132 ELEMENTS OF ELECTRICAL ENGINEERING. 



is connected to the armature terminals, and the two rheostats are 

 adjusted until the machine runs at 1,200 revolutions per minute, 

 and shows an electromotive force of 100 volts between the 

 brushes. The ammeter then shows that 2.4 amperes is flowing 

 through the armature. The resistance of the armature (hot) is 

 found by measurement to be 0.25 ohm. 



From these data the power P delivered to the armature is 

 found to be P 2.4 amp. x I oo volts = 240 watts, and the arma- 

 ture loss corresponding to the current of 2.4 amperes is (2.4amp.) 2 x 

 0.25 ohm = 1. 44 watts, so that the stray power loss under the 

 condition of the test is 240 watts 1.44 watts, or 238.56 watts. 

 A slight correction to this result may be applied to find the value 

 of S, corresponding to a speed of 1,200 revolutions per minute and 

 E a equal to 100 volts, as follows : Under the conditions of the 

 test, 100 volts = E x = E a + RJ a , where ^ = 0.25 ohm and 

 f a = 2.4 amperes, so that E n of the test is equal to 99.4 volts. 

 Therefore, according to equation (30), the value of S, corres- 

 ponding to a =ioovolts, is approximately 



100 



238. 56 watts x - - = 239.8 watts 

 99-4 



If it were desired to find from the above test the approximate 

 value of S fora speed of 1,100 revolutions per minute and a 

 field excitation which gives with this speed E a = 105 volts, 

 equation (30) would give 



S= 238. 56 watts x - = 25 1. 8 watts 

 99-4 



65. Efficiency of a Generator. The efficiency of a generator is 

 defined as the ratio : output of power divided by intake of power. 

 This ratio is sometimes called the true efficiency or the commer- 

 cial efficiency to avoid confusing it with the two partial efficien- 

 cies defined later. 



Calculation of true efficiency. When the stray power loss of a 

 dynamo has been determined for a given speed and a given volt- 



