Operation on unbalanced voltage: one motor´s experience and more

Derating a motor for operation on an unbalanced voltage system is still much of a black art although NEMA and IEC have generic derating charts for this application. Numerous papers have been written in an attempt to provide a theory for this operating mode and the losses that may be generated. This paper discusses the instrumentation and testing of a 4-pole 1500 hp induction motor that was tested at three different levels of unbalanced voltage. Results is presented on the stator and rotor heating, the speed-torque and locked rotor performance as well as noise, vibration and shaft voltage. The problem has also been analyzed using time-stepping finite element analysis. Additional information on other machines and their performance is also be discussed.