THE OPERATION OF THE DYNAMO AS A MOTOR. 9/ 



upon the reversed armature currents in the same direction as 

 before, thus leaving the direction of rotation unchanged. To 

 reverse the direction of running of a motor the connections must 

 be changed so that the current through armature (or field) is 

 reversed while the direction of the current in the field (or arma- 

 ture) remains unchanged. This requires the reversal of the arma- 

 ture connections, or a reversal of the field connections, but not of 

 both. 



53. Fundamental equation of the motor. Let E x be the elec- 

 tromotive force applied at the terminals of a motor. This is called 

 the impressed electromotive force. This electromotive force is 

 partly used to overcome the resistance R a of the armature. The 

 part so used is equal to ^ a / a , where I a is the current flowing 

 through the armature. The remainder of the impressed electro- 

 motive force balances the electromotive force, E a (= <&Z'ti) 

 which is induced in the rotating armature by the field flux, and 

 it is numerically equal thereto. Therefore : 



E x =3>Z'n + RJ a (23*) 



in which E x is the impressed electromotive force acting on a 

 motor, R a is the armature resistance,* 7 o is the current flow- 

 ing through the armature, < is the magnetic flux entering the 

 armature from one field pole, n is the speed of the motor in rev- 

 olutions per second, and Z' is written for the factor pZ ' jp r io 8 . 

 (See Art. 30.) 



The electromotive force, QZ'n, which is induced in the rotat- 

 ing armature of a motor, is opposite in direction to the impressed 

 electromotive force, E x and to the current, 7 o . Therefore QZ'n 

 is called the counter electromotive force of the motor. 



In the generator the voltage is determined by the fundamental 

 equations (21) and (22), inasmuch as all the other quantities in 

 these equations are fixed in value by the conditions under which 



* In the long-shunt compound motor the combined resistance of armature and series 

 field coil, Ra, -(- J?ci is to be used for R a . It is hardly necessary to formulate the 

 very slightly different behavior of the short-shunt compound motor. 



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