Basically, the difference is because of the configuration. With the wye connection, what you're calling a star, you have the connections made on the outside of the three lines, with a neutral in the middle. On the delta connection, the triangle, the connections are also on the ends, but without a neutral in the middle. The disadvantages of star-star connection of three-phase transformer are the following. If the load on the secondary side of the transformer is unbalanced, the phase voltages of load side change unless the load-star point is earthed. The difficulty of shifting neutral can be overcome by connecting the primary star point to the star point of the Four years late but I didn't like the answers so far, so I'm posting mine. From what I understood of the answer by Andy, he said the phase shift is due to the typical 30° difference between a line-to-line voltage phasor and the corresponding phase voltage phasor, but that's a partial answer; you also need to consider how the delta winding is interconnected and how the windings are connected Voltage is defined as the amount of work done in bringing a unit positive charge from infinity to a point in the electric field. The terms line and Phase voltage are usually used in Alternating Current (AC) power supply systems, especially in the case of star and delta connection systems and three-phase electrical power systems. Three phase motor connection different work in hindi - why motor connect in star - induction motor connection types - benifit of delta connection - Electric P=0.4kV*1.73*40A = 27kW, which is already the rated power of described motor. For example a delta/star motor of 400/690V, suitable for star/delta starter should have a power 1/3rd in star connection, compared to nominal power when delta connection is established, so something arround 7kW. But it hard to say without any nameplate data. The emf induced in 3 phases is shown above. In star connection, there are two windings between each pair of outer, and due to the joining of similar ends together the emf induced in them are in opposition. The potential difference between the two outer known as line voltage, is the phasor difference of phases emfs of the two phases. So, the motor is actually only capable of operating at 58% of the rated current continually when connected in star. You simply have to look at 6-lead motor data to see that this current ratio is true - the higher voltage rated star connection will have less current. The ratio of delta to star current is 33% and 58% divided by 33% is 175%. Y6fyM.