THE TRANSFORMER 



217 



voltages from the point O (Fig. 205 (c) and Fig. 206 (6)) to a, 6 

 and c are all equal. Therefore, point is the neutral of the pri- 

 mary system. Point is two-thirds the way down the teaser 

 transformer winding from ai to d, Fig. 206 (6). 



In these connections the voltages become slightly unbalanced 

 even under balanced loads. This is due to the unsymmetrical 

 phase relations among the voltages and the currents in the 

 individual coils. 



IK,. L'07. T-connected transformers giving quarter-phase, 4-wire system with 



balanced voltages. 



94. Constant -current Transformers. The transformers here- 

 tofore considered are constant potential transformers; that is, 

 the secondary voltage remains substantially constant and a 

 change of load is accompanied by a corresponding change of 

 current . There are instances where a constant current is desired, 

 the most common being series street lighting. It will be recalled 

 that constant direct current is obtained from a series generator. 

 Constant alternating current is ordinarily obtained from a con- 

 stant current or "tub" transformer. 



Tip const ruction of the transformer is such that the primary 

 and the secondary can move with respect to each other. The 

 primary coil may le fixed and the secondary may move or the 

 secondary roil may he fixed and the primary may move. Both 

 are found in practice. FI'LHIH- 'JOS shows a transformer in 

 which the primary is stationary and the secondary is movable. 

 The load consists of a number of lamps connected in series. The 

 secondary is suspended from a lever which is counter-weighted. 

 A da.-hpot i- provided to prevent rapid iluct nations in the posi- 

 tion of the movini: coil. 



The operation of t he Iran-former is as follows: Assume that the 

 <eeond:iry coil ifl " floating "; I hat is. it i- free to move eit her up or 

 doun and is deli\ n'n current to a sen.- load. The 



