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This is  op amp  based amplifier used to amplify the difference between two input signals. In other words it can be said that, A  difference amplifier  is a specially designed  op amp  based amplifier circuit, which amplifies the difference between two input signals and rejects any signals common to both inputs. Let us consider the above  op amp  circuit. Now, applying  Kirchhoff Current Law  at node 1, we get, This equation is written assuming that there is no  current  entering in the inverting terminal of the  op amp . Now, by simplifying the above equation, we get, Now, applying  Kirchhoff Current Law , at node 2, we get, We know that, in  ideal op amp  ,  voltage  at inverting input is same as the voltage at non inverting input. Hence, So, now from equation (i) and (ii), we get, The  difference amplifier  must reject any signal common to both inputs. That means, if polarity and magnitude of both input signals are same, the output must be zero. This condition must be satisf

Section III    Synchronous Machine Model The schematic diagram of a synchronous generator is shown in Fig. 1.15. This contains three stator windings that are spatially distributed. It is assumed that the windings are wye-connected. The winding currents are denoted by  i a   ,  i b   and  i c . The rotor contains the field winding the current through which is denoted by  i f   . The field winding is aligned with the so-called direct (  d  ) axis. We also define a quadrature (  q  ) axis that leads the  d  -axis by 90°. The angle between the d-axis and the a-phase of the stator winding is denoted by  θ d . Fig. 1.15 Schematic diagram of a synchronous generator. Let the self-inductance of the stator windings be denoted by  L aa ,  L bb ,  L cc   such that (1.80) and the mutual inductance between the windings be denoted as (1.81) The mutual inductances between the field coil and the stator windings vary as a function of θ d  and are given by (1.82) (1.83)