A four-pole, 25kW , 250V separately excited DC motor is mechanically coupled to a three phase four-pole, 25kW, 460V, cylindrical pole synchronous generator. The DC motor is connected to a constant 250V DC supply and the ac generator is connected to a 460V fixed voltage, fixed frequency, three phase grid. The synchronous reactance of the synchronous generator is 6.6 Ω. The armature resistance of the DC motor is 22 mΩ. All unspecified losses are to be neglected.

1. If the two machines act as a motor-generator set receiving power from the DC source and delivering power to the ac grid, what is the internal phase voltage ESM of the ac machine when it delivers 25kW at unity power factor? What is the internal voltage (back-emf) of the DC motor?
2. Leaving the field current of the ac machine at the value corresponding to the condition of part (a), what adjustment can be made to reduce the power transfer between the two machines to zero? Under this condition of zero power transfer, what is the armature current of the DC machine? What is the armature current of the ac machine?
3. Leaving the field current of the ac machine as in parts (a) and (b), what adjustment can be made to cause the transfer of 25kW from tha ac source to the DC source? Under these conditions, what are the armature current and internal voltage of the DC machine? What will be the magnitude and phase of the current in the ac machine?