Motor Issue

Does the delta contactor close?
Or does the supply trip?

As the motor is able to start in Star, there is no possibility that it is stalling due to mechanical overload in delta (the torque in delta is 3 times that in star).

You say there is a spark in the Υ contactor, but is it very severe? It may have welded, thereby preventing changeover to delta ?

Is there short-circuiting of the incoming supply when it changes from Υ to Δ ? This is quite common if there is no 'transition delay' of at least 30 ms between Υ contactor dropping and Δ switching on.

sounds like you have your connections the wrong way around, check those first then the SD starter checking the contacts, also the time taken for the change over, is it manual or automatic?