34 



COMMERCIAL ELECTRICAL TESTING 



Maximum Output 



The maximum output of direct current compound 

 wound generators is dependent upon their commutation, 

 or heating limitations; hence the maximum output 

 test on these machines is usually a commutation test, 

 which will be described later. Since in shunt wound 

 generators the voltage falls with the load at constant 

 field excitation, the maximum output is not always 

 limited by commutation. It is not usual to make 

 maximum output tests, however, on compound wound 

 machines, since they possess little practical interest. 



In the case of induction motors, the maximum output, 

 or breakdown point, is a matter of considerable impor- 



Fig. 10. Field Compounding Curves on a 150 Kw M 250 Volt, 225 R.P.M., 

 6-Pole, D-C. Generator (6 Bar Brush Shift) 



tance. If sufficient power is available, the motor is 

 loaded in successive steps up to the breakdown point, 

 beginning at zero load. During this test readings of 

 volts armature, amperes armature, speed, and motor 

 output are taken and plotted. It is essential that the 

 voltage and frequency of the power circuit from which 

 the motor is operating be held constant. It is also 

 important that the readings be taken quickly at over- 

 load, and that the motor be allowed to cool between 

 such readings, else it will overheat. Where sufficient 

 power is not available to take a breakdown test with 

 normal voltage impressed on the armature of the motor, 

 a voltage considerably below normal is used; viz., f, 

 |, or even \ voltage. It is then necessary to calculate 

 the full voltage results from those obtained at the lower 

 voltages. This may be done by increasing the power 

 output in proportion to the square of the ratio of normal 

 voltage to the lower voltage. 



