Single-phasing protection of line-operated motors of different efficiency classes

Single-phasing is one of the major causes of motor failure in the industry. The electric motor market is changing fast due to the minimum energy performance standards. In a large number of cases, particularly when the original old motors are retrofitted with motors of higher efficiency classes, the respective overload protection devices are not properly tuned/configured or upgraded/replaced. In this paper, an overview of the motor protective devices and the main results of an experimental study on the behavior of five three-phase 7.5-kW, 400-V, 50-Hz, 4-pole motors, namely, four squirrel-cage induction motors of IE1, IE2/EPAct, IE3/NEMA Premium and IE4 classes and one line-start permanent-magnet of IE4 class, under single-phasing supply at no-load, locked-rotor and part-load operation, are presented. On the basis of the obtained results, it can be concluded that, in general, modern commercial thermal overload relays or thermal-magnetic circuit-breakers are able to protect line-operated motors against single-phasing operation, tripping within a safe time window, as long as they have differential mode mechanism, their trip class is properly selected, and the current setting correctly adjusted as a function of the motor efficiency class, technology, service factor, load, rated power and rated current.