188 ELECTRICAL MACHINERY 



according to the size and location of the power station 

 and the amount of power used by the customer). The cost 

 of power per ten-hour day would be equal to $.06 X 83 X 10 = 

 $49.80. 



If now the motor has an efficiency of 80% we may calcu- 

 late in the same way the cost for power to run the factory 

 a ten-hour day and find it to be $56.00. The difference in 

 cost in operating these two motors for a year would be 

 $1860.00, and this is more than the cost of the motor. 

 This one example serves to show how important the efficiency 

 of a dynamo-electric machine is commercially. 



41. Losses and Their Variation with Load. Mechanical 

 Losses. There are several so-called losses in any electric 

 machine. For example, in the operation of a motor some 

 of the power input is used in overcoming the friction of the 

 shaft turning in the bearings, the friction of the brushes on 

 the commutator, and the friction of the revolving armature 

 on the air (called windage). These are usually designated 

 as mechanical losses. 



Electrical Losses. There are aiso other losses due to 

 ohmic resistance, hysteresis, and eddy currents; these are 

 called electrical losses. As losses due to ohmic resistance 

 may be mentioned the I 2 R losses in the field windings, in 

 the armature windings, and in the brushes and brush con- 

 tacts. Then there are the hysteresis loss in the armature 

 core, and the eddy-current losses in the armature core and 

 pole faces, where power is used and given out in the form 

 of heat. 



Variation of Losses with Load. Some losses are inde- 

 pendent of the load the machine is carrying and some vary 

 with the load. If the speed of a machine (either motor or 

 generator) remains constant as the load varies, we may 

 assume that all of the mechanical losses are constant, i.e., 

 independent of load. If they are measured at any one 

 load they may be regarded as having the same value for 

 any other load. 



