ACTUAL LOAD TESTS 



39 



motors. Cases sometimes occur when the speed of 

 the motor, due to armature reaction, increases with 

 increase of load. In pumping back, this condition is 

 shown by the motors taking an overload at no load 

 speed, in which case the speed of the loss supply must 

 be increased. 



In the method of electrical loss supply, two machines 

 are direct connected or belted together and the losses 

 supplied electrically. Should two shunt motors be 

 tested by this method, one machine should be run at 

 normal voltage, current, speed and full field; the other 

 motor to be run as a generator with a little higher 

 current and slightly stronger field than for normal 

 conditions. The fields of the generator may have to 

 be connected in multiple. Connections should be made 

 as in Fig. 13. The motor should be started first from 

 the electrical loss supply circuit and its brushes shifted 



Fig. 13. Connections for Electrical Loss Supply Pump Back 



for commutation and speed. After exciting the field 

 of the generator and adjusting the voltage between 

 the machines to zero, the circuit is closed. The machines 

 are loaded by increasing the field current of the gener- 

 ator. Care should always be exercised when shifting 

 the brushes while the machines are under load, since 

 a slight change in shift will at once change the load. 

 After the heat run has been finished and all motor 

 readings taken, the wiring should be changed and motor 

 readings taken on the machine which ran as a gener- 

 ator. 



