What is the significance of vector grouping in Power Transformers?

Every power transformer carries a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. EG. DYN11 means Delta primary, Wye Secondary and the current is at 11 o clock referred to the voltage.

Name power losess in rotating electrical machines.

Power losess in rotating electrical machines are Copper losses, core losses, mechanical losses and stray losses.

In the motors starting purpose why the armature rheostats at maximum position while the field rheostat is kept in minimum position?

In the time of starting a motor the field rheostat is kept in minimum state for the high starting torque.

In the time of starting a motor the armature rheostats is used to control or reduce the high starting current. This high starting current can damage the motors starting systems.