Interaction of synchronous machines: A graphical solution by means of a new circle diagram

When two synchronous machines are in operation as generator and motor, the amount of power demanded of the motor causes it to assume a definite phase relation to the generator, somewhat dependent also upon their relative excitation and upon the impedance of the circuit including the two machines. When such machines are operating in parallel there is theoretically no power to be transferred, but this ideal condition can only exist when the excitation and phase relation of the machines are identical; if these differ, even by a small amount, transfer of power must take place between generator and motor. The power required by the motor causes a certain vector difference; conversely, the presence of a vector difference points to a definite transfer of power from one machine to the other.