In a synchronous machine, the ratio of the field current required to produce rated voltage on an-open circuit to the field current required to produce rated armature current with the armature terminals shorted while the machine is mechanically run at synchronous speed is termed as:

The short circuit ratio (SCR) of a synchronous machine is defined as the ratio of the field current required to generate rated voltage on an open circuit to the field current required to circulate rated armature current on short circuit.

\(SCR = \frac{{{I_f}\;for\;rated\;O.C.\;voltage}}{{{I_f}\;for\;rated\;S.C.\;current}} = \frac{{Oa}}{{Od}}\)

• For the small value of the short circuit ratio (SCR), the synchronizing power is small.
• As the synchronizing power keeps the machine in synchronism, a lower value of the SCR has a low stability limit.
• Thus, a machine with a low SCR is less stable when operating in parallel with the other generators.
• A synchronous machine with the high value of SCR had a better voltage regulation and improved steady state stability limit, but the short circuit fault current in the armature is high.