Why does the Dual Speed Dahlander motor need a bridge in the fast speed configuration?
Bridging U1, V1 and W1 reduces the voltage across each winding to 86.6% of the voltage without bridging. That reduces the load torque that the motor can drive, but that is compensated by doubling the number of poles. The motor may seem to operate satisfactorily, but the motor will probably draw more current for any given load. The design is probably designed to be most efficient with the high speed connection. Reducing the voltage per winding at the lower speed may be necessary to keep the motor operating current and temperature within safe limits with a less efficient winding configuration and and the reduced cooling air flow due to the lower rotor speed. Whatever the detailed design reasons are, the motor should be connected as the nameplate indicates.