2-B] EFFICIENCY. 61 



ticular field current / we will, therefore, have the same speed in the 

 no-load run as in an assumed load run with current I, if in the no- 

 load run the counter-electromotive force (which in this case is the 

 impressed* armature voltage) is equal to the counter-electromotive 

 force of the assumed load run. But for the load run we can com- 

 pute the counter-electromotive force, E' = E RI, for any assumed 

 load current /. (Here E is the rated or assumed constant line volt- 

 age for which the losses are desired; R is the hot resistance of the 

 armature and field, including brushes, etc.) Hence this is the proper 

 voltage to supply the armature in the no-load run when the field 

 current is /. 



In testing a series motor by this method the field is excited with 

 current I, which is given successive values, and the armature is sup- 

 plied with the corresponding proper voltage, E RI. (Or the 

 armature can be given successive voltages and / adjusted to cor- 

 respond.) 



37. A convenient method sometimes used for adjusting field cur- 

 rent and armature voltage to their proper corresponding values is 

 to connect the field and armature in series as a series motor with 

 one regulating resistance in series with the line and one in shunt 

 with the armature. For the first reading the series resistance is 

 adjusted; after that, adjusting the shunt resistance alone will tend 

 to cause the field current and armature voltage to assume automat- 

 ically their correct relative values. (For this condition the series 

 resistance is made equal to the armature resistance.) For modified 

 ways of conducting the test, see R. E. Workman, Electric Journal, 

 I., 169. 



38. No-load Run for Friction. When the field current is very 

 small, hysteresis and eddy current losses are so small that W gives 

 practically the friction and windage loss; compare paragraph (&), 

 20. A run at low field excitation can be made as in 34. This 

 run, however, can most conveniently be made with the field and 

 armature in series, the motor being run as a series motor on no load 

 at a low voltage. The voltage and the speed are controlled by a 

 series resistance; no shunt resistance is used. At no load the cur- 

 rent through the field is so small that iron losses in the armature 

 are negligible. 



* The copper drop due to armature resistance at no load can be neglected 

 or a small correction made. 



