TRANSFORMERS 183 



treated as a single-phase transformer. Use as many 

 thermometers on one three-phase transformer as on 

 three single-phase transformers. A higher pressure is 

 necessary to force air through the ducts in the iron of 

 the three-phase transformer than in the single-phase, 

 and the damper must be carefully adjusted. 



In calculating the voltage required to supply the 

 primary current for the heat run, the following rule may 

 be used : If the primaries are connected in delta, multi- 

 ply the impedance voltage by three; if the primaries 



are connected in Y, multiply the impedance voltage by 

 3 

 -~- If overload is required, add 50 per cent for 50 



per cent overload and 25 per cent for 25 per cent 

 overload. 



If it is impracticable to open the primary connection 

 in order to take the heat run by applying three-phase 

 voltage to the secondaries and single-phase current to 

 the primary, the following method may be used: Con- 

 nect the transformers as shown in Fig. 62. Auxiliary 

 transformers A, A', A" are used to supply secondary 

 voltage, and transformers B, B r , E" as series transformer 

 to supply the impedance voltage to the secondary 

 circuits. The voltage necessary to supply full load cur- 

 rent is twice the impedance voltage of one transformer. 

 The impedance voltage is the same percentage of the 

 total voltage, irrespective of the circuit to which it is 

 applied. The figure shows two three-phase alternators, 

 one supplying core loss and exciting current, and the 

 other the copper losses. When two alternators are 

 used they must be run at nearly the same frequency, 

 otherwise the superposition of the impedance voltage on 

 the core loss voltage will impart a slight swing to the 

 meter needles. Instead of the three transformers B, B f , 

 B", a three-phase transformer or an induction regulator 

 may be used. If a regulator is used, all the losses may be 

 supplied from one alternator. 



Double Potential Test 



If any repairs or alterations are required, such as 

 making primary delta or Y connections permanent, or 



