what will happen if the motor that is designed for delta connected in star connection
When you connect it in a star configuration, the voltage across each of the stator windings will be reduced by \$\sqrt3\$ and correspondingly, the peak mechanical power that could be delivered to a load for a given slip and supply frequency will be lowered significantly.

So, for a comparable slip, the current and voltage will both be reduced by about \$\sqrt3\$ and hence the power delivered to a load is reduced by about 3.