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ELECTRICAL EQUIPMENT 



where the low-voltage winding may be connected for 115-230 

 volts. This makes possible the following connections: 



For transformers of very high voltages it is often requested 

 that the high-voltage winding be designed for series-parallel con- 

 nection. So, for example, by designing a transformer with a high- 

 voltage of say 110,000-55,000 volts, it is possible to operate the 

 system at the lower voltage until the load has increased to a point 

 necessitating a change-over to the higher transmission voltage. 



Magnetizing Current. The effect of the magnetizing current 

 in transformers sometimes leads to the question of considering its 

 proper limitations. It was previously shown that this current is 

 wattless with the exception of a small PR loss and has little influ- 

 ence on the values of the total current in the transformer when it 

 is operating at full load, but as the load decreases the effect be- 

 comes more prominent until at no load it is most noticeable, 

 and the power-factor naturally very low. This is an important 

 point where a large number of small transformers are operating 

 on a system, and for such cases it has become quite common to 

 limit the magnetizing current to a value not exceeding about 10 

 per cent of the full-load current, a value which cannot be consid- 

 ered detrimental to the system. 



There is also another limitation which is given consideration in 

 connection with large transformer units. Such transformers are 

 nowadays built of high-grade steel, which has a core loss per 

 pound much less than formerly, and this has in many instances 

 made it advisable to increase the core densities. If, however, 

 these are increased much above the bend of the saturation curve 

 an unstable operation is liable to follow, and for such conditions, 

 the limitation of the magnetizing current is governed by the per- 

 missible core density, usually around 90,000 lines per square inch. 

 With over-voltages causing a saturation of the core the mag- 

 netizing current increases very rapidly, but with the above limi- 



