EFFICIENCY OF A DYNAMO-ELECTRIC MACHINE 189 



When the speed of a machine varies as the load changes, 

 the mechanical losses may be considered to vary directly with 

 the speed. If, for example, on a series motor, the mechanical 

 losses are measured and found to be 800 watts when the 

 armature is turning 1200 r.p.m., they may be assumed to 

 equal 400 watts when the motor is turning 600 r.p.m. 



The current flowing in a shunt field is nearly independent 

 of the load. In the case of a shunt motor connected to a 

 constant potential line the shunt field current will be entirely 

 independent of the load and hence the I 2 R loss in the field 

 coils will be constant. This loss will be larger when the 

 machine is first started than when it has run a sufficient 

 length of time to get warmed up; in Chapter I the effect 

 of an increasing temperature upon the resistance of a con- 

 ductor was noted. 



The current in the armature and series field of a motor 

 (or generator) is proportional to the load the machine is 

 carrying; the ohmic resistance loss is, therefore, propor- 

 tional to the square of the load and if the PR loss is plotted 

 as one co-ordinate with the load as the other, the curve 

 will be a parabola. 



Loss at the Brush Contacts. The resistance of the brush 

 contacts (carbon brushes are assumed) is not a constant 

 quantity but depends upon the current density at the 

 contact surface. If the current density is low, the resist- 

 ance is high and vice versa. This is entirely different from 

 the armature and field circuits; except for the temperature 

 change effects the resistances of these circuits are constant 

 and not affected by the current density. It has been found 

 by experiment that the resistance of brush contacts varies 

 somewhat with the current density at the contact surface; 

 the IR drop through two brush contacts in series may be 

 calculated approximately by the formula, 



IR drop = .8+ (.2 X ampere per sq. cm.) . . (36) 



