Reliability problems were encountered with the prototype units.  Bearing failures were occurring too early and rotors were rubbing on the stators causing catastrophic failures.  The manufacturer requested assistance in data acquisition and analysis.  The generator manufacturer had already acquired data indicating a rotor critical speed near 1.5X RPM.  The entire unit was mounted on rubber isolators.

The initial readings were taken on the generator set with a full range of load conditions.  The outboard end of the generator generated the highest vibration amplitudes in the vertical direction due to 1X RPM sources and was not load dependent.  The horizontal direction vibration was dominated at low loads by 1X RPM vibration but was dominated by 1.5X RPM vibration at high loads.

½X RPM harmonics are characteristic of V6 reciprocating engines.  The intensity of these pulses increases significantly with load.  This coincides with the findings that the 1.5X RPM amplitude increases significantly with load.  The rotor/stator contact was likely a result of excessive rotor deformation when the rotor critical at 1.5X RPM was excited by high load 1/2X RPM harmonics from the engine.  Also these pulses often excite torsional natural frequencies of attached machinery.

To determine if torsional natural frequencies were excited, a spectrum/sideband analysis was performed on the signal generated from the 14 generator fan blades as they passed a photo cell.  The high resolution spectrum is included along with the “zoom” view of the data which clearly shows sidebands spaced at 1.5X RPM (90 Hz).  The sidebands are caused by torsional vibration of the rotor.