Assuming 100 amperes and 1,000 volts in each phase winding, the pressure between any two conductors is the same as the pressure in the winding, and the current in any conductor is equal to the current in the winding multiplied by √3, that is 100 × 1.732 = 173.2 amperes, that is, disregarding the fraction, 173 amperes.
Ques. What are the relative merits of the star and delta connections?
Ans. The power output of each is the same, but the star connection gives a higher line voltage, hence smaller conductors may be used.
Fig. 2,170.—T connection of transformers in which three phase current is transformed with two transformers. The connections are clearly shown in the illustration. The voltage across one transformer is only 86.6% of that across the other, so that if each transformer be designed especially for its work one will have a rating of .866 EI and the other EI. The combined rates will then be 1.866 as compared with 1.732 EI for three single phase transformers connected either star or delta.
When it is remembered that the cost of copper conductors is inversely as the square of the voltage, the advantage of the Y connected system can be seen at once.
Assuming that three transformers are used for a three phase system of given voltage, each transformer, star connected, would be wound for 1 ÷ √3 = 58% of the given voltage, and for full current.
For delta connection, the winding of each transformer is for 58% of the current. Accordingly the turns required for star connection are only 58% of those required for delta connection.
Ques. What is the objection to the star connection for three phase work?