The following case study looks at the damaging effects of an electrical current finding a path to ground through a bearing, due to a loose rotor bar.  

The issue was identified during an initial condition monitoring inspection performed by SDT at a new customer site. The data collection and analysis was performed by Trevor Bruinix and Gilles Lanthier, from SDT’s Partnered Ultrasound Services Team. 

When a rotor bar becomes loose, electromagnetic forces cause it to lift and drop as it passes each stator winding slot. This repeated movement produces characteristic electrical and mechanical signatures. The rotor bar pass frequency (RBPF) can be calculated by multiplying the number of rotor bars by the RPM of the asset. 

Rotor bar-related defects are commonly associated with elevated amplitudes at rotor bar pass frequency (RBPF) with harmonics. In advanced stages, these defects are frequently accompanied by strong electrical signatures at multiples of line frequency due to arcing and intermittent current discharge. 

During the site visit, SDT’s service team was informed that the motor was operating at an abnormally elevated temperature and was asked to collect ultrasound condition data. The resulting spectrum, shown in Figure 1, revealed a dominant peak at 2× line frequency (7,200 cpm) along with multiple higher-order harmonics, including prominent peaks near 21,600 cpm and 28,800 cpm. 

The motor operates at approximately 1782 RPM, driving a pump through a reduction gearbox at approximately 288–300 RPM. While the driven equipment operates at low speed, the electrical and electromagnetic fault mechanisms originate within the motor itself. 

The presence of strong, repeated line-frequency harmonics at these amplitudes is a recognized indicator of electrical arcing and unstable current flow. When evaluated alongside excessive heat and previously confirmed RBPF-related defect activity at the motor, this spectral pattern strongly indicates that electrical current is discharging through the bearing.