4 6 DIRECT CURRENT MOTORS. [Exp. 



9. Copper Losses. The copper losses for any circuit can be 

 computed, if the current and resistance through which it flows 

 are known, being equal to RP where R is resistance and / is cur- 

 rent. The armature copper loss is thus computed; it is a vari- 

 able loss, changing with load. 



The field copper loss is a constant loss and does not vary with 

 load. It also can be computed by the formula RP, or more con- 

 veniently from the formula El, the product of current in the field 

 circuit and voltage supplied at its terminals. (The formula El 

 cannot be thus used unless copper loss is the only expenditure 

 of energy; it cannot be used for determining copper loss of an 

 armature or other circuit in which there is a back electromotive 

 force.) 



In a self-excited machine, in which a field rheostat is used 

 under normal operation, the loss in the rheostat is to be included 

 in the field circuit loss. 



10. Iron Losses. The iron losses are losses due to hysteresis 

 and eddy currents ;* they are independent of load, but vary with 

 the speed and with the flux density in the armature. At con- 

 stant speed, hysteresis loss (within the usual working range) 

 varies approximately as the 1.6 power of the flux density; eddy 

 currents as the square of the flux density. At constant flux 

 density, hysteresis loss varies directly with the speed and eddy 

 currents with the square of the speed. If the field current of 

 the motor is held constant, the flux density in the armature will 

 be practically constant for all loads. It will be modified under 

 loadf to a small extent by armature reaction, the effect of which 

 will be neglected. Hence in a shunt motor run with constant 



* This includes eddy currents in the pole pieces and in armature copper 

 as well as in armature iron. 



t ( loa). Load Losses. Losses which occur under load in addition 

 to copper losses and to the no-load iron, friction and windage losses are 

 termed load losses. Any loss due to field distortion constitutes such a 

 loss. Load loses are usually neglected as small or are estimated. See 

 Standardization Rules 114-7. 



