190 ELECTRICAL MACHINERY 



The normal current density at full load is 5 amperes 

 per sq.cm. so that the brush contact drop = .8 +(.2X5) = 1.8 

 volts, for both brushes together. This drop is nearly the 

 same on all sizes of machines as it depends only upon the 

 current density in the brush contacts and not upon the cur- 

 rent itself; the current density is practically the same for 

 all sizes of machines. 



The loss due to brush contact resistance is small compared 

 to the other losses in a machine and it is often computed by 

 taking, the IR (contact resistance} drop as equal to 2 volts 

 at all loads. This makes the calculation of brush contact 

 I 2 R very simple and even though it is a rough approxima- 

 tion, but very little error is produced in the final result. 

 In a motor taking 40 amperes at full load the contact resist- 

 ance loss may be approximately obtained by putting PR 

 loss = 40X2 = 80 watts, etc. 



Hysteresis and Eddy Cwrent Losses. In a shunt generator 

 or motor the hysteresis and eddy current losses are nearly inde- 

 pendent of the load providing the speed does not change with 

 the load. They depend upon the flux density in the arma- 

 ture core and the speed with which the armature revolves. 

 The speed will generally change somewhat as the load 

 changes and these losses may be considered to vary directly 

 with the speed, the same as the mechanical losses. 



Stray Power. All of the mechanical losses, and the 

 hysteresis and eddy current losses (commonly called core 

 loss) are measured together and the whole loss is called 

 the stray power. The stray power therefore comprises all 

 mechanical losses and core losses and may be assumed to 

 vary directly with the speed. 



Loss Curves. We have, then, four losses to take into 

 consideration when determining the efficiency of a motor 

 or generator; the stray power, which varies slightly with 

 the load, the shunt field loss which is generally independent 

 of the load, the brush contact resistance loss which varies 

 directly with the load, and the armature and series field 



