82 ELEMENTS OF ELECTRICAL ENGINEERING. 



generator is not constant but increases very rapidly as the current 

 output increases. When the resistance of the receiving circuit is 

 large, the electromotive force is small and the current output is 

 very small. When the resistance of the receiving circuit is small, 

 the electromotive force is large and the current is very large. 



Power output of a shunt or compound generator. The power 

 output of a generator, being equal to the product of the terminal 

 voltage of the generator and the current output, is small when 

 the current is small and large when the current is large. Fur- 

 thermore, since the generator armature is driven in opposition to 

 the side push which the field magnet exerts upon the armature 

 wires, and since this side push is proportional to the current, it 

 follows that the amount of power required to drive a generator 

 increases with its current output. When the current output (to 

 external receiving circuit) is zero, the only power required to drive 

 a shunt generator, for example, is the power required to supply 

 the field current and to supply the friction and magnetic losses in 

 the armature. 



Power rating. From the above discussion it may appear that 

 any desired amount of power however great can be delivered by a 

 given generator. In a certain limited sense this is true, but when 

 the current output of a generator is increased more and more, 

 the temperature of the machine increases more and more, on 

 account of the heat generated by the current in the windings, and 

 the tendency to spark at the brushes increases. The deterioration 

 of the insulating materials caused by heat sets a more or less defi- 

 nite limit to the permissible rise of temperature of a generator, 

 and the smooth running and durability of the commutator depend 

 upon sparkless commutation. Therefore, there is a practical 

 limit to the output of a given generator beyond which either the 

 heating or the sparking becomes excessive. This practical limit 

 to the output of a generator determines what is called its rated 

 output. 



46. Conditions which affect the electromotive force of a generator. 



(a) Primary conditions. Speed and field excitation. The 



