Fault Mechanism Research And Test Of Rotating Machinery With Variable Frequency Drives

Operation of pump and fan with variable frequency drives can reduce energy cost. However, faults occur on many equipments after VFD modification. It influences the safe and reliable operation of the equipment largely. The paper researches faults mechanism of equipment with variable frequency drives from the viewpoint of lateral vibration and torsional vibration to reduce system vibration.The finite element model of a cooling pump of Dabieshan Power Plant was set up. System natural frequencies and modes were calculated and the influence of boundary condition on system dynamics was calculated. Results show that system is in its resonance state when it is running in low speed zone. It causes large lateral vibration of motorA sets of test on the cooling pump with VFD were done. Change of vibration with rotating speed during run up and run down process were test and analyzed. Impulse test was also done to measure cylinder and rotor natural frequencies. Test results show that typical peak occurs at 1100rpm-1300rpm. The measured result is coincided with the calculated result. Balance test at the pump was done thereafter. Motor vibration was reduced successfully.Torque simulation model of induction motor with SPWM was set up to analyze fault mechanism of fan after VFD modification. Voltage harmonic component distribution was calculated. Many components including integer harmonic components and interharmonic components occur on VFD motor. It will cause torsional resonance if the harmonic component is coincided with system natural frequency.Torque during run up process, run down process and external disturbance was test on rotor-bearing rig. Torsional vibration and dynamic torque of a fan with shaft crack and coupling crack faults was test in field. Large torque pulsation occurs if rotating speed is higher than a threshold speed value. The pulsation frequency is 17.5Hz, which is shaft natural torsional frequency. It is the main reason of shaft crack.