Transient characteristics of a laboratory universal machine operating as an alternator

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.