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The paper describes experimental results obtained from transient tests on a laboratory universal machine, connected to operate as a 3-phase alternator. During these tests, which were carried out in order to measure the parameters, severe harmonic distortion was found in the armature-current waveform under conditions of sudden symmetrical short circuit. This appeared to be mainly second and third harmonic together with a very low-frequency subharmonic. The paper gives an analysis of the machine with computed values of transient current and shows that, with the compromise design used to achieve a measure of generalisation, such harmonics are to be expected. It also shows mathematically that the usual transient d.c. component in the armature current may become a transient, very-low-frequency oscillation, depending on the machine parameters. These effects observed on a `universal machine? were also found during tests carried out on a second generalised machine of different design. It was found that damping and field time constant have considerable effect on the transient harmonic distortion in the short-circuit armature current.

The paper analyses the influence of various types of field regulators on the dynamic performance of synchronous generators. Both conventional and dual-excitation generators are considered, with basic controls provided by voltage and angle regulators, which may have proportionate and derivative feedback. In the case of dual-excitation machines, both bus-rotor-angle and terminal-rotor-angle signals are employed in the quadrature-axis regulator. Stability is initially defined by computed regulation curves, and additional information is then obtained by calculating the variations of the synchronising- and damping-torque coefficients (Ts and TD) with regulator gain. These graphs are superimposed on the regulation curves, to show how Ts and TD determine the dynamic stability of the machine, and to illustrate the underlying reasons for stability.

Current interest in doubly excited synchronous machines makes it relevant to consider the basic physics of a synchronous machine which has more than one field winding. In this paper, the synchronising and damping torques, which may arise in the individual windings of a large turbogenerator, are examined analytically for a number of different rotor-winding configurations.

The paper investigates synchronous-machine damping and synchronising torques analytically and experimentally, in two distinct reference frames. The first is that of Park and the second is a freely rotating frame used by Kron, Ku and others. it is found that the uniformly rotating free frame presents a clear picture of the nature of the torque components and the sources of these during hunting conditions. The analysis has been confirmed by tests on a laboratory machine.