128 COMMERCIAL ELECTRICAL TESTING 



Running light is taken by applying normal voltage to 

 the stator and reading the amperes input to the motor. 

 Static impedance is taken by blocking the rotor and 

 applying such a voltage to the stator as will give about 

 full load current, reading the current in each leg, to- 

 gether with the voltage between each of the legs. If 

 the motor is of the form L type, impedance is taken with 

 the resistance all in and then all out, always holding the 

 same voltage across the stator. This practice has been 

 found to give the best results. End play should be 

 tested both with and without voltage on the stator, and 

 on all motors particular care should be taken to see that 

 the rotor is in perfect balance. 



When cutting out the internal resistance, the start- 

 ing switch of form L motors should be watched closely 

 for sparking or any other defects. The brushes must 

 make good contact on the resistances in all positions and 

 the switch must not work too easily, otherwise the 

 resistance may be cut out too rapidly. 



On form M motors, the brushes must fit the collector 

 rings perfectly, as a successful test on this type of motor 

 depends considerably on a good fit. The voltage ratio 

 should be taken on form M motors by impressing normal 

 voltage on the stator and measuring the voltage between 

 the rings of the rotor on open circuit. Volts and amperes 

 stator, and volts between rotor rings should be read and 

 recorded. 



Two speeds on a motor can be obtained by changing 

 the connections on the stator by means of a switch and 

 connection board, these changes altering the number of 

 effective poles. The rotor must have the correct num- 

 ber and ratio of slots in the stator and rotor, otherwise 

 dead points may occur at certain starting positions, or 

 again the motor may operate at subsynchronous speeds. 

 These machines are usually run at the lower speed during 

 test. 



Excitation 



The tests for excitation and impedance are important, 

 and the following precautions must be observed in all 

 cases. The calculation of the characteristic curves of 



