210 ELECTRICAL ENGINEERING 



(3) The characteristics of a compound motor are (a) variable 

 speed and (b) torque greater than in the shunt motor but less 

 than in the series motor. 



At light loads compound motors approach a limiting speed 

 which is fixed by their shunt excitation. 



They are used for elevators and in classes of work where the 

 load is variable and constant speed is not necessary and where a 

 fairly large starting torque is required, except in those cases where 

 series motors are necessary on account of their very large starting 

 torque. 



In rolling mills where the load fluctuates very rapidly a com- 

 pound-wound motor is used with a heavy flywheel attached to it. 

 When a heavy load comes on the speed falls and the flywheel 

 gives up part of it's energy. A similar motor with a flywheel is 

 used to drive shears, punches, etc. 



130. Power Losses in Dynamos. The power losses occurring 

 in dynamos may be divided into copper losses, iron losses and fric- 

 tion losses, and these may again be subdivided as follows: 



Copper losses. 



(a) Shunt-field copper loss. 



(b) Series-field copper loss. 



(c) Armature copper loss. 

 Iron losses. 



(d) Hysteresis loss. 



(e) Eddy current loss. 

 Friction losses. 



(f) Brush friction loss. 



(g) Journal friction loss, 

 (h) Windage loss. 



(a) The shunt-field copper loss is 7/ 2 r/ watts, where // is the 

 current in the shunt-field winding and r/ is the resistance of the 

 winding at the running temperature of the machine. This loss 

 can be represented by #///, where E f = I f r f is the voltage im- 

 pressed on the winding. Thus all the energy supplied to the field 

 winding is transformed into heat and is wasted, since no energy 

 is required to maintain the magnetic flux after it is once established. 



The shunt-field loss is constant under all conditions of load and 

 it ranges from 1 per cent of full-load output in large high-speed 

 machines to 5 per cent in small low-speed machines. 



