Experimental analysis of industry standards on derating of a three-phase induction motor due to thermal stress caused by voltage unbalance

This paper presents the results and analysis of an empirical verification of the NEMA standard MG-1 voltage unbalance derating graph using temperature sensors embedded in a 5 Hp motor. This verification process identifies a reference coil temperature and automatically regulates the load to maintain the reference temperature at the single hottest coil for a wide range of incoming voltage unbalances such that design lifespan is maintained. The experimental results are disparate with that of the NEMA MG-1 standard and display evidence that conditions of unbalance play a role in motor derating. An investigation between IEC and IEEE style unbalance calculations is presented with a clear indication that both styles represent the same physical system nearly identically.