SOLUTION

• The rate at which synchronous power, ‘P’ varies with load angle δ is called the synchronizing-power coefficient.
• It is also called stiffness of coupling, stability, or rigidity factor; It is represented as P_syn.

$\begin{array}{l} {P_{syn}} = \frac{{V{E_f}}}{{{X_s}}}cos\delta \\ \\ P \propto \frac{1}{{{X_s}}} \end{array}$

In a synchronous motor, the synchronizing power comes into action when rotor speed is either less or more than synchronous speed.

• Synchronizing power of synchronous machines is inversely proportional to the synchronous reactance
• A synchronous machine, when synchronized to infinite busbars has an inherent tendency to remain in synchronism
• At perfect synchronization, their synchronizing power is zero.
• Hence, an over-excited synchronous machine is more rigidly coupled than the one which is under-excited. A large air gap decreases the value of (synchronous reactance) X_s, thus a synchronous machine with a longer air gap is more stiffer than the one with smaller air-gap units of synchronizing power coefficient are watt per electrical radian.
• The variation of synchronous power with the change of load angle is called the synchronizing power. It exists only during the transient state, i.e. whenever there is a sudden disturbance in load (or steady-state operating conditions).
• Once the steady-state is reached, the synchronizing power reduces to zero.
• The synchronizing power flows from or to the bus in order to maintain the relative velocity between the interacting stator and rotor field, zero, once the equality is reached, the synchronizing power vanishes.