Star Delta Motor Starter, at higher ampere in Delta Configuration
As Marko Buršič pointed out, the motor is rated 3.7 kW (5 Hp) not 15 Hp. Either the motor's power rating is not high enough for the load, or the transition from star to delta is being done too quickly. If the load is a fan or centrifugal pump, the load will be very low at first and then it will increase very quickly as the motor speed increases. The very low current in star seems to indicate that the motor can supply the initial starting torque, but, in delta, the motor very quickly accelerates the load to a speed where the motor can no longer supply enough torque for further acceleration. The motor draws a high current attempting to accelerate the motor to full speed, but it does not have enough torque.

With a weak (high impedance) power distribution system, the higher current may cause the voltage to drop if the transition is done too quickly. In that case also, the motor may not have enough torque due to lack of sufficient voltage.

There is also the possibility of a mechanical problem with the load, some problem with the motor or a wiring error.