We have installed a 50Hz Fischbach Disc-Rotor-Motor with VFD on a 60Hz grid. The VFD is only to bring the 60hz net to the 50Hz motor. However this is the second motor in half a year where the bearings wear out and the whole fan ensemble start to shake and brake.
Anyone any idea what goes wrong?

 

We have installed a 50Hz Fischbach Disc-Rotor-Motor with VFD on a 60Hz grid. The VFD is only to bring the 60hz net to the 50Hz motor. However this is the second motor in half a year where the bearings wear out and the whole fan ensemble start to shake and brake.
Anyone any idea what goes wrong?

The VFD in routing current thru the bearings to ground. This pits the bearings and race. Install a motor shaft grounding kit and this should resolve your issue.

 

What is alternater and generator

 

The premature bearing failures and vibration issues in your 50 Hz Fischbach Disc-Rotor motors, operated via VFD on a 60 Hz grid, are most likely caused by a combination of electrical and mechanical factors rather than random defects, as evidenced by two motors failing within six months. VFDs produce high-frequency PWM voltages that induce common-mode shaft voltages, which can discharge through motor bearings, causing pitting or fluting and leading to early failure. Mechanical imbalance or misalignment of the fan or coupling can further amplify vibrations and accelerate bearing wear, while improper lubrication, incorrect grease type, or seal failures can exacerbate the problem. Long VFD-to-motor cable runs without filtering may introduce voltage reflections, stressing bearings, and resonance at operating speed may amplify vibration in the fan assembly. Immediate inspection should focus on bearing damage patterns, grease condition, and fan assembly integrity, and motor grounding must be verified. Mitigation measures include installing shaft grounding devices or insulated bearings, using dV/dt or sine-wave output filters for long cables, reducing carrier frequency on the VFD, dynamically balancing the fan, checking coupling alignment, ensuring correct lubrication schedules, and employing shielded cables with proper grounding. Implementing these actions, along with vibration monitoring, can prevent recurrence and protect motor reliability.