Houston and Thomson.] &® [Nov. 1, 



Circumxtunces Influencing the Efficiency of Dynamo- Electric Machines. 

 By Prof*. Edwin ./. Houston and Eiihu Thomson. 



{Read before the American Philosophical Society, November 1st, 1878.) 



During the recent competitive trials made at the Franklin Institute as to 

 the relative 1 efficiency of some different forms of Dynamo-Electric Ma- 

 chines, the authors having been entrusted with the work of determining 

 the relations between the mechanical power consumed and the electric and 

 thermic effects produced, took the opportunity thus afforded to make a care- 

 ful study of many interesting circumstances which influence the efficiency 

 of these machines. 



It is proposed in the present paper to select from the many circumstances 

 thus noticed, a few of the more interesting, reserving the others for a fu- 

 ture consideration. 



It will readily be understood from the comparatively new field in which 

 we had been working, no reliable data of the electrical work of these ma- 

 chines having before been obtained, that difficulties constantly arose owing 

 to necessary conditions of operation, and new developments as to the be- 

 havior of the machines under varied conditions, were constantly met. 



A convenient arrangement of the particular circumstances we are about 

 to discuss may be, 1st, Those affecting the internal work of the machine ; 

 2d, Those affecting the external work, and 3d, The relations between the 

 internal and external work. 



The mechanical energy employed to give motion to a Dynamo Elec- 

 tric Macbine is expended in two ways, viz., 1st, In overcoming fric- 

 tion and the resistance of the air ; and, 2d, In moving the armature of the 

 machine through the magnetic field, the latter of course constituting solely 

 the energy available for producing electrical current. The greatest amount 

 of power expended in the first way Avas noticed to be about 17 per cent, of 

 the total power employed. Tliis expenditure was clearly traceable to the 

 high speed required by the machine. The speed therefore required to 

 properly operate a machine is an important factor in ascertaining its 

 efficiency. 



The above percentage of loss may not appear so great, but when it is 

 compared with the total work done in the arc, as heat, constituting as it 

 did in this particular instance over 50 per cent, of the latter, and about 33 

 per cent, of the total work of the circuit, its influence is not to be disre- 

 garded. In another instance the work consumed as friction was equal to 

 about 80per cent, of that appearing in the areas heat, while in the Gramme 

 machine experimented with, this percentage fell to 20 per cent, of 

 that which appeared in the arc as heat, and was only about 7 per cent, of 

 the total power consumed in driving the machine. 



In regard to the second way in which mechanical energy is consumed, viz. : 

 in overcoming the resistance necessary to move the armature through the 

 magnetic field, or in other words, to produce electrical current, it must not 

 be supposed that all this electrical work appears in the circuit of the ma- 



