Precision measurement of rotor angle of a microsynchronous alternator

A precision instrument used for the measurement of rotor angles of a laboratory microalternator is described. The measurement is performed by integrating digital signals derived from a phase-locked loop (PLL) reference (i.e. laboratory infinite busbar or the terminals of another machine in a multimachine system) and an optical encoder mounted on the shaft of the machine. The integration is executed by synchronous up-down binary counters. A 12-b digital-analog converter enables both transient and dynamic movement of the rotor to be displayed on an oscilloscope or other suitable recorder. When used on a four-pole 3-MVA microalternator, the resolution of the encoder is 0.2 electrical degrees. Relatively slow and fractional movements of the rotor angle arising from dynamic power imbalances as well as fast and large rotor excursions arising from severe transient disturbances, which sometimes result in pole slipping, are reliably and precisely recorded. It is possible to obtain constant information on the stability or lack thereof for the machine under every operating condition. With appropriate transducers, the principle may easily be applied to continuously monitor changes in the rotor angle of large alternators in interconnected power systems