RECTIFIERS. Till- *Y\<'H !tn\uus CONVKltT/-:/! 365 



Usually, the transformer primaries may be connected either in 

 Y or in delta. The most common six-phase connections for the 

 transformer secondaries are the "diametrical," the star, and the 

 double- Y. (See Fig. 321, page 350.) The difference between 

 the diametrical and the star is that the secondaries are con- 

 1 together at the neutral point in the star, whereas three 

 separate secondaries are connected across diametrically opposite 

 IM .juts in the diametrical connection. There is no difference 

 between the double-Y and the star if the neutrals of the two Y- 

 s\ stems are connected together. Other than a slight effect on 

 harmonics, and the fact that a neutral is available in the double- 

 Y, there is little ditTerence in the use of the three connections, 

 pt with the split-pole converter. 



If the induced electromotive force of the converter armature 

 contains harmonics, there will be no circulatory current within 

 the armature itself, for in the direct-current type of armature 

 such as is used for the converter, any electromotive force induced 

 under a iziven pole in one part of the armature is opposed by an 

 opposite and equal electromotive force induced under an opposite 

 pole. However, if the line voltage is practically sinusoidal and 

 the indncul electromotive force of the converter contains har- 

 monics, there will be unbalanced harmonic voltages. The 

 current due to these unbalanced voltages will consist entirely of 

 harmonics which contribute no energy but do heat the armature 

 and transformers. 



This effect is neJii:ible in the ordinary converter, but in the 

 split-jH)le type, the voltage control depends on the introduction 

 of larjrc harmonic voltages into the electromotive force wave. 

 Therefore, when this type of convener i- used, the transformer 

 connect ions must be so chosen that as many as possible of the 

 harmonic cure < liminated. Mo<t three-phase transformer 



connections eliminate the third harmonic current and its multi- 

 ple-, with the following The primaries cannot be 

 connected in delta if the secondari. Miected either diamet- 

 rical, Stt-p ! doiiMe-Y, with the neiitraU of the two 

 Y-system- conned her. for the third harmonic cunvni- in 

 . unbalanced harmonic \ < .It .i^es. will cause 



third harmoni' ofarOUlatein 1 he primary delta, producing 



extra heating in the convei 'ure and in the transformers. 



