It is defined as the ratio of field current required to produce rated voltage on open circuit to the field current reqd. to circulate rated current on short circuit.

Explanation

The Fig shows open Circuit and short Circuit characteristics of an alternator.

According to definition,

Effect of SCR on Machine performance

 1. Voltage regulation

 2. Stability

 3. Parallel operation

 4. Short circuit Current

 5. Cost and size of the machine

 Parallel operation: 

SCR  = 1/ Xs,  as  SCR↑     Xs ↓  IXs ↑ V ↓  Psync ↓

 Short circuit current

Cost and size of the machine

as SCR ↓ Xs ↑ Zs ↑           Isc ↓        and hence cost of control equipment reduces

Size and cost of the machine as SCR Xs Zs Isc and hence cost of control equipment reduces

For salient pole machines SCR value varies from 0.9 to 1.3

For turbo alternators SCR value varies from 0.7 to 1.1