Wye (star) connections on a 3ph induction motor put the windings in series; to get from one phase to another, current passes through 2 or 3 windings, (dependant on the exact point on the cycle or phase)- which reduces the amps drawn; this is the reason for doing it on starting. An induction motor rpm is dependant only on the hz(cps) and the number of poles, (because the field of the stator revolves to match that of the generator in simple terms); less a certain amount of 'slip' by the rotor, usually reckoned at about 4% and which is essential for a pure induction motor to operate). An induction motor can draw up 6 times the amps pulled at full power, even if started unloaded, so a Y(STAR) start reduces the amps drawn and thus the size of the starter and wiring, and causes less of a shock to the mechanical parts of the driven machine too. Once up to speed, the motor is switched over,( by a special starter- these are usually automatic with a variable timer, but manual versions can be seen), to a Delta (ie coils wired directly across the supply) configuration for normal running. If left at the Y (star) wired configuration it can't develop its full potential HP. Hope this helps.