Q6Electrical Machine Design
Question
Q.6. Define the short circuit ratio of synchronous machine and no-load current of a transformer.
Answer
SCR of a synchronous machine is the ratio of field current for rated open-circuit voltage to field current for rated short-circuit current; no-load current of a transformer is the small current drawn on open circuit, needed to establish the core flux and supply core loss.
The Short Circuit Ratio (SCR) of a synchronous machine is defined as the ratio of the field current required to generate rated voltage on open circuit to the field current required to circulate rated armature current on sustained short circuit, and is approximately the reciprocal of the per-unit synchronous reactance, serving as an indicator of voltage regulation and stability margin as discussed in more detail elsewhere in this paper.
The no-load current of a transformer is the (relatively small) current drawn by the primary winding when the secondary is open-circuited (unloaded) and rated voltage is applied to the primary. It consists of two components: the magnetizing current (the larger, reactive component, needed to establish the working flux in the core) and the core-loss (working) component (a smaller, in-phase-with-voltage component, supplying the hysteresis and eddy current losses in the core), and is typically only about 2-5% of the full-load rated current for a well-designed power transformer.