for compounding, (a) Flat-compounding. Consider 

 a given shunt generator driven at constant speed and having a 

 terminal voltage equal to no volts when the current output is 

 zero. Under these conditions the field excitation in ampere-turns 

 has a definite value. Let the current output of the generator be 

 increased more and more ; let the field rheostat be manipulated so 



