What does the open-circuit (no-load) test reveal about a generator?

• A. The active power output under no-load conditions.
• B. The short-circuit current and stator winding resistance.
• C. The relationship between generated emf and no-load current, helping determine the synchronous emf and reactance.
• D. The rotor mechanical losses.

Answer: (C)

Open-circuit testing on a generator shows how the internal generated emf relates to the excitation current when there is no electrical load on the terminals. By exciting the field and observing the generated voltage at the terminals with the load open, you map the relationship between the internal emf behind the synchronous reactance and the no-load current. From these data you can determine the synchronous emf (the internal generated emf) and the synchronous reactance that the armature current sees. This is why this test is used to establish E_f and X_s. The other points don’t fit as well: active power under no load is essentially zero; the short-circuit test reveals short-circuit current and winding resistance; rotor mechanical losses aren’t determined by this electrical open-circuit test.