What does the open-circuit (no-load) test primarily determine?

• A. The relationship between generated emf and no-load current, helping determine the synchronous emf and reactance.
• B. The short-circuit current.
• C. The rotor inertia.
• D. The heating rate.

Answer: (A)

The open-circuit test shows how the machine’s internal generated emf responds to excitation when there is effectively no load on the terminals. With no load, the armature current is negligible, so the terminal voltage available at the terminals equals the internal generated emf behind the synchronous impedance. By varying the field excitation and recording the resulting emf, you map the relationship between the generated emf and the excitation (the no-load current). This information lets you determine the synchronous emf (the internal voltage the machine tries to produce) and, when used with other data, the synchronous reactance that lies behind that emf. The short-circuit test, rotor inertia, and heating rate address other aspects of machine behavior, so the open-circuit test is specifically about the emf that can be generated at no load as excitation changes.