144 THE niRErr-rrKKENT MOTOT? < H. VI 



wheel. One machine is run as a motor, and a loud is 

 put on the other acting as a generator, by means of a 

 water or other rheostat. The two magnet circuits are in 

 series, so that the same current passes in both, and the 

 induction factors are equal. The full rated tension is 

 maintained on the motor terminals. 



The motors are first heated up to the normal working 

 temperature, and the resistances taken ; the test is then 

 commenced at once. The readings observed are the ter- 

 minal tension and current of motor and generator. 



The difference between the line watts and the motor 

 heat watts is equal to the mechanical watts in the motor. 

 The sum of the electrical watt output of the generator and 

 its heat watts is equal to the mechanical watts in the 

 generator. Hence the total mechanical loss is the difference 

 between the two mechanical watts, and assuming that 

 the losses are equally distributed in the two machines, 

 we get finally the torque loss in watts by halving this 

 quantity. 



If c, is the current in the motor, and c 2 that in the 

 generator, E the terminal tension of the motor, and 7? the 

 resistance in the motor circuit, u; the losses of the two 

 machines in watts, and t the torque losses in inch-pounds, 

 then .1.41c,M=l-41e a 2l/"+< y and Mn=Ec l li, so that we 

 have 



7r,=( Cl _f 2 ) (E- Cl R) (G2). 



We can thus find the torque losses simply by observing 

 Cj,c 2 and E, and it is not necessary to observe the terminal 

 tension on the generator. 



Example 40. A railway motor when tested in this 

 manner gave the following results. Motor terminal 



