164 ELECTRICAL MACHINERY 



where K is a constant involving the number of conductors 

 on the armature, etc. So we have 



E K&N 

 /= tT , ...... (30) 



or 



In the case of a shunt motor <i> is nearly independent 

 of the armature current, /, so that by inspection of equa- 

 tion (31) it may be seen that the decrease in speed as the 

 armature current increases is due to the factor IR a . Also it 

 is evident that the amount of decrease depends directly 

 upon the value of the armature resistance. 



Compound Motor. In the case of a compound motor 

 the equation for speed becomes 



N _E-I(Rg+R s ) f 



- K(*+*.) * 



With an increase in / (i.e., with increase in load) the speed 

 of the compound motor must decrease because of two 

 effects; the term I(R a +Rs) increases directly with the load 

 and the term K($ S ^+<E> S ) increases somewhat with the load. 

 Although 3> s >, is independent of the current 7, the term 

 <J> S is directly proportional to the current, I. 



Series Motor. With the series motor we have 





This equation shows that the speed of a series motor must 

 vary greatly as the load is changed. The flux < s is directly 

 proportional to the current, /, so we may write 



(34) 



