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

Abstract

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.

Experimental study of the effect of positive sequence voltage on the derating of induction motors under voltage unbalance

“…Therefore to maintain the operational life of the motor, the international standards [6], [7] recommend the derating of the motor. Researches about the derating of induction motors under unbalanced voltage conditions have been reported in the literature since 1959 [8][9][10][11][12][13][14][15][16][17][18][19][20]. The most accurate method for derating under unbalanced voltage conditions is to reduce the induction motor load so as to limit the temperature rise to the normal value.…”

“…In 2009, several authors [16][17][18] have presented studies about the performance of induction motors under different voltage unbalance conditions. In [16] a 1.5 kW standard induction motor has used but without a thermal monitoring system.…”

“…In [17] the load carrying capacity of two induction motors of 3 kW and 5 kW was studied under voltage unbalance combined with over-or under-voltage. In [18] an empirical verification of the NEMA MG1 derating graph was presented using temperature sensors embedded in a 5 HP motor.…”