TRANSFORMERS 377 



tation, based on normal voltage, an over-voltage of around 10 per 

 cent, which is to be expected, should not cause an excessive mag- 

 netizing current. 



With regard to efficiency and regulation the effect of the mag- 

 netizing current is insignificant. 



Reactance. The percentage of the total flux that links with 

 the primary but does not link with the secondary winding, plus 

 that which links with the secondary but not the primary, is the 

 per cent reactance of a transformer. Thus, if 95 per cent of the 

 primary flux cuts both primary and secondary, the transformer is 

 said to have a 5 per cent inherent reactance. 



The factors affected by the reactance of a transformer are its 

 regulation, parallel operation, mechanical stresses and eddy- 

 current losses. A low-reactance transformer has naturally a 

 better regulation than one of high reactance, especially for highly 

 inductive loads, and in order to obtain a good voltage regulation 

 it was formerly the custom to design transformers with a reac- 

 tance as low as 1 1 to 2 per cent. Such a low reactance is, however, 

 often detrimental to the safe operation of a transformer from the 

 mechanical point of view. If a short circuit should occur at the 

 secondary terminals of a transformer, and the power supply at 

 the primary is sufficient to maintain the primary terminal voltage,* 

 as may be the case in very large generating systems, the primary 

 and secondary currents of the transformer are limited by the 

 impedance only, and with the exception of very low reactance 

 transformers it is essentially the reactance which determines the 

 total impedance and thus the short-circuit current. 



As the primary and secondary currents are opposite in phase, 

 they repel each other, the force being approximately propor- 

 tional to the square of the current. It therefore follows that the 

 repulsion, which is small at full load, may reach enormous values 

 under short-circuit conditions if the transformer reactance is low. 

 For example, in a transformer having a 2 per cent reactance the 

 short-circuit current will be 50 times normal and the mechanical 

 stresses will increase as the square of this or 2500 times, amount- 

 ing to many hundred tons. This clearly illustrates the necessity 

 of a very rigid construction and also the advisability of reducing 

 the short circuit to a safe value. This may be done by increasing 

 the transformer reactance, and modern practice tends toward the 

 use of considerably higher internal reactances than was formerly 



