16 COMMERCIAL ELECTRICAL TESTING 



the circuit breaker controlling the motor should there- 

 fore be so arranged that it can be readily operated by the 

 tester reading the speed. 



On alternating current apparatus, the machine is 

 run as a motor and the impressed voltage varied as 

 already described. The speed is independent of the 

 motor field in this case, and instead of regulating the 

 motor field for speed, it should be regulated to give 

 minimum input current at each voltage. Readings 

 should be taken of voltage impressed, amperes armature, 

 amperes field, and volts field. With induction motors 

 it is only necessary to impress variable voltages at con- 

 stant frequency and record readings of volts armature, 

 impressed amperes armature, and speed. 



The calculation of saturation tests is very simple, 

 as it only consists of applying instrument correction 

 factors and ratios, and plotting upon coordinate paper, 

 volts armature as ordinates and amperes field as abscis- 

 sae. Fig. 4, which is plotted from the values given in 

 Table I, shows the results of a saturation test made by 

 one of the above methods. 



Ballistic Saturation 



In order to make a generator or motor saturation 

 test, the machine must run at, or near, its normal 

 speed. Sometimes it is desirable to test for saturation 

 without the delay and expense of setting up and oper- 

 ating the machine. In this case a test known as "bal- 

 listic saturation" can be used, obtained as follows: 



First, a machine of given armature winding, armature 

 core, and field structure is operated for a running 

 saturation test as already described. The machine is 

 then shut down and its brushes shifted 90 electrical 

 degrees from their no-load neutral position when the 

 machine was running. The shunt field of the machine 

 is wired to a source of excitation and provided with a 

 field reversing switch so that the field circuit may be 

 quickly opened and reversed. This permits of the field 

 current being interrupted at any value, quickly reversed, 

 and allowed to rise to the same value again. Leads 

 from a ballistic galvanometer are then connected to 



