Impact of stator grounding in low power single-phase EC-motors

Bearing currents cause a reduction of ball bearing´s lifetime in electronically commutated permanent magnet motors. To compensate the lifetime decreasing factors, a tradeoff between special bearings and additional changes in the machine design have to be found. Stator grounding, for example, keeps the bearing voltage low and thus guarantees a long fan system´s lifetime, however, results in a higher input filter effort. On the other hand, instead of reducing the bearing voltage, hybrid bearings could be deployed, which feature a high dielectric strength. The major disadvantage is that hybrid bearings are much more expensive than conventional metal bearings, thus the extra cost for the hybrid bearings has to be weighed up against the higher filter effort needed with stator grounding. In this paper bearing damage mechanisms are analyzed and the impact of stator grounding to avoid bearing currents in low power singlephase motors is presented. Based on the presented filter design procedure, the additional filter effort for stator grounded motors is determined, which shows that hybrid bearing are undesirable in mass application.